Soils: Nature, Fertility Conservation and Management

Soils: Nature, Fertility Conservation and Management

Ezekiel A. Akinrinde Agronomy Department, University of Ibadan, Ibadan, Nigeria

AMS Publishing, Inc. 2004 Tel: +00921 231 13333, Fax: +00921 231 13334 Vienna, P. O. Box 1123, Austria

Copyright © 2004 Lulu, Inc.

All rights reserved.

First published in July 2004 Second impression, May 2006

No part of this publication may be produced or transmitted in any form by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing. Address requests for permission to reproduce materials from the book or for further information to: Akinrinde E.A., Agronomy Department, University of Ibadan, Ibadan, Nigeria Comments and observations can also be directed to the editors: Prof. Victor Chude, National Programme for Food Security, PCU Headquarters, Federal Ministry of Agriculture and Water Resources,Near VIO Office MABUSHI District, Abuja, Nigeria.

& Prof. M. A. Amakiri Department of Forestry and Environment, Rivers State University of Science and Technology, Port Harcourt, Nigeria.

AMS Publishing, Inc., 2004

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TABLE OF CONTENTS Chapter / Contents Pages Preface ------------------------------------------------------------------------------------------------------------- iv Introduction --------------------------------------------------------------------------------------------------------1 1. Rocks and their Weathering------------------------------------------------------------------------------3 Types of rock and their minerals --------------------------------------------------------------3 Rock weathering ---------------------------------------------------------------------------------8 2. Soil Composition and Formation - ---------------------------------------------------------------------11 Soil Components -------------------------------------------------------------------------------11 Factors influencing soil formation ----------------------------------------------------------- 19 3. Soil Profile and Properties------------- ------------------------------------------------------------------21 Soil Profile Study -------------------------------------------------------------------------------21 Soil Properties -----------------------------------------------------------------------------------27 4. Soil Fertility Conservation and Management----------------------------------------------------------39 Introduction -------------------------------------------------------------------------------------39 Chemical dynamics of mineral soils ---------------------------------------------------------44 Measures of soil chemical dynamics ---------------------------------------------------------51 General principles of soil management ------------------------------------------------------55 Soil erosion, desertation problems and Control -------------------------------------------- 60 5. Soil Biology and Fertility---------------------------------------------------------------------------------67 Soil Biology ------------------------------------------------------------------------------------ 67 Soil Fertility ------------------------------------------------------------------------------------ 72 Fertilizers --------------------------------------------------------------------------------------- 77 6. Soil-Water-Plant Relations-------------------------------------------------------------------------------- 83 Water use by crop plants ----------------------------------------------------------------------- 83 Irrigation and Management of Irrigated Soils ------------------------------------------------87 Principles of land Drainage --------------------------------------------------------------------- 93 References ------------------------------------------------------------------------------------------------------------ 97 Appendix ------------------------------------------------------------------------------------------------------------- 101 Subject Index ---------------------------------------------------------------------------------------------------------113

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PREFACE The soil is a very crucial factor in food production. Its impact can result to food crises. The most important problem of tropical agriculture is the inability of the land to sustain annual food crop for more than a few years at a time. Soil science as a discipline is represented by the sub divisions of soil physics, soil chemistry, soil mineralogy, soil microbiology, soil fertility, soil genesis, soil morphology, classification and survey, soil technology and soil conservation. These sub divisions generally aim at providing the basis and idea of maintaining or improving the productivity of farmlands. Recognizing this situation, agricultural establishments (State and Federal Ministries of Agriculture, Agricultural Research Stations and Colleges or Schools/ Departments of Agriculture) are putting increased emphasis on the research into and the teaching of Soil Science. It is an obvious fact that a potential agriculturist should be well educated on the basic principles of soil science. For quite a long time, the need for a comprehensive but concise introductory textbook on soil science for undergraduates has been felt. This book is intended to provide basic but yet thorough introduction to the study of soil science that is involved in the new course content provided under the minimum standard created for universities in Nigeria. It is based on each topic of the course content with some additions to suit the undergraduates and graduate students in the university system. The topics have been subjected to daily classroom teaching. As such students’ difficulties have been taken into consideration. Indeed, efforts have been made to treat some interesting topics – which most students seem to find difficult in such a way that learners can follow up without either Teachers’ guidance or reference to other textbooks. In most cases, graduate students making use of this book will need to make very few references to other advanced textbooks in order to have thorough conception of the discipline. The author wishes to put on record his gratitude to Messrs Akinpelu, Iyiola and Gbadamosi for their respective assistance in typing the original manuscript, and encouraging the printing of the book. Finally, it is a pleasure to express our gratitude to God Almighty with whose support the efforts have been successful. Ezekiel A. Akinrinde (July 2004).

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The King of kings

v

vi

INTRODUCTION Agricultural development is crucial to the survival of mankind in as much as the provision of food, shelter and clothing is closely associated with it. Food, in particular, is necessary for growth, energy production for good health and normal; development of the populace. All living things (Plants and animals) depend on their environment for survival – to remain alive, thrive and reproduce their kinds – As could be expected, nearly all green plants including our farm crops (having their roots fixed in the soil) depend on the fertile and productive soils that provide anchorage and conducive environment on one hand and supply all the essential materials which they need for their growth. Since animals, in turn, depend on plants, it becomes obvious that all agricultural activities directly or indirectly depend on the soil. It is from the soil that plants obtain their food (called nutrients) and water. It also contains air needed for respiration of the roots. Plants are able to stand upright because their roots are firmly held by the soil. Certain organisms that may affect the growth of the plant are also found in the soil. Thus, soil is far from being a simple substance. It is a mixture of several things – mineral matter, humus, water, air, animals and unicellular plants including bacteria. Physically, the soil is a mixture of mineral particles of varying sizes – coarse and fine. It can also be taken as a natural body on the surface of the earth, which supports the growth of plants. In present day Agriculture, considerable emphasis is given to the inorganic nutrition of the plant in some cases with seeming disregard for massive role of carbon dioxide and light. Keeping the latter two factors in perspective, however, it is appropriate to discuss mineral nutrition. The mineral elements are critical indeed, and facet of the environment is one readily changed by the agriculturist through soil management and fertilizer application practices. There is no doubt, the need for a more intensive crop production to feed the ever-increasing human population. As such, yields of genetically improved crop varieties should be further enhanced by optimum plan nutrition – the process by which living organisms obtain their food materials from their environment. A soil may be regarded as fertile when it supplies adequate plant nutrients. Absence of any one of the ESSENTIAL NUTRIENTS acts as a limiting factor and thus affects normal growth of the plant. The plants have the ability of assimilating large amounts of certain elements out of proportion to their abundance in the soil. Plants usually take in simple materials and build them into more complicated substances, which can be used as human/animal food. Such materials are H2O, CO2 and mineral salts (e.g. NO3, SO4 and PO4). From these they build up carbohydrates, oil and protein. The process of building up of chemical substance from simpler substances is known as SYNTHESIS. The two basic criteria for establishing the essentiality of an element are: (i) If the plant (when grown in a medium devoid of that element) fails to grow and to complete it’s life cycle, whereas in the presence of a suitable concentration of that element it grows and reproduces normally. In this wise, an indirect or secondary beneficial effects on some other elements, do not qualify an element as essential. (ii) If the element is shown to be a constituent of a molecule which is known as an element metabolite. It is important to keep in mind that the quantities of nutrients taken from the more readily available supply in the soil. Furthermore, the quantities removed by a single crop may seem rather small in some instances, but when the quantities contained in all the crops of a rotation are summed or when the amounts removed by crops for several years are considered, the necessity of supplying plant nutrients in the form of fertilizers and manures to maintain soil fertility is apparent. Before a farmer applies fertilizer to his farm for replenishing of nutrients, he has to know the deficient elements and at which quantity should it be used to produce optimum yield because

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

different quantities for its optimum production. Appropriate fertilizer types should be applied to the soil so as to avoid chemical imbalance because non-availability of others to the plants while it is possible for the same thing to happen if one element is in excess in the soil. Hence, soil fertility evaluation is like drawing up a nutrient balance sheet of crop – soil relationship in effort to produce at optimum level and yet maintain the integrity of the soil for many years. Since human survival depends so much on productive and fertile soil, preservation and conservation methods must be ensured to avoid soil mineral losses through various degradation processes. Soils should not be over used and they should be kept at an optimum productivity level if supply of food and fibre for the ever-increasing human population will be maintained. Furthermore, high yields are necessary for farming to be economic and to raise world food production. For these reasons, it is highly essential and desirable for agriculturists to be knowledgeable in SOIL SCIENCE – the study of soil physical, chemical and biological properties. Indeed, the study of agriculture logically begins with the study of the soil and proper understanding of soil leads to its wise management. The study of the soil as a science involves the knowledge of the more basic sciences (geology, chemistry, physics and biology). Hence, soil science is the application of the science of the theoretical basic sciences. The focus in the first section of this book is on the following: Soil components, Types of rock and minerals, Soil formation and weathering of rocks, Factors influencing soil formation, and Properties of soil (type, texture, structure, aeration, temperature, pH). Subsequently, attention is given to Nutrient cycling and Maintenance of soil fertility.

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CHAPTER 1 ROCKS AND THEIR WEATHERING Types of Rocks and their Minerals The knowledge of rocks that form the earth’s crust as the soil parent materials is essential to the study of soil formation. Such a body of knowledge is termed GEOLOGY. Similarly of great importance in soil formation is the knowledge of landscape – forming processes that have resulted in the relief and the formation of secondary deposits, which are also parent materials. The study of the landscape formation is GEOMORPHOLOGY (Physical geology). A rock may be described as an igneous or stratified mineral constituent making up the earth’s crust. It is the base on which the sub – soil and the soil parent material immediately lies. The classification of rocks involves the placing of the rock in the right category according to their origins (the ways in which they were formed), colours, texture, shapes of crystals, hardness, reaction, to HCl, presence of fossils, presence of metals and concentration of sand and clay particles. In this way tow major types of rock have been identified. 1. Original or Primary Rock. This is the rock from which the others are ultimately formed. It is otherwise known as IGNEOUS ROCK. This name was originally formed from Latin word “Ignis” which means, “fire”. The rock is formed by heat from molten magma. When it cools down it hardens. This means that rocks are derived from an original molten material or magma transferred from the lower regions f the earth’s crust to layers near the surface. The rock can either be “intrusive” i.e. formed in situ in the earth or pushed up even to the surface (in the case of Extrusive Igneous rocks). As the cooling occurs, crystals combine to form the rock. Differences in this type of rock are due to the method and the speed of the cooling process. The slower the cooling of the magma, the larger would be the crystals since slow rate of cooling permits growth before the rock become hard. If, however, sudden cooling occurs, it gives rise to very minute crystals of individual minerals. They may be minute making it difficult to be seen with the naked eye and such rocks appear to be uniform and without individual mineral crystals except when viewed through a microscope. Examples of Igneous rock include granites, diorites, basalts and gabbros. 2. Secondary rock There are two categories of this type of rocks. When the original rocks are exposed at the surface, they can be weathered and eroded and the detached materials transported and later deposited as sediment with the aid of wind or water. Rocks derived from such sediments are known as SEDIMENTARY rocks. The way in which such rocks are built up layer gives rise to the characteristics stratified nature. Examples of rocks so formed by the consolidation of sediments that are accumulated by wind or water at the surface level are sandstones, shale, limestone and conglomerate. Occasionally, previously existing rocks, (igneous or sedimentary) can be greatly affected and changed by heat and pressure to form the second category of secondary known as METAMORPHIC rocks. Examples are Gneiss, Slate, Marble, Quartzite and graphite. The Inorganic Framework of Rocks Rocks differ in their mineral contents. They also vary in the size, arrangement and chemical composition f the constituent minerals. A mineral is naturally occurring substance having a fairly uniform chemical composition and a regular well defined crystalline structure, though a particular

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

mineral can vary slightly in its exact chemical composition as a result of the substitution of one element for another in the crystal structure. In most cases, the minerals are silicates – combination of silicon and oxygen with other elements. Knowledge of the structure of a mineral helps in comprehending how easily it can weather and what elements it is likely to release. The basic structural unit is, however, very simple – silica tetrahedron or pyramid (a four – sided unit) in which one relatively small silicon atom at the centre is linked (by bonding) to four much larger oxygen atoms that surrounds it and which form the four corners of the regular tetrahedron. The silicate minerals can thus be classified on the basis of the way the fundamental tetrahedron units have linked up to form the mineral. The four different types are: A. Nesosilicates: Silicate minerals in which the silica tetrahedron remains from each other with no shared oxygen atoms, but is linked by intermediate cations. This is the reason why the name was coined from the Latin word “nesos”, which means, “Island”. Thus in the olivine group of minerals, the silica tetrahedral are linked by divalent magnesium (Mg2+) and iron (Fe2+) ions. Olivine (termed ferro – magnesium silicate mineral) is easily weathered since the ferrous iron and magnesium cations are exposed at the edge of the crystals and can oxidized or hydrated to cause the disintegration of the mineral. Some other neso-silicates that contain other cations can be more resistant. It can be concluded, therefore, that Olivine is an example of a group of minerals that is rich in iron ad magnesium and weathers relatively easily and releases magnesium and iron. It is usually dark in colour ad may include minerals of other structural. B. Inosilicates: Coined from the Latin word “inos” meaning “fibre”, these silicate minerals have their silica tetrahedral joined to form chains. Those that occur in single chains are members of the pyroxene family of minerals while those in double chains are the amphiboles. Both types have Ca2+ and Mg2+ cations as the link of the chains. With hornblende as the most important member of the group, pyroxenes and amphiboles are, thus, calcium silicates. Due to isomorphism, however, other cations like Fe2+, Mn2+ or Na+ can exist in the crystalline structure to give rise to different minerals within the family. They are known to be dark – colored minerals that weather relatively easily and expectedly release large amounts of Ca and Mg to the soil. C. Phyllosilicates: These include both the common rock – forming minerals (the micas) and the silicate clay minerals. The silica tetrahedra share three of their oxygen atoms to form flat sheets of tetrahedral. The sheets are tied to each other (above and below) by linking cations. Since they appear as leaves the name was taken from the Latin word “phyllon”, meaning, “leaf”. D. Tectosillicates: In this extreme case of linking up each silica tetrahedral shares all of its four oxygen atoms with other ones above, below and on the sides of it. With every oxygen atom shared by two adjoining tetrahedral, there are half the oxygen atoms in relation to silicon compared to the case in nesosilicates where the tetrahedral are all separate and non of the oxygen atoms are share. The general composition for the tectosilicate is SiO2 compared to SiO4 for the nesosilicate. A very typical example of a tectosilicate is quartz - a mineral s\consisting if silica tetrahedral and of nothing else. This simple and regular structure of quartz makes it extremely resistant to chemical weathering. Another very important group of tectosilicates are the feldspars having a more complex formula and less regular structure than quartz due to “isomorphism”, during rock formation, of one ion in the crystal lattice by another of approximately the same size. Though “isomorphism” means “same shape” to imply that the ions introduced are approximately of the same size as those being replaced, in practice they are either a little smaller ad may also have a different valency. For an ion of a slightly different to be fitted into a crystal, the structure becomes imperfectly regular to the extent that the extra stresses lattice and cause decomposition or weathering more rapidly than what occurs for a more regular one. In the same vein if a cation substitutes another one of a different 4

ROCKS AND THEIR WEATHERING

valency (e.g Al3+, replacing Si4+ or Mg2+ replacing Al3+), there will be net negative charges from the oxygen left unsatisfied. For electrical neutrality, an additional cation (such as Na+ or K+) has to be introduced. Such additions modify the structure. The relevance of isomorphous substitution can be further illustrated by comparing two groups of common minerals, the micas and the feldspars, in which isomophous substitution occurs to a considerable degree with quartz in which the simple structure (without the possibility of isomorphous substitution) gives the mineral a very high degree of resistance to chemical weathering. Micas are sheet silicates (phyllosilicates) having many years, each consisting of two sheets of silica tetrahedral held together by a layer of aluminium and hydroxyl ions (in the case of white mica). The silica – aluminium layers are held to each other relatively weakly by potassium ions and can be separated easily to give the micas their characteristics cleavage that permits them to be separated very thin sheets. White mica (muscovite) is therefore a potassium aluminum silicate – KAl2 (AlSi3O10) (OH)2 – one quarter of the silica tetrahedral have had the silicon atom at the centre replaced by aluminium and in each case this has been balanced by bringing in one potassium ion. Biotite (black mica) is formed when the aluminium in white mica is replaced by iron or magnesium, so that (Mg. Fe)3 replaces Al2 in the formula, Biotite is therefore, the richer of the two types of mica as regards plant nutrients and is also more easily weathered than the relatively resistant muscovite. Biotite is also one of the ferro – magensian minerals with the typical dark color. Sericite is a form of white mica, usually but not necessarily of muscovite composition, occurring as flakes and is often a constituent of the metamorphic rocks. Feldspars being a typical example of tectosilicates, have a three – dimensional block structure whereby all the silica tetrahedral share oxygen atoms with all adjacent tetrahedral and each oxygen atoms is therefore shared between two tetrahedral (as in quartz) since a proportion of the central silicon ions (valency of four) have been replaced by Al3+ there is an excess negative charge to be satisfied by a cation. A potassium ion is introduced to satisfy the excess charge. The potassium feldspar (KAlSi3O8) is thus formed when a potassium ion is introduced to satisfy the excess charge. The potassium feldspars are of two types (orthoclase and microcline) with similar composition but simply different crystalline forms because of different temperatures or formation. It is, however, possible for the excess negative charge not to be satisfied by a single cation but by a combination of cations (K+ and Na+ for alkali feldspars or Na+ and Ca2+ for plagioclase feldspars). It is, therefore, evident that feldspars are not fixed composition though in physical appearance they are known o be similar having whitish, grey or pink color. Calcium feldspars are believed to be grey or pink color. Calcium feldspar is believed to be the most easily while potassium feldspar is the least. Quartz (SiO2), also a tectosilicate (with the same 3 – dimensional block structure as the feldspars) consists of silica tetrahedral and nothing else. In essence, there is no isomorphous substitution. It is extremely resistant to neither chemical weathering since there is neither a basic control to be attacked nor isomorphous substitution to weaken the simple regular structure of the mineral. As such, it usually merely breaks down physically to smaller particles and may accumulate in soil after other minerals have been broken down. It is usually hard, transparent and the sole component of the sand fraction of the soil. Mineral content and physical properties of typical rocks On the basis of their chemical composition alone, rocks can be grouped into: (i) The more basic rocks (those containing relatively high proportions of the basic metallic cations) and (ii) The more acid rocks (having an increasing proportion of the total composition as silica).

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SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

All silicate minerals (except quartz) contain silica. Acid rocks are known to contain more than 66% silica. The other recognized types include intermediate rocks (52 – 66% silica), basic rocks (45 – 52% silica) and ultra-basic rocks (with less than 45% silica). Rocks can also be sub-divided into: (i) Alkaline cations predominated rocks (containing K+ and Na+) (ii) Calcic elements predominated rocks (containing Ca++ and Mg++) The basic elements predominated rocks are believed to be more common. The more basic rock is, the more its content of the ferromagnesian minerals. On the other, the more acid a rock is, the more its content of feldspars and quartz. As a result, basic and ultrabasic rocks have olivine and pyroxene plus some hornblende or biotite. The intermediate rocks contain hornblende, biotite and plagioclase feldspars while acid rocks contain quartz and feldspar (usually mainly orthoclase) and some biotite. The essential components and physical properties of some typical examples of the major types of rocks are presented in Table 1 below. Metamorphic rocks are usually derived from sediments rocks or from the metamorphosis or pre-existing igneous rocks. The following are typical examples of the transformations: (i) Gneiss – Metamorphosed granite (ii) Slate – metamorphosed shale (iii) Marble – metamorphosed limestone (iv) Quartzite – metamorphosed sandstone (v) Graphite – metamorphosed coal T able 1 : R o ck typ es and their p ro p erties R o ck T yp e

T yp ical E xam p les

M ineral C o ntent

1.

Ig neo u s

G ranite

D o m inant m inerals are L ig ht in co lo u r. H ave co arse to Q u artz and Felsp ars m ed iu m p article sizes. T hey are so m e m icas, A m p hibo les and iro n o xid es.

2.

Ig neo u s

D io rite

L ittle o r no q u artz bu t rich G rey to d ark co lo u red . C o arse to R ich in felsp ars, am p hibo les, m ed iu m textu red . M icas and iro n o xid es

3.

Ig neo u s

B asalt

N o q u artz bu t there is little Felsp ars, p yro xene and iro n O xid es.

D ark /B lack co lo u red . D ense to fine g rained .

4.

S ed im entary

S and sto ne

H ave q u artz and so m e C em ents C aC O 3 , F eO 2 and clays.

L ig ht to red co lo u red and g ranu lar o r p o ro u s in stru ctu re

5.

S ed im entary

S hale

H ave clay m inerals, so m e q u artz and so m e o rg anic m atter

L ig ht to d ark co lo u red and w ith thinly lam inated stru ctu re.

6.

S ed im entary

L im esto ne

H ave calcite and d o lo m ite W ith so m e iro n o xid es, C lays p ho sp hate and O rg anic m atter.

L ig ht o r g reen in co lo u r. F ine g rained and co m p act.

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P hysical P ro p erties

ROCKS AND THEIR WEATHERING

Heat, Pressure and chemical changes are attendants to the metamorphosis of rocks at some depth within the earth’s crust. The folding of the earth, other earth movement as well as contacts between rocks and intrusions of molten magma can subject rocks to great heat and tremendous pressure. The result is the formation of a new structure and change of the components into new (secondary) crystalline minerals. The size of such crystals varies from very fine (microscopic) to coarse. Another characteristic of metamorphic rocks is the orientation of the constituents to produce a banded effect. Coarse-grained rocks showing only a rough banding are grouped as banded gneiss.

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SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

ROCK WEATHERING Originally, the earth consisted of nothing but rocks, some of which are still exposed today. Most of the elements of the earth’s crust have combined with one or more other elements to form compounds called “minerals”. The minerals usually exist in mixtures to form the rocks of the earth. Gradual processes have formed soils formed from the rocks by erosion. The origin and development of soil is known as “soil genesis”. Soil is derived from decomposition of mineral particles of rocks as well as plant and animal residues. The product of the wearing away (weathering) of rock particles in the absence of organic matter is termed “Crust of weathering”. Soil is formed only if the weathering of minerals occurs in the presence of organic matter. When formed as a result of deposits (by streams and rivers) of more or less weathered and sorted material the soil is called “alluvium” while the term “ colluvium: refers to the soil forced as a result of the movement of materials down a slope largely under the influence of gravity. When a soil appears to have been developed from materials similar to the underlying rocks, it is referred to as “sedentary’ or “residual” soil. The material (hard rock or any unconsolidated deposit) in which soil develops and in which a soil profile begins to form is termed “parent material” while the parent material that are rocks are known as “parent rocks”. Rock Weathering Rocks may, in the process of soil formation, be acted upon and broken down by the action of rain, running water, frost, wind, action of micro and macro – organisms (such as bacteria, fungi, protozoa earthworms, etc.) interactions of various chemical substances and numerous other agents to form soils. If limestone is the rock material exposed, the agents enumerated above and a lime can break it down rich soil will be produced. This is also true of sandy and clayey soils, the former being formed from sandstones and the latter form shale, granite or similar rocks. The weathering of rocks is known to be a combination of two processes: (i) Destruction and (ii) Synthesis Destruction involves the breakdown of rocks to give the parent material while synthesis is the changing of the parent materials into new materials such as silicate clays and very resistant products like iron and aluminium oxides. Associated with the two processes of rock weathering are the major forms of weathering itself – physical and chemical weathering? Physical weathering ensures the disintegration or destruction process as rocks are merely broken down by mechanical means to smaller and smaller particles without their chemical composition being changed, though the fragmentation may make chemical attack easier later. This is so because it causes the exposure of inner and larger surfaces to water and other agents for further breakdown. This predominates in dry climatic zones as in deserts where temperature – changes cause contraction (shrinking) and expansion and hence cracking and breaking up of rocks. These processes happen because the rocks are aggregates of minerals with different coefficients of expansion. Physical forces (e.g. winds, expansion of roots in rocks crevices and water) may also break up rock particles by rolling impact and so on. On the other hand, during chemical weathering, the rock is decomposed chemically to liberate the constituents of that it is composed and such are either removed to form new substances as in the formation of clays. This form of weathering is prevalent wherever rainfall is moderate to heavy as in most parts of West Africa. The main agent of chemical weathering is the percolating soil water. Rainwater dissolves some quantities of atmospheric constituents such as nitrogen oxide, sulphur dioxide, oxygen, and carbon dioxide and perhaps traces of ammonia, sodium chloride and other compounds. Nitrous, nitric and sulphuric acids aid the chemical breakdown of rocks.

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ROCKS AND THEIR WEATHERING

Oxygen and Carbon dioxide, which attack weathering rock by oxidation and the formation of carbonates, are of major importance. Various organic acids derived from the decay of plant and animal materials can also be added to the soil water as it seeps downwards. The solution that can attack exposed rock fragments in the soil and penetrates into massive un-weathered rocks along cracks and joints. The various mechanisms of chemical weathering include: (i) Solution: Water as a universal solvent can dissolve easily soluble minerals present in rocks. Alkali metals such as Ca and Mg are easily solubilised while Fe, Si, Al are not. (ii) Hydration: The simple combination of water with another substance such that the substance formed is not very much different from the original form e.g. hematite can be hydrated to form limonite. 2Fe2O + 3H2O = 2Fe2O3.3H2O (Hematite) (Limonite) (iii) Hydrolysis: The reaction of a substance with water while hydrogen serves a catalyst. It is essentially a decomposition reaction because the water molecule displaces any cation present in the minerals. e.g. KAlSi3O8 + H2O = HAISi3O8 + KOH (iv) Oxidation: the taking up of oxygen from the atmosphere by an element or a compound e.g. the conversion of iron to ferric iron. 4FeCO3 + O2 2Fe2O3 + 4CO2 (v) Carbonation and related acid forming processes: This is the formation of carbonates and bicarbonates. Carbon dioxide can dissolve in water to form carbonic acid, which can dissolve marble and other carbonates. H2O + CO2 = H2CO3 H2CO3 + 2CaCO3 = 3CaHCO3 (vi) Reduction (vii) Attack by acid and alkaline solutions (viii) Removal of the soluble products liberated From the above, it is obvious that chemical weathering is a complex process, the details of which vary according to the soils, rocks and climate involved. The following five descriptive stages have been recognized in the development of tropical soils: (i) Initial stage – the un-weathered parent material (ii) Juvenile Stage – weathering has started but much of the original materials is still un-weathered (iii) Virile Stage – easily weatherable minerals have largely decomposed: clay content has increased and a certain mixture is discernable. (iv) Senile stage – decomposition arrives at a final stage and only the most resistant minerals have survived (v) Final stage – soil development has been completed and the soil is weathered out under the prevailing conditions. The weathering of Igneous Rocks One of the minerals in igneous rocks is often more easily attacked than the others. The softening and breaking down of such easily attacked minerals usually result in the disintegration of the rock and the separation of the remaining mineral constituents that are then more exposed to further attack. Granites (one of the commonest groups of crystalline rocks) contain quartz, feldspars and a third mineral either mica or hornblende. The feldspars (K, Na or Ca, Al silicates) are the first to weather as the metallic bases they

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SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

contain are removed and the remaining silica and alumina combine to form kaolin. The less easily weathered mica and the very resistant quartz may remain as part of clay. The order or weathering of the common mineral constituents of igneous rocks has given as follows: 1. Olivine – most easily weathered. 2. Calcium feldspar 3. Pyroxenes and amphiboles (hornblende) 4. Sodium feldspar 5. Black mica (biotite) 6. Potassium feldspar 7. While mica (muscovite) 8. Quartz – most resistant to weathering.

The Weathering of Sedimentary and Metamorphic Rocks Secondary materials (already weathered, transported and deposited) lead to the formation of sedimentary rocks. As such, sedimentary rocks often contain very resistant residues. Thus, sandstone that is largely quartz sand will break down on weathering to the original sand or a poorer sandy soil. If some feldspar or sand (other than quartz sand) is contained in the sandstone, weathering may result in the formation of some clays and the soil will be less lights-textures. It is known that sedimentary rocks are much less likely to contain crystalline silicates, which can weather to give nutrients to the soil that in the case of igneous and metamorphic rocks. It is very difficult to make general statements on metamorphic rocks both in respect of the rate of weathering of their minerals and the release of plant nutrients. This is because of their tremendous range of characteristics. At one extreme, quartz-schist (obtained from quartz sand) is usually nearly sterile and breaks down to more quartz sand. At the other extreme, certain base-rich metamorphic rocks resemble the more basic igneous rocks and give rise to very fertile soils.

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CHAPTER 2 SOIL COMPOSITION AND FORMATION THE CONCEPT OF THE SOIL Soil is difficult to define precisely. Yet, different people have different ideas about the soil which is one of the natural resources with which mankind is endowed. The geologists and mining engineers are concerned with the rocks and minerals below. The soil may be of little interest to them. In fact, it is a nuisance and must be disposed off in order to get at the mineral wealth that must be dug out. To the highway engineer, the soil is the material for the construction of roads. If the properties are suitable, the soil is useful. If not, the soil must be removed and gravel put in place. The farmer or the soil scientist is not usually concerned with what is deep down in the soil except in as much as it helps him to understand the formation and parent rock of the soil itself. He is interested in that part of the earth’s covering, which supports plants and animal life. The soil scientist can this define the soil as being that natural covering of the earth’s surface in the soil as a habitat for plants and animals. He makes his living from it. Hence, it is more than useful; it is indispensable being the major source of the nutrients, air and water for the growing plant apart from giving mechanical support to it. Other definitions of the soil by soil-scientists include the following: (i) Soil is the collection of natural bodies that have been synthesized in profile form from a variable mixture of broken and weathered minerals and decaying organic matter which cover the earth. (ii) Soil is a thin layer that covers the earth; supplies mechanical support and sustains plants when containing proper amounts of air and water. (iii) Soil s the collection of natural bodies (on the earth’s surface), which supports the growth of plants and is the principal source of man’s food and clothing. (iv) Soil is a loose surface of the earth as distinguished from solid rock (v) Soil is an unconsolidated material derived from rock weathering which has been acted upon by climate and vegetation. (vi) Soil is a natural body of loose, unconsolidated material, which constitutes a thin layer over several meters deep of the earth’s crust. It is evident from the various definitions that although the soil can be studied in may ways – some of more practical value than others – soil scientists are mainly interested in aspects of the soil influence on plant growth. Soil Components Soil is a heterogeneous material and may be considered as consisting of the following three major components: (a) Solid phase (b) Liquid phase and (c) Gaseous phase. All the three phases influence the supply of nutrients to plants roots. The solid phase is the main nutrients reservoir. The inorganic particles of the solid phase contain cationic nutrients elements such as K, Na, Ca, Mg, Fe, Mn, Zn and Co while the organic particles of this phase provide the main reserve of N and to a lesser extent also of P and S. The liquid phase of the soil (the soil solution) is mainly responsible for nutrient transport in the soil to plant roots. Nutrients transported in the liquid phase are mainly present in ionic forms, but Oxygen and Carbon dioxide are also dissolved in the soil solution. The gaseous phase of the soil mediates in the gaseous exchange, which occurs between the numerous living organisms of the soil

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

(plant roots, bacteria, fungi and animals) and the atmosphere. The percentage composition of each of the three phases is given in Figure 1. The natural bodies in soils can also be classified into organic matter (mostly the remains of plant and animal tissues), inorganic matter (mostly minerals), living forms (micro – and macro flora and fauna), air and water. Mineral salts are compounds that normally release nutrients for plant’s absorption while microorganisms make the decaying processes possible in soils. Both the organic materials and mineral particles are intimately associated in the topsoil. If the organic material is removed or destroyed, the mineral particles will remain. Microorganisms play

SOLID PART Air 20 – 30%

Mineral 45 %

AIR / PORE SPACE

Water 20 – 30%

5% Organic Matter

Figure 1: SOIL COMPOSITION an important role in the uptake of plant nutrient elements from soil. Shortage of energy substrates makes it unlikely for n fixers in the soil microbial population to fix significant amounts of nitrogen. Yet, Mg and Fe uptake by plants can be altered by microbial activity while non-nutritional bacteria effects can also influence growth of plants. Air is a mixture of gases such as oxygen, carbon dioxide, nitrogen, etc. Oxygen is required for the germination of seeds as well as for respiration by roots of plants and the soil macro and micro – organisms. Carbon dioxide is usually a product of respiration. Water plays a major part in almost all the physical, chemical and biological processes in the soil. It is involved in most forms of mechanical weathering, redistributes materials throughout the soil profile, and carries away both soil particles and solute and transports nutrients to plants. Soil water is variable in quantity over time and space.

12

SOIL COMPOSITION AND FORMATION

Classification of soil water As a matter of convenience, various forms of soil water can be recognized as illustrated Figure 2. These are: (a) Run off water (b) Gravitational / Percolation water (c) Capillary water (d) Hygroscopic water (e) Water of Crystallization/Structural water Water enters the soil through rainfall or irrigation. It infiltrates the soil by moving through the air/pore spaces. The rate of infiltration depends on the intensity of water supply and the amount and state of pores in the soil. If already saturated by previous rainfall of irrigation water, infiltration is reduced. Infiltration is also adversely affected when the soil surface is compact and dense, or the pores are small and few in numbers. If water cannot infiltrate the soil, it tends to run off over the surface (especially on steep slopes). This is referred to as RUNOFF WATER. This form of soil water usually flows to meet rivers, streams oceans, seas and other large bodies of water. It is not available for plants use because it runs off from and does not reach the plants’ roots. Although on rough ground or low angle slopes, its movement is also and it may be stored in latter case, gullies may develop and considerable losses may occur. The water that enters the soil pores is affected by GRAVITY and MATRIC (CAPILLARY) forces downwards and is only effective in very large pores or macro-pores (> 0.06mm in diameter). The matirc or capillary forces are responsible for the retention of water in the soil since they lead to the attraction of water under the influence of force of gravity is called GRAVITATIONAL OR PERCOLATION WATER. It sinks so freely such that no plant root can absorb it within the micro-pores (<0.06 mm) in forces of retention and called CAPILLARY WATER. This latter form of soil water can move through the soil only very slowly and in theory can not readily drain put of the soil profile. It is the form of soil water that is available for plants’ roots absorption since its movement is also towards the roots, which do not need to exert too much force in order to reach it. Rain Irrigation Water a S o il P a r t ic le A ir / p o r e S pace (d ) (b ) (c)

P e r v io u s L ayer

(e) I m p e r v io u s L ayer

Figure 2: The Fate of Soil Water 13

W a t e r t a b le Or B ed ro ck

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

Some of the water in the micro-pores (called HYGROSCOPIC WATER) is held so tightly to the soil particles to the extent that it can be assumed immobile. In fact, it is adsorbed on to the soil particles by an electrochemical bond such that it can only be removed by heating or prolonged drying. In a similar fashion, a small proportion of water is also bound up within the structure of the soil particles e.g. within the crystal lattice of the clay minerals. This is called WATER OF CRYSTALLIZATION or STRUCTURAL WATER. It can only be released by destruction of the clay particles. The hygroscopic and structural forms of soil water are unimportant in terms of processes of water movement and availability to plants but they are significant when the moisture content of the soil is being measured. Summary: In this section, soil components have been discussed. They include: (a) Air (b) Water (c) Organic matter / humus (d) Micro organisms and (e) Mineral salts. Laboratory Techniques for Quantitative Determination of Soil Components There are several experiments that can be performed to show the presence of each of these components in the laboratory. In some cases, quantitative assessments may also be involved. For this introductory book on soil science the following simple methods have been carefully selected for ease of comprehension and performance in the laboratory. A. Soil Air: Materials: Two Transparent beakers, Soil sample, Water. Procedure: Carefully collect soil from a farm with minimum disturbance. i. Put some soil in a transparent glass beaker ii. Pour in some water contained in the other beaker …. …. .…….. ….. … ‘. .. . . .. . .. ….. .. .. ... . .. . . .. . . .. . .. ..

Water

Beakers Air Bubbles Soil Samples

Figure 3: Identification of Air as Soil component Observation: Bubbles of air could be seen escaping from the soil as water enters the soil. Conclusion: The air that escapes should have been in the soil’s pore spaces that did not contain moisture initially. On adding water to the soil, the molecules of water replace the air. 14

SOIL COMPOSITION AND FORMATION

B. Soil Water MATERILAS: Evaporation basin, stirring (glass) rod, Balance, Soil sample, Bunsen burner, Tripod stand, wire gauze. Procedure: Determine the mass of an empty evaporating basin following the steps: i. Collect some soil from a depth of about 10cm in a farm ii. Put a sample of the soil into evaporating basin and weigh iii. Then heat up using Bunsen burner flame to a temperature of about 1050C. iv. Occasionally stir the soil with a glass rod, weigh the evaporating basin and content again and after about 1 hour when all the moisture contained in the soil should have been evaporated. v. Repeat this process until a constant weight is obtained.

Glass Rod Evaporating basin with soil sample Wire Gauze Tripod Stand

Bunsen Burner Figure 4: Quantitative and qualitative assessment of soil moisture. Results and Calculation: If mass of the empty evaporating basin = A grams And mass of the basin and fresh soil = B grams Therefore, mass of the fresh soil = (B – A) grams If mass of the basin and heated soil = C grams Mass of the heated soil = (C – A) grams Loss in Weight (Mass of water driven off) = (B – A) – (C – A) grams Therefore, % Moisture content = (Mass of moisture driven off x 100) % Mass of fresh soil = [(B - A) – (C – A) x

1 100] %

(B – A)

1

15

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

Conclusion: The boiling point of water is 1000C. Hence, at 1050C water turns into vapour which when driven off reduces the weight of the soil. C. Soil Organic Matter / Humus Materials: Evaporating basin, dried soil sample, Balance, Tripod stand, Wire gauze, Bunsen burner, Stirring rod, Desiccators. Procedure: Place the evaporating basin with the dry soil in the last experiment over the Bunsen burner and heat strongly while observing the change in the appearance of the soil. Keep the soil well stirred but take care not to loose any of its particles. Continue to heat for about half an hour Cool the basin and its content in desiccators and weigh. Repeat the process until a constant mass is obtained Calculation: Mass of evaporating basin and soil after ignition = X grams Mass of ignited soil only = (X – A) grams Mass of organic matter / humus = Mass of dry soil – Mass of ignited soil = [ (C –A) – (X – A)] grams % Organic matter in dry soil sample = [(C – A) – (X – A) x 100] % (C – A)

1

Conclusion: Continuous heating of dry soil removes the humus / organic matter in soil by converting it into gases which escapes into the air. Thus, there would be reduction in the mass. D. Soil Micro – Organisms Materials: Two 250cm3 conical flasks, Rubber corks, two pieces of fresh meat, fresh soil sample, Sterilized soil, 2 strings / threads Procedures: Set up the apparatus as shown in Figure 5 below. i. Put some fresh soil sample in conical flask A, suspend a piece of fresh meat in it with the aid of a string and cork it. ii. In conical flask B, Put some sterilized soil sample to serve as control, suspend another piece of fresh meat and cork. Leave the experimental materials for four days. Observation: A bad odour similar to that of rotten eggs resulted from flask A when opened on the fourth day because the fresh meat has started decaying. In the conical flask B, no decaying was observed. Discussion: The decaying of meat in A is an indication that microorganisms are present in the fresh soil. This is because microbes are known to be agents of decay resulting in bad irritating odour.

16

SOIL COMPOSITION AND FORMATION

Conclusion: The fresh soil contained microorganisms, which do not respond actively while microorganism

Rubber Cork

String

Fresh Soil

Sterilized Soil

Figure 5: Identification of microorganisms as soil components.

in a sterilized soil should have been rendered inactive. E. Soil Mineral Salts Materials: A round bottom flask, delivery tube, Rubber Cork, Dilute HCl, Dry soil, Conical flask. Procedure: Set up the apparatus as shown in Figure 6. Pour some soil in the round bottom flask and carefully pout HCl on to it. Observation: As soon as dilute HCl reaches the soil effervescence occurs resulting in the production of a gas whish led through the delivery tube to the conical flask contains limewater. Subsequently, the limewater turns milky. Discussion: The gas that can turn limewater milky is carbon dioxide, which can be produced when dilute HCl reacts with carbonates of Ca, K, and Na etc. The observation in this experiment indicates the presence of carbonates (which are mineral salts) in the soil. The resulting reaction can be represented as follows:

17

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

2CaCO3 + 2HCl

CaCl2 + 2H2O + 2CO2

D ilute H Cl T histle funnel D elivery tube Rubber co rk

Ro und Bo tto m flask Co nical flask ….

. . .. . ...…

. . … … .. … . . . . . . . . . .. … … .. .. . . . . . ……. … …. . . .

D ry So il

Figure 6: Identification of mineral salt as soil component

18

Lime W ater

SOIL COMPOSITION AND FORMATION

FACTORS INFLUENCING SOIL FORMATION The characteristics of a soil are as a result of the influence of five main groups of soil-forming factors: (i) The parent material from which the soil is developed. (ii) The climate (past and present) of the area (iii) The vegetation supported by the soil (influenced by climate) and the soil fauna (bacteria, other micro-organisms, worms and termites) that live the soil and (iv) The type of relief associated with the soil and (v) Time – i.e. the length of time during which the other factors have been influencing soil formation. The factors of soil formation can therefore be summarized as climate, living organisms (biotic factors), topography, parent materials and time. Climatic and biotic factors are termed “active” or “causal” factors while topography and parent materials are passive period when climatic and biotic factors act on the parent materials under a given topographic condition to produce soil. Climate exerts the most important effect on soil formation. Rainwater is one of the major agents of chemical weathering. Rainwater is one of the major agents of chemical weathering. It facilitates the washing down of the products of disintegration deeper into the soil. Rainfall and temperature can also influence the type of vegetation that grows, dies and decays to form part of the soil. Heat aids chemical reactions to the extent that the greater the heat, the faster the rate of reactions. Wind is involved in the transportation of soil particles. The greater the wind velocity, the more the particles transported. Living organisms help in one way or the other in the disintegration of rocks. Microorganisms may secrete certain substances that can help in various chemical reactions in the soil. When finding their ways into small cracks in the rocks, roots of plants widen the cracks and cause greater breakdown of the rocks. The larger animals also help to break the rocks into smaller particles through their activities on the surface of the earth. In the same vein, the roots of the plants penetrate parent materials and open up channels for air and water circulation at death. On decomposition, they add many nutrients to the soil and become part of structure of soils. The chemical composition of the soil parent material can give an indication of the type of soil formed. Thus, a parent material having only quartz will form a very poor sandy soil, whereas if micas and feldspars are composed in the parent material, the soil will contain some clay. Furthermore, the rate of soil profile development is faster in parent material that are permeable to water than in the case of parent materials that are impermeable. Topography affects the soil formation as a result of its relation with water movement. It, thus, influences erosion, temperature as well as the composition and density of vegetation. Rains falling on steep on steep slopes tend to run off and collect in depressions. As a result of this the rate of soil profile development on steep slopes is less than in depressions since the latter receive more water and too rapid run off (on steep slopes) tend to delay soil profile development. Time as a soil formation factor indicates the period that it takes a soil to develop fully at the instance of the factors already discussed. It is well known that soil development under warm, humid, forested conditions will occur faster than under cool, dry and scantily vegetated area. Similarly, soil developing under bedrock will take a longer time to develop than ones developing under disintegrated type of parent material. The earth’s crust is composed of elements; the major ones in their order of predominance are oxygen, silicon, Aluminium, iron, calcium, sodium, potassium, magnesium, titanium, hydrogen, carbon, phosphorus and sulphur. These elements combine in various proportions to form the rocks of the earth, which are the parent materials. The parent materials differ in structure, composition and rate of decomposition. These differences are due to the difference in the proportions and types of elements that form the parent material. 19

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

The parent material disintegrates to form the soil whose properties will be greatly influenced by the properties of the parent material. A sixth “soil – forming factor” is man who uses the soil and causes important changes in the process.

Sun

C lo u d

R ain

B u rro w ing earthw o rm R ain-w ater actio n o n a crevice

ROCK

Fig u re 7 : S o m e so il fo rm ing facto rs

20

P lant ro o ts g ro w ing Insid e ro ck

CHAPTER 3 SOIL PROFILE AND PROPERTIES SOIL PROFILE STUDY The following are some of the uses to which the soil can be put. (i) As a medium for plant growth (Farmer’s view) (ii) As a structural material in the making of highways, dams foundations for building or for other engineering purposes (civil engineer’s view) (iii) In manufacturing bricks and tiles – (Mason) (iv) For waste disposal systems (sanitary engineering) The suitability of soil for the various uses man can put them is highly dependent on their physical properties. It is important and beneficial for any one involved with the use of the soil to know what extent and by what means its properties can be changed. In knowing the use to which a soil should be put, one may consider the following: (i) The rigidity and supporting power of the soil (ii) Wet and dry drainage of the soil (iii) Moisture – storage capacity of the soil (iv) Plasticity of the soil (v) Ease of penetration of the soil by roots (vi) Aeration of the soil (vii) Retention of plant nutrients in the soil All the above factors are closely associated with the physical condition of the soil and it is the consideration if such properties that indicate the type of soil found in any location. The physical properties of the soil that are of prime importance to the soil scientist are: (i) Texture (ii) Structure (iii) Weight, pore-space and air-relationships (iv) Colour (v) Temperature

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

SOIL SAMPLING TOOLS Digger profile pits.

A digger is used for making openings on the earth. It is useful in digging up soil

Chisel end

Soil chisel is similar to the flat chisel widely used in Nigeria for cutting through oil palm roots. It could be made locally from pieces of car springs and fitted to a long, wooden handle by the blacksmith. It is used for digging inspection holes in soil survey.

22

SOIL PROFILE AND PROPERTIES

Square- pointed spade is used in collecting soil samples, especially after the preliminary excavations has been made either with a digger or a chisel

Square – pointed spade

Post-hole Spade

Posthole Spade is a useful spade in mapping, since it could take samplesto a depth of 30cm. Its use could be limited on gravely soils.

Screw –auger

The ordinary screw auger is 100 to 150cm long, with provisions for adding extra lengths for deep boring. The screw or “worm” part should be about 16.5cm (7ins.) long, with the distances between flanges about the same as the diameter. If the distances between flanges are narrower than the diameter, it will be difficult to remove the soil with the thumb. The soil sample is clogged within the screw of the auger. It is convenient to have a scale marked on the shaft of the auger from the tip. The screw auger is very useful in probing soils to depths of 150cm and beyond. Typical Soil Profile In determining and describing the properties of a soil, it is useful to examine both the top few centimetres, the part which most affects plant growth and the entire soil profile (Fig. 8) (Showing the various layers/horizons). This is usually done for sufficient information on the soil. A SOIL PROFILE is a vertical cross section through the soil showing the various horizons of which the soil is made up. The horizons represent the different zones or layers of soil material that together form the entire soil. Each of the horizons is different in some uppermost few centimetres of the soil is called TOP SOIL and is made up of 23

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

plant materials and humus as well as minerals and other inorganic matter. The layer is dark coloured due to the presence of humus, which varies in amount with the vegetation and other factors. Good humus topsoil is typically dark and greyish-amorphous and glue-like substance derived from plant remains (leaves, flowers and branches) that fall on to the soil surface. It can also be derived from dead and decomposing roots and the bodies of microorganisms and other soil fauna. Apart from giving the soil particles the dark colour, humus helps to bind them together such that they are frequently more or less loosely combined to form soil crumbs. It usually decomposes or mineralizes to release plant nutrients. It, thus, has great influence on the physical and chemical properties of the soil and expectedly on the soil fertility and productivity. A very thin layer (THE LEAF LITTER LAYER) remains continuously thin on the soil surface because decomposition of the great quantity of materials (leaves, twigs, flowers, fruits, branches and other pant parts) is rapid that there is very little time for their accumulation. The attack by bacteria, fungi, termites, worms and other animals and insects starts the decomposition as soon as a leaf falls. It is, however, possible for the leaf litter layer to be thicker in cooler parts of the world than in tropical soils, as decomposition rate is lower. Climatic factors (particularly high temperature) evidently speed up bacterial activity and it therefore responsible for the rapid decay of leaf litter in tropical climates. To examine the topsoil, the following steps should be used: (a) Collect a small handful of the topsoil and view it with a hand lens. (b) Feel the soil by pressing it lightly between thumb and first finger. (c) Rub it between fingers (d) Take it apart carefully; examine the structure, the pores and natural spaces in it. The topsoil may contain gravel stones and a large number of very small roots. This is because the topsoil is the home of the feeding roots of grasses, herbs, shrubs and trees usually having dense but shallow root mats. The second major horizon called SUB SOIL consists mainly of inorganic materials with colour markedly changed from that of the topsoil above it. The subsoil colours are more striking and varied (brown, brownish yellow, red, grey etc.) than relatively uniform colours of humus stained topsoil. The red and brown colours are associated with the occurrence of iron compounds in the subsoil. The subsoil may also be relatively compact, with fewer roots, pores and channels than the topsoil, which is often more opening, porous and easy to work. The subsoil horizon could, therefore, be said to be much thicker than the first (topsoil) and its colour varies according to its parent material and the amount of organic matter present. The subsoil merges into a third horizon, the weathered substratum consisting of parent materials. The transition between the second and the third horizon is usually a gradual one and the boundary between them is very diffuse, irregular and difficult to see. It may take considerable experience to separate the two horizons accurately and their differences depend on the type of soil ad the differences depend on the type of soil and the nature of the present rock below. The weathered substratum may in turn merge into hard, fresh rock below, usually referred to as BED ROCK, which may or may not be similar to the rock from which the upper parts of the profile were ultimately derive. It may occur at such great depths that it cannot often be seen in a soil pit.

24

SOIL PROFILE AND PROPERTIES

Geologist’s hammer A geologist’s hammer, or small hand pick, one end of which can be used as a hammer, cut along roadsides. For most soils and those containing many wooded root, a chisel – pointed hammer is better, whereas for dry soils a sharp – pointed hammer is better. A more detailed consideration of the typical soil profile than the simple view presented above is given in Figure 8. The entire profile can be divided into two portions – the Solum and the Regolith. The regolith is composed of the parent material and the bedrock while the other upper sections form the solum. There are the O and A horizons, usually referred to as zones of elluviation (where materials are removed, washed or leached away). The Bhorizon of illuviation, which acquires the materials, moved or leached away from O and A horizons such that the material accumulates in this zone. An inclusion of deep feeders in a crop rotation scheme ensures that such plants tap

25

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

H – H o rizo n

O rganic Top M aterial

O - H o rizo n Z o ne o f E lluviatio n M ineral o r A H o rizo n T o p so il S ub so il o r B - H o rizo n (Z o ne o f illuviatio n)

P arent m aterial o r C - H o rizo n B ed ro ck o r R - H o rizo n

Figu re 8 : A T ypical so il P ro file.

the nutrients that have accumulated in the zone of illuviation. This is the advantage of crop rotation to nutrient recycling. The zone of elluviation (Top soil) can be classified into: (a) Organic horizon. This can be subdivided into H, O1, O2, O3, etc. it is essentially the horizon in which the original forms of most plant and animal matter cannot be recognized with the naked eyes due to decomposition. (b) Mineral horizon: This can be subdivided into A, B and C sub horizons. The A horizon consists of organic matter formed or forming adjacent to the surface. It is also an horizon that has lost clay, iron or Al and having a resultant concentration of quartz or other resistant minerals of sand and silt size. The B-horizon is one in which the dominant feature is an illusion concentrate of silicate clay, Fe, Al or humus alone or in combination. The C-horizon is a mineral layer excluding the bedrock. It is either like or unlike the material from which the solum is formed. Methods of Soil Profile Study There are the following major techniques of studying the soil profile: (a) Using a soil auger to bore a deep hole into the soil. The vertical sample of materials extracted is then carefully laid on a clean sheet of white cardboard in the order brought out. If one needs to reach the bed rock, however, a well-like hole (a typical soil profile) needs to be dug. 26

SOIL PROFILE AND PROPERTIES

(b) Observing an exposed vertical cutting such as road cuttings, quarries, mines or building sites. In all cases, the following steps are crucial in the profile study: (i) Take measurements of the various horizons and have accurate scale drawing. (ii) Identify the parent rock and note the different colours of the layer above the rock. The junction between dissimilar layers should be noted / recoded. (iii) Study the amount of vegetation and humus on the surface and the amount of gravel that will affect drainage. (iv) Study the textures, structures, and consistencies of the different layers. (v) Compare the different samples taken from different locations, discuss and record the differences to allow the development of a soil map of the region concerned. It is important to stress, here, that in writing a soil profile description, soil scientists focus on soil colour, texture, structure, thickness and pH.

SOIL PROPERTIES SOIL COLOUR The colour of soil serves both farmer and soil scientists, provided that they understand the causes of the various colours and is able to interpret them in terms of soil properties. Organic matter content, drainage and aeration are soil properties related to colour that are of interest. Soil colour is the first soil characteristic that is observed during profile study and it is often used to describe the soil. Soil colour has indirect effect on plant growth through its effect on temperature and moisture. Colour under which a soil has been developed or its parent material. In most cases, the productive capacity of a soil can be judged from its colour. While the topsoil is of similar uniform, dark colour due to the presence of humus, the subsoil are usually of more striking colours. Practically all colours (white, red, brown, grey, yellow, black, bluish and greenish tinges, etc.) occur in soils. Predominantly, soil colours are not pure, but mixtures, such as grey, brown and rust. Pure blue and green are not known to exist in soils. When two or three colours occur in patches, “mottling” is said to take place. The colour of the soil is usually a composite of the colours of its components. The colloidal material has the greatest impact on soil colour. Thus, humus is black or brown; iron oxides may be red, rust – brown or yellow depending on the degree of hydration. Reduced iron is blue-green. Quartz is mostly white. Limestones are white, grey, or sometimes olive green. Feldspars have grey, white or red as determined by the type and the amount of iron coatings. Wet and moist soils look darker than dry soils. As earlier mentioned, colour can serve to tell much about a soil. Generally, the darker a soil, the higher is it’s productivity due to the amount of organic matter present. Light colour often results from the preponderance of quartz, a mineral that has no nutritional value. With some exception, the sequence of decreasing productivity is black, brown, rust-brown, grey – brown, red, grey, yellow, white. In “young” soils, colour is an indication of the parent material. In “mature” soils, it is an indication of the climate in which they have developed. Practically, all soil profiles reveal a change in colours from one horizon to the next. The changes are most obvious in mature soils, while both in young and very old soils they are less pronounced. This is because, in young soils there has not been sufficient time for much differentiation, while in the very old ones, leaching has proceeded to considerable depth and has left only the least soluble components. The specific colour of the horizons makes it possible to recognize erosion. In many fields, the eroded spots stand out clearly from the rest of the land. 27

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

In the classification of soils, colour is frequently very helpful. Since colours are good indicators of soil characteristics, they serve well in the study of the genesis of soils and in arriving conclusion concerning their best use and management. The colour of the soil affects other soil conditions through its effect upon radiant energy. Black and dark colours absorb more than light colours or white. As such, dark soils tend to be warmer than light coloured soils when the sun shines or when the atmosphere is warm and the soil is able to absorb energy from it. The greater amount of heat energy available to the soil results in higher rates of evaporation. A dark soil will therefore dry out faster then a light-coloured one under identical conditions. A cover of vegetation or much will expectedly reduce or even eliminate this on heat balance as it affects temperature and moisture of the soil and indirectly plant growth microbial of the soil, and indirectly plant growth, microbial activity, and soil, and indirectly plant growth, microbial activity, and soil structure. It is important to note, however, that it is only the colour of the soil surface that can have an influence on other soil conditions since the colour that is not exposed cannot be of significance. The colour of an object depends upon the kind of light, which it is capable of reflecting to the eye. Description of colour of light is usually associated with the measurement of its three principal properties, hue (the dominant wavelength of colour of the light), value (i.e. brilliance or total quantity of light that increases from dark to light colours) and chroma (the relative purity of the dominant wave – length of light which increases with decreasing proportions of white light). The three basic factors or components of light (hue, value and chroma) underlie the construction of THE MUNSELL COLOUR CHARTS. The Munsell notation of colour is a systematic numerical and letter designation of each of the three variable properties of colour. The Munsell notation for a given soil sample can be determined by comparing the sample with a standard set of colour chips. The chips are mounted in a notebook with all the colours of a given hue on one page. Each page then corresponds to a slice through the colour cube parallel to it’s front. The pages are arranged in the order of increasing or decreasing wavelength of the dominant colour in order to facilitate the matching of the unknown soil colour with the colour of the standards. It is important to note that the relationships of the colours to one another can be shown by use of a solid (e.g. a cube) in which hue, value and chroma are plotted along the three edges. Each possible colour represents a point in this cube and is completely defined by the three co-ordinates of that point, which is its Munsell notation. Many soil horizons have a single dominant colour. Horizons that exhibit mottled colour condition (when several colours are present in a spotted or variegated patter) have a mixture of two or more colours, as they are dry during part of the year and wet on the other part of the year. Generally, it is not economical to attempt to change the colour of the soil surface. In some cases, however, black or white powders or sand or coloured plastic sheets can be placed on the soil. The plastic is for mulching to affect temperature and moisture of the soil and the colour change will be incidental. The commonest use of a colour treatment is the use of crop residue mulch on top of the ground. When it decomposes early, it can be dark at the season when it is necessary to absorb sunshine energy. Fresh straw, on the other hand, is usually lighter than the soil.

28

SOIL PROFILE AND PROPERTIES

SOIL TEXTURE Texture is the most permanent and important characteristics of the soil. The term “soil texture” refers to the fineness or coarseness of the soil. It is the relative size of the soil particles i.e. the relative proportion of the various ultimate grain-size fractions (sand, silt and clay) in the soil. It can also be said to relate to the relative percentages of sand, silt and clay in a soil. The names of the predominant size fractions are normally used as texture designations (Table 2) while word “loam” refers to a situation whereby all the three major size fractions occur in sizeable proportions. Thus, soils having 85% or more particles of sand are called “sandy soils”. Those with 7 – 27% clay, 28 – 50% silt and less than 52% sand are “loamy soils” while those with 40% or more clay particles, less than

Table 2. Major Soil Texture Designations and the Corresponding Percentages of Sand, Silt and Clay contents. Texture Designations

% Sand

% silt

% Clay

Sandy Soil

? 85

< 15

<15

Loamy Soil

<52

28 – 50

7 - 27

Clayey Soil

<45

<40

? 40

45% sand and less than 40% silt are “clayey soils”. For greater precision, soils that come between these categories are described as loamy sand, sandy clay loamy, silt clay etc. The term “silt clay” describes a soil in which the clay characteristics are outstanding and which also contains much silt. A “silt clay loam” is similar to the silt clay except that it contains more sand and therefore mellower. The articles can be distinguished as presented in Table 3 below: Table 3. Approximate sizes, textures and properties of the various ultimate grain size fractions in soils. Size (mm)

Texture

Properties

0.002

Sticky

plastic, easy to mould and absorbs water

clay

0.002 – 0.02

Slimy

Not plastic, not moldable But sticks together

silt

0.02 – 0.2

rough

Does not hold together

Fine sand

0.2 – 2.0

coarse

Very loose

Coarse sand

2.0 – 20.0

stony

very loose

gravel

29

Particles

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

The three major types of soil are: (i) Clayey (water logged) soil: In this type of soil air and water do not move easily and the particles tend to form lumps when dried. As such, they are difficult to work as they drain poorly and tend to crack in the dry season. They have a colloidal nature that enables them to hold large numbers of mineral ions which are however not available to plant roots because of poor drainage. Such soils can be improved by organic matter and lime additions that allow more effective water percolation. (ii) Sandy (artificially dried) soils: These are well aerated, light and very easy to work with. They are however, termed hungry soils because nutrients are easily leached away in them. Due to the very minute quantity of water that may contained in them, they are referred to as the artificially died soils. (iii) Loamy (well drained) soils: These contain a fair balance of clay, silt and sand particles and are suitable for most crops. Their clay content enables them to retain nutrients and water while the sand content permits adequate drainage. They can also be referred to as moderately drained soils. As a guide to the textural classification or identification of soils, the soil-texture triangle (Figures 9 & 10) having 12 main textures with their compositions is usually employed as a strategic instrument. A glance at the texture triangle indicates the importance of specific surface. It takes more than 80% of silt to call a soil ”silt” and more than 85% of sand to call a soil “sand” but only 40% clay is required to call a soil “clay”. In the consideration of the mechanical composition of soil, the terms “clay”, “sand”, and “silt” are used both for soil separates and for texture designations. The percentage of the individual fractions is calculated on the basis of organic – free, oven-dry soil particles less than 2mm in diameter. Although the organic matte content is mineral soil is not indicated in the texture designation, it is of great importance in determining the value of the soil. Soils containing over 15% organic matter are designated as mucky or muck (for well – decomposed materials) and peaty (for soils with only partially decomposed plant residues). The knowledge of soil texture is evidently of immense significance for the following reasons: (i)

(ii) (iii)

It is a guide to the value of land since land use capability and soil management techniques are dependent on it. It is widely accepted that the best agricultural soils are those containing 10 – 20% clay, 5 – 10% organic matter and the rest divided approximately equally between sand and silt. It is helpful in the study of the morphology and genesis of soils as well as for their classification and mapping. It governs the rate and extent of many important physical and chemical reactions in soils since it controls the amount of surface on which the reactions can occur.

30

SOIL PROFILE AND PROPERTIES

Figure 9: Soil Textural Triangle

31

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

Figure 10: Simplified Soil Textural Triangle

Determination of soil texture There are several techniques for soil texture determination. Some provide approximate or rough estimations while others give more accurate descriptions of the texture of soils. The following two major categories of the techniques are recognized: (a) Field or Feel Method: This is a rapid method that can be used on the farm although not accurate. It involves moistening the soil sample on the palm and rubbing it gently with the thumb or forefinger. The extent to which it can be shaped is an indication of the texture. Fine textured clayey soils will roll into a long “worm” than can be bent into a ring without cracks. It will be sticky to touch and with no feeling of grittiness. Light clay forms a circle with cracks in it. Medium textured loamy soils will not be mouldable but may be slimy and with a very fine gritty texture. Coarse textured sandy soil will feel very gritty and will not hold together at all. Sand cannot, thus, be shaped or worked at all and the most that can be done is to heap it up into a pyramid or cone. If the sand contains sufficient finer material to enable it to be shaped into a ball, it is loamy sand. If the sample can be rolled into a cylinder, but breaks when bent further, it is a loam (a light loam forms a short, fat cylinder, an ordinary loam one which is full length). If the cylinder can be bent into a U – shape but no further, the sample is a heavy loam. The above practical test is of much value as it gives an indication of how easy or difficult to handle the soil during cultivation. High clay content makes the soil relatively hard to work with and may be described as “heavy”. On the other extreme, sandy soils are usually much easier to dig, hoe or plough and are referred to as being “light”. (b) Mechanical Analysis: The determination of the amount of the various “separates” present in the soil is called a mechanical analysis of particles size analysis. Mechanical analysis can also be defined as the relative distribution of the size groups of ultimate soil particles. The three major steps involved in all types of quantitative mechanical analyses are: (i) Destruction of soil organic matter where necessary 32

SOIL PROFILE AND PROPERTIES

(ii) Separation of all particles from each other (iii) Measuring the amounts of each size group in the sample. The various types of quantitative mechanical analysis that permits varying degrees of accuracy are: (i) Soil Mechanical Analysis by riddle: This involves placing a weighed dried sample in the top section of a soil riddle, which is a set of sieves with different mesh sizes. The sieves should correspond to the desired particles – size (2mm, 1mm and 0.5mm) out of the largest particles, sand. The next mesh is smaller and it is for separating out the smaller (silt) particles while the third is very fine and allows only the smallest (clay) particles to pass through. Soil crumbs should be broken down with hand while the content of each of the sieves are separately weighed and the percentages found to determine the respective proportion. For finer materials (≤ 0.05mm), this method is unsuitable. (ii)

Soil Mechanical analysis by sedimentation techniques: The sedimentation techniques are based on the long known fact that the velocity of fall an object in a liquid medium is influenced by such conditions as: (a) The viscosity of the medium (b) The difference in density between the medium and the falling object (c) The size and shape of the object. Stokes’ law (formulated in 1851) describes the rate of setting of spherical particles in a various medium. It states that the resistance offered by a liquid to the fall of a rigid spherical particle varies with the circumference of the sphere and not with its surface. Conversely, the force of fall by the particle is proportional to its weight and consequently to it’s volume. The components that make up the equation of strokes’ law, thus involve the cause of settling i.e. Cause of settling = resistance of settling. Thus, Volume of particles x Density difference x Acceleration = Circumference of particle x velocity of sedimentation In summary, the sedimentation techniques are based upon the particle that the rate at which a particle settles from a suspension is proportional to its diameter. Large particles settle more rapidly than small ones and by measuring the time it takes particles to settle from a suspension of water, it is possible to estimate their diameter. Hence, in reality, the sedimentation techniques (just like sieving) merely subdivide the soil into several size fractions but do not measure the diameter of individual grains. Soil Mechanical analysis by fractional sedimentation is a typical and simple laboratory procedure that can give a detailed analysis and subdivide a soil sample into a large number of fractions and thereby obtain a very fine and precise estimation of particle size. The following steps are involved: i) Pour water on about 20g soil contained in a glass jar and stir thoroughly. The water becomes dirty and some soil particles sink to the bottom of the glass jar ii) Pour away the dirty water into another jar. iii) Add more water to the first jar, stir again and pour the dirty water into the second jar. iv). The last process is continued until the water remains clean and the first jar contains pure sand. v) Allow the second jar containing the dirty water to stand for about 48hrs undisturbed. Silt will settle and clay particles will remain in the dirty part. The dirty part can be poured into another container and the clay contained therein obtained by heating to dryness. One can then weigh the sand, clay and silt separately to find out the proportion of each in the 20g soil sample. Numerous other specific method of mechanical analysis (based on sedimentation) has been developed. The two that have found widespread acceptance are the pipette method and the hydrometer method. The pipette method is preferred for accurate work, while the hydrometer method is considerably faster and still accurate enough for most purposes. 33

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

The pipette method In this method, a sample of the soil suspension is taken at a given depth and predetermined time such that the sample contains all fractions still in suspension at that depth. By sampling at different intervals after the suspension has rested, the mechanical composition of the soil can be calculated. Using Strokes’ law for the case of water at 250C and soil particles of 2.65g/cc density in the gravitational field (980.15 g/cm – sec2), V = 40285r2 i.e. a particle of 0.05mm in diameter would fall at a velocity of 0.25cm/sec or 5cm in 20sec. However, an accurate pipette sample of the suspension cannot the taken 20seconds after the suspension has come to rest. Two or three seconds would be needed from the beginning of the Sedimentation period after the suspension has been stirred. Yet, pippeting requires 10 sec after it is placed at the correct elevation. Hence, silt plus clay fractional sampling is an approximation. However, for particles of 0.01mm in diameter, the rate of sedimentation of 0.01cm/sec or 5cm in 8min. and 20 sec., this is sufficient for, fine silt plus clay sampling in a satisfactory manner by the pipette technique. Bouvoucos or Hydrometer Method This method (originally suggested by Bouyoucos in 1927) is based on the continuous reduction of the density of soil suspension with time at the rate soil particles drop below the level of the hydrometer. At anytime, the density is lowest near the top and increases towards the bottom. This has to be taken into account in the calibration of hydrometer for the determination of the amount of soil in suspension. Procedure: i. Weigh 50g oven dry soil sample into a baffled cup, fill the cup half full with distilled water and add 5ml of Neutral Sodium hexametaphosphate or calgon or hydrogen peroxide to help in dislodging soil lumps. ii. Transfer suspension into a Bouyoucos cylinder and fill to the lower mark with distilled water while the hydrometer is in suspension. iii. Stir with a stirrer until all soil aggregates are broken down. % Sand is obtained as follow: (a) At the end of 20 seconds, carefully insert the hydrometer and read it at the end of 40 seconds. (b) Remove the hydrometer and record the temperature with a thermometer (c) For each degree above 670F add 0.2 to the hydrometer reading to get the corrected hydrometer reading. For each degree less than 670F, subtract 0.2 from the reading. The hydrometer is calibrated in such a way that the corrected value so obtained gives the amount (in grams) of soil materials in suspension. The sand particles settle to the bottom of the cylinder within 40 seconds. Hence, the corrected 40 seconds hydrometer reading actually gives the amount of silt and clay in suspension. The weight of sand in the original sample is obtained by subtracting the corrected hydrometer reading from the total weight of the sample wile the % sand is calculated by dividing the weight of sand by the weight of the sample and multiplying by 100. % Clay in the sample can be obtained as follows: i. Re-shake the suspension and take (hydrometer and thermometer) readings at the end of 2hrs. Correct the hydrometer reading as usual. At the end of 2hrs, the silt should have settled out of suspension in addition to the sand. The corrected hydrometer reading, therefore, represents the grams of clay within the sample. 34

SOIL PROFILE AND PROPERTIES

ii.

and the % clay is calculated by dividing this weight by the weight of the original sample and multiplying by 100. Calculate the % silt by difference i.e. subtract the sum of the % sand and the % Clay from 100 to get % silt.

iii.

POROSITY This is closely related to bulk density. It is the volume of pore spaces within the soil and thus inversely related to the density of particle packing. It is also the amount of space occupied by air to the total volume of sample. It can be expressed by the relationship: Porosity, P =

(Vv

x

100)%

V 1 Where Vv = Volume of pore space and V = Bulk Volume.

Therefore, P =

(Volume of pore space x 100) (Bulk volume

1)

The small voids (existing between soil particles due to imperfect “fit” of the particles) and the large voids) occurring between the soil aggregates) are both important as the main passages for air and water movements through the soil. They control aeration and drainage. They also provide space in which soil organism can live and into which plant roots can extend. With all these, the pore spaces can influence the chemical conditions of the soil. The large voids (non capillary or macro-pores or aeration pores) hold air while the small voids (capillary or micro pores or water pores) hold only water. Porosity is controlled by soil texture and soil structure. Coarse textured soils have low porosity while a well-aggregated soil has a higher total porosity than a compact soil. It ranges from 30 to 60% about 33% for sandy soils, 40 – 50% for sandy loam and 50 – 60% for clay soils. It can be calculated as follows: P = 100 (1

-

bd) pd

Where P = total porosity bd = bulk density and pd = particle density. Sandy soils have low total porosity and yet allow water to move rapidly whereas clay soils with high total porosity will not allow rapid passage of water due to the size of the pores. Sandy soils have large pores while clay soils have abundance of micro pores. The significance of the small capillary / water pores stems from the importance of water to crop plants. The importance of soil water can be summarized as follows: i) Large quantities of water must be supplied to plants to offset the evapo-transpiration of the plants 35

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

ii) iii) iv) v) vi)

Soil water acts as a solvent which dissolves the nutrients to ensure their availability to plants Soil water aids enzyme activity Soil water is involved in transpiration which helps to cool plants Water is an important constituent of the protoplasm of cells Soil water helps to dissolve carbon dioxide for use in photosynthesis.

The various components of the soil atmosphere (soil gaseous phase) are necessary determinants of the soil productivity just like the solid and liquid phases of the soil. Oxygen is needed to the respiration of plant roots, microbes and the soil fauna. Carbon dioxide helps to dissolve nutrients and to make them available to the plants. Nitrogen gas serves for the production of combined nitrogen by symbiotic and non-symbiotic bacteria. Water vapour prevents the desiccation of roots and microbes and aids in the transfer of water within the soil. Adequate amounts of oxygen in the soil are of particular importance since roots and microbes are constantly exhausting it. When insufficient, the normal functions of most crop plans and of the aerobic microbes come to a standstill. Anaerobic bacteria use oxygen in organic and inorganic compounds, reducing them to sulphides, nitrites, ferrous compounds and other reduced compounds that are toxic to the plants. An excess of oxygen in soils is also undesirable because the organic matter would be oxidized too rapidly. Semi-aerobic decomposition is best for the production of the largest amount of true humus and for the steady supply of organic compounds that serve to stabilize soil aggregates.

SOIL ARCHITECTURE The mineral grains or particles (constituting soils solid phase) do not usually occur individually or loosely. Rather they are bound together by certain cementing agents (clay particles, organic matter, and some chemical cements – Calcium Carbonate and organic gums produced by soil animals and decaying plant materials) to form aggregates or peds. The resulting architecture i.e. the relationships of the various physical properties and Ingredients to each other – are governed by: Relative locations: (a) Physiographic location, micro climatic influence of upland, valley depression, ridge, and steepness, configuration, and aspect of slope. Availability of precipitation water, opportunity for evaporation and condensation, temperature situation, likelihood of freezing. (b) Location with respect to ground water (c) Location with respect to other locations 2. Vegetation: Type and amount of vegetation as they affect temperature and structure of the soil. Decayed roots leave open channels. 3. Animal activity: The burrows of animals supply a system of communication for water and air in the soil. Animal droppings represent the largest part of well – aggregated surface soil. 4. Consistence: The holding together of soil particles. 5. Structure: The arrangement of individual soil particles with respect to each other. Ultimately, however, the character of the individual grains and the way in which they are held together in pods determined the structural properties of the soil. Out of the enumerated factors, only consistence and structure are of crucial importance to the understanding of soil behaviour. This is because the aggregates or peds have numerous implications for soil 36

SOIL PROFILE AND PROPERTIES

processes and as such many important ecological effects. They give the soil the chemical and physical stability considerably greater than that of loose sediments. They help to control water movement through the soil, allowing excess water to drain away, but remaining moisture in the small pore spaces within the aggregates. Furthermore, the architecture of the soil produces a range of environments, which differ in terms of their aeration, humidity and temperature. These conditions provide a diversity of habitats, suitable for many different soil organisms. Soil consistence involves the attributed of soil material indicated by it’s degree and kind of cohesion (molecular attraction) and adhesion (surface tension) or by it’s resistance to deformation or rupture. In addition to the two main forces (cohesion and adhesion) that the responsible for soil consistence, organic compounds, iron and aluminum oxides and hydroxides, calcium carbonate and freezing of soil water can also be involved. The phenomena involved in soil consistence are friability, plasticity, stickiness and resistance to compression and smear (soil strength or bearing strength). Soil consistency can also be said to refer to the resistance of a soil to the insertion of an instrument such as a cutlass, knife, or hoe or it’s resistance to deformation (when handled), and its cohesion. It depends on whether the soil is wet moist or dry. Wet soils are usually described in terms of their stickiness (extent to which they stick to other objects) and their plasticity (ability to be pressed into any shape without breaking and to retain that shape). The two properties are related to the type and amount of clay contained by the soils. The resistance of the soil to breakdown upon wetting is also termed aggregate stability and it of immense importance. When low, the soils concerned break down rapidly under the influence of rainfall and tend to form compact surface crusts, which prevent water infiltration that may cause serious erosion problems (especially on slopping land). Crust formation also hinders plant growth since seedlings would be unable to penetrate the compact surface layer. Plants may, therefore, fail to reach maturity. Thus, aggregate stability gives a direct measure of the susceptibility of the soil to structural deterioration under the influence of rainfall. Being prone to physical damage by farm machinery and trampling by animals soils can also be indirectly judged. This is because soils with low structural stability tend to be prone to physical damage. The use of tractors or heavy cultivators or harvesting machines on such soils may destroy the aggregates and cause surface compaction to reduce plant growth by hindering root growth, slowing dam water movement and reducing the rate of air circulation. All these occur mostly in wet soils where aggregate stability is at a minimum. It is important that aggregates are stable in order to be able to resist dispersion. Nevertheless, the larger a soil aggregate is, the less stable it is. Most soils are of lower moisture content. They break into smaller aggregate more easily than wet soils though such aggregates may come together when pressed against each other. A very friable soil is easily crushed with gentle pressure, but a very firm soil is hardly crushable between thumb and finger and an extremely form one cannot be crushed in this way. Dry soils are often very rigid, brittle and hard. They may break into angular fragments that do not adhere to each other again when pressed together. If a soil mass is easily broken between thumb and forefinger, it is slightly hard. A very hard soil (when dry) cannot be broken between thumb and forefinger and can be broken in the hands only with difficulty. The term “soil structure” refers to the arrangement of the soil into natural aggregates or the arrangement of the individual soil particles with respect to each other into a pattern. Given that the pores in a soil are a important as the solid particles. Soil structure may also be defined as the arrangement of the air/pre spaces (small, medium and large ones) into a structural pattern. It focuses on: (a) The shape and arrangement of the structural units (b) The size of the structural units (c) The degree of development, distinctness and durability of the structural units. It refers to the aggregation of primary soil particles (sand, silt and clay) into compound particles, clusters of primary particles, which are separated, from the adjoining aggregates by surface weakness. It is a dynamic 37

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

property of the soil and it determines or dictates the soil productivity. Though not a plant growth factor, soil structure influences practically all plant-growth factors and it is, indeed, the “key” to soil fertility. This is because, water supply, aeration, availability of plant nutrients, microbial activity, root penetration, drainage, water infiltration, etc. are affected by it. As a result, poor soil structure may be an indirect factor limiting plant growth while good structure permits the plant-growth factors to function at optimum efficiency. Soil structure depends on the following three main factors: (i) Capability of the soil to form aggregate (ii) Size and shape of aggregates and (iii) Stability of the aggregates. As earlier explained, aggregates are made up from primary/individual soil particles and the degree of structures is closely related to the colloidal content of the soil. Light textured soils (low in soil colloids are structure less as coarse sand material does not form aggregates. When classified on the basis of shape, the following two major classes of soil structure are recognized: 1. Simple Soil Structure: In this category of soil structure; natural cleavage planes are absent or indistinct. There can be two main subdivisions of the simple structure. They are: (a) Single-grain structure: Particles involved are unattached (separate or loose). This occurs mainly normally only in sands and silts of low organic matter content. In sands, this allows for aeration and maximum capillary movement. In other soils, it is undesirable as it results in complete absence of large pores needed for satisfactory aeration. (b) Massive structure: The soil particles are joined together in large/massive lumps without a visible sign or line of weakness as in clay. Massive structure is similar to single grain structure, except that it is coherent. 2. Compound/Aggregated Structure: The natural cleavage planes are distinct and the individual soil particles are joined together into small structural units called aggregates. They arise from the single grained and massive structures, which are termed non structural soil conditions since there is no specific arrangement and no cementing agents in them. Soil aggregates or peds can be classified on the basic of shape as follows: (b) Plate-like: The groups of soil particles and pore spaces forming the aggregates are arranged in horizontal plates or leaflets with various thicknesses. It is most common in the surface soils although the subsoil can also it especially if inherited from shale and slate. The plates often overlap and impair permeability. (c) Prism-like: (Columnar and Prismatic): This is like a cylindrical pillar, which varies in length and width. When with rounded tops they are referred to as columnar while level and clean out tops are designated prismatic. It is common in sub soils of arid and semi arid regions. (d) Block-like: (Block and nuciform or sub angular blocky). The aggregates are like blocks having irregular edges that may be sharp (blocky) or rounded (nuciform or sub angular blocky). It is most common in sub soils of tropical soils. (e) Spheroidal (granular and crumb): These are rounded, lumped up particles that are no more than 1 – 2.5cm in diameter and commonly found in the furrow slice (surface soil). Non-porous ones are called granular form while porous ones are called crumbs. The preservation and encouragement of spheroidal form of soil structure is very important in crop production as they regulate the air and water movements that are of immense importance to the survival of plants. The process of granule formation in the surface soil (termed granulation) is encouraged by: (i) Wetting and drying of soil. (ii) Plants’ root physical movement (iii) Soil organism (burrowing animals) physical activities (iv) Organic matter accumulation 38

SOIL PROFILE AND PROPERTIES

(v) (vi)

Adsorbed salts Man’s activity e.g. tillage

(vii)

Freezing.

Soil structure also depends on the vegetative cover. Under permanent grassland, very good soil structure occurs as a result of the effect of the high organic matter content and soil fauna. The earthworm contributed considerably to the formation of the stable aggregates. Arable soils are often lower in organic matter and soil fauna and for this reason they are often poorer in soil structure. The stability of the aggregates depends mainly on the cations adsorbed to the soil colloids. Poor structure occurs where Na+ is dominant in the exchange complex as it has a dispersing effect and aggregation of soil particles is prevented. Calcium plays an important role in soil structure due to its flocculating ability and the resultant contribution to the formation of stable aggregates with minerals. Flocculation is the process by which soil particles come together because of the electrostatic attraction between the colloidal particles (clay and organic matter) and calcium ions. This encourages granulation. In combination with humic acid and clay minerals, calcium also forms very stable organo-mineral complexes. The presence and amount of certain cementing or binding agents determine resistance to disintegrative forces. Some of the agents are: (a) The amount and kind of clay minerals (illite, smectite / montmoriIlonite and Kaolinite). The kaolinite is the most stable and the most common in tropical soils (b) Sesquioxides – Fe2O3 + Al2O3 (c) Organic matter and its product of decay (d) Root hair

SOIL AERATION AND DRAINAGE Soil Aeration and drainage are affected by the weight of soil, the amount of pore space and air relationships. This is why they are all usually discussed together. Soil is made up of various- sized particles packed together, with the spaces between particles known as “voids” or “pores/air spaces” which are mixtures of air and water. In certain circumstances, the voids may be completely water. Void ratio is the ratio of volume of voids to volume of solids i.e. Void ratio, Vr = Vv Vs

39

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

Figure 11. Soil Structure (FAO, 1984) Water and air (gases) move through the voids such that the supply of water and oxygen for plant growth and the rate of water movement through the soil are related to the amount and size of the voids. The weight and voids of soil vary from horizon to horizon just like the other soil properties and both are affected by soil texture and structure. Soil weight measurements are usually done with respect to the individual soil particles (excluding the voids) and the bulk soil (including the voids). Thus, Particle density, pd is the mass (weight) of solid particles per unit of soil volume excluding the pore spaces i.e. pd =

weight of material Volume of material

Specific gravity =

weight of a certain value of material Weight of an equal volume of water

Thus, the particles density of any soil is a constant and does not vary with the amount of space between the particles. It usually ranges between 2.4 and 2.75gcm-3 density while high amounts of haematite and limonite increases it. Most of the minerals in soils (e.g. quartz, feldspars, mica) have a density of about 2.6 – 2.7gcm-3. Hence, on the average, the particle density (also known as the true specific gravity) is usually taken as 2.65gcm-3. It does not vary much for different soils unless there is considerable variation in content of organic matter or mineralogical composition. The bulk density (apparent density or volume weight) of the soil, bd is the ratio of mass of dry soil to the volume of bulk soil including pore spaces. It is usually expressed (as gram par cubic centimetre) by the relationship, Bulk density, bd =

mass of dry soil Volume of bulk soil (including pore spaces) 40

SOIL PROFILE AND PROPERTIES

The two main factors affecting the relationship are the composition ad packing of the soil. While the relative proportion of mineral and organic matter (composition) affect the intrinsic density of the soil voids within the soil. Thus, care should be taken when collecting core soil samples for bulk density determinations so that the natural structure (arrangements of particles) of such soils is preserved. Any change in the arrangement of the soil particles, will change the amount of pore space as well as the weight per unit volume. Bulk density of mineral soils varies from 1.0 to 2.0gcm-3 depending on the differences in the volume of voids (void ratio or porosity). Recently cultivated and well-structured topsoils with abundant pore spaces within and between the aggregates have low values. On the other extreme, compaction or consolidation and highly impermeable soils have high values due to the destruction of pore spaces have high values due to the destruction of pore spaces in such soils that have been compacted by agricultural activity. Sandy soils or coarse textured soils also have high bulk densities (1.5 – 1.7gcm-3) due to lack of cementing agents while clays have bulk densities ranging from 1.1 to 1.3gcm-3. Most of the common soil minerals (quartz, feldspars, mica and clay minerals) have a density of 2.6 – 2.7gcm-3. Thus, as the organic matter content of soil increases, its bulk density tends to fall. The average bulk density for most mineral soils is about 1.25 g cm-3. Many peats (organic soils) have a value of about 0.5gcm-3 or less since it is the variations in the packing of the soil particles that are more important. Four or more cores are usually needed from each soil horizon to obtain a reliable value. The bulk density cores obtained in the field are dried and weighed in the laboratory.

SOIL TEMPERATURE Just like soil water, air and nutrients, soil temperature is an important plant growth factor. It affects plant growth directly as seeds, plant roots and microbes live in the soil and the temperature of the soil directly influences their life processes. It also influences plant growth indirectly by affecting soil moisture, aeration structure, microbial and enzyme activity, the decomposition of plant residues and availability of plant nutrients. The most important influence of soil temperature on plant growth is through it’s effect on soil moisture. Soil aeration is affected by both temperature and moisture content differences. On soil structure, temperature is important because of it’s influence on the growth of plants and moisture changes and through freezing and thawing. Raising or lowering the temperature or by freezing the soil water has much influence on the decomposition of organic and mineral components of the soil, the resulting release of plant nutrients elements as well as on clay formation. The rate of chemical reaction doubles with every 100 rise in temperature. As temperature decrease, the life processes of plants and animals are slowed down until they finally stop altogether. Growth processes of most plant are very slow at about 400oF. The slow growth plants become more and more pronounced until temperatures of 700 to 900F. Due to the variation in temperature from season to season, the planting period for most crops in temperate climate arrears is governed by soil temperature since seeds will no germinate at low soil temperatures. The length of the growing season is also partially dependant on soil temperature. In warm climates, the effect of soil temperature on the planting or sowing time of crops and the duration of the growing season is not as pronounced as other factors, such as the beginning and ending of the rainy season, the total rainfall during the growing season. The light intensity and the climatic conditions existing at the time the plants mature. This does not imply, however, that temperature may not be a significant soil factor in warm climates. Bacteria action and mineral weathering are both more rapid in warmer climates. Soils in warmer climates also contain higher percentages of iron and aluminium oxides than soils in temperate climates. The red and yellow-brown colour of some soils in warm climates may be an indication of the presence of these oxides. Soils temperatures can be altered y the interaction of numerous factors.

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SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

The following are of practical importance: i. Removal of excess water ii. Provision of drainage iii. Use of mulches and various shading devices to alter the amount of solar radiation absorbed by the soil, loss of heat energy from the soil by radiation, infiltration of water and loss of water by evaporation. It is however, important to note that soil heat comes from two sources (radiation from the sun and sky and conduction from the interior of the earth). Thus, theoretically both external (environmental) and internal (soil) factors contribute in bringing about changes of soil temperatures. The environmental factors are: (a) (b) (c) (d) (e) (f) (g) (h)

Solar radiation Radiation from the sky Conduction of heat from the atmosphere Condensation Evaporation Rainfall Insulation Vegetation

The soil factors are: (a) Thermal capacity. On equal weight basis, the specific heats of organic soils are usually larger than those of mineral soils. The differences are normally small on equal volume basis. (b) Thermal conductivity and diffusivity. Dry loosely packed soils have very low thermal conductivity as conductivity of air is minute (c) Biological activity. The greater the activity, the more the heat produced. (d) Radiation from the soil. This increases with the temperature of the soil. (e) Structure, texture and moisture. Soils have higher conductivity in their natural structure than when disturbed. (f) The differences of thermal conductivity due to structure and texture are, however, small compared with those due to moisture changes. (g) Soluble salts. Very high and low concentrations affect evaporation and hence indirectly influence soil temperature.

42

CHAPTER 4 SOIL FERTILITY CONSERVATION AND MANAGEMENT Introduction Being highly complicated systems, soils should be studied with definite approaches. In agronomic, horticultural and silvicultural studies, the ultimate purpose of soil science is the growing of plants. With just a casual study of the soil, it can be realized that the production of a crop depends on the cooperation or interaction of many heterogeneous factors. Hence, for the understanding of the factors, soil science as a discipline is divided into various fields: Soil physics, soil chemistry, soil mineralogy, soil microbiology, soil fertility, soil genesis, soil morphology, classification and survey, soil technology and soil conservation. Yet, no definite boundary can be made between the fields and none could be said to be outstandingly of importance without being related to the others. They are all studied with overriding idea to maintain or increase land productivity. This is more so considering the fact that a soil rich in all essential plant nutrients is a desert and unsuitable for crop production if water is insufficient. Furthermore, the resources of the subsoil remain inaccessible to crop plants if a dense plough soil layer inhibits root penetration. High yield of crop is a major determinant of successful farming and is dependent on optimum plant growth, which in turn is controlled by soil fertility and productivity. Soil fertility is the quality or property that enables a soil to provide the proper compounds or mineral salts, in the proper amounts, and in the proper balance, for the growth of crops capability of a soil to produce a specified crop, or sequence of crops under a specified system of management. For a soil to be productive, it must of necessity be fertile. Yet, it does not follow that a fertile soil is productive. The fact that human life depends on less than one meter of mixed organic and inorganic debris may surprise modern man. Yet, it is so. The soil, the atmosphere and the oceans constitute the biosphere, which a thin layer around the earth in which living things exists. The soil is the most complex of these three constituents. It is also the most easily destroyed. Without soil, there would be no food apart from what the rivers and the seas can provide. In the developing countries, where nearly three quarters of the world’s population now live, the soil also provides most of the fuel in the form of firewood and a great deal of fibre needed to make clothes, ropes and other essentials. The soil is thus, the world’s most precious stones command prices, which have led us to treat soil as mere dirt. If famine and malnutrition are to be defeated, the value of soil has to be reassessed. We are losing, through soil degradation of one form or another, nearly as much new land as we are now bringing into production every year. It has been estimated that about 2,000million hectares of cropland have been lost to soil erosion. Nevertheless, farming itself is not to blame. It is possible to farm good land, produce ample crops and still maintain and improve the soil. In fact the better a piece of land is farmed, the healthier and more prolific its soil becomes. However, when the wrong techniques are used, or the wrong crops are grown, a chain reaction of disaster soon begins. For example, falling production is associated with increased erosion, which leads to yet lower production and even more erosion. Cropland becomes a wasteland. One of the most important causes of soil erosion is poverty. When the poor cause soil erosion, they do so because they have no alternative but to exploit the soil. Soil erosion is closely connected with the problems of rural development is impossible. Soil erosion is one of the most fearsome threats confronting mankind, though this threat ca be confronted and defeated.

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

Apart from soil erosion, the other causes of land degradation (turning of once productive fertile soil into a waste land) are: i. Chemical poisoning ii. Salt accumulation / salinization and iii. Building and mining Land can often be improved. Thus, arid areas can be irrigated and waterlogged ones drained. It is obvious that continuous cropping for about half a decade on tropical soils, without a stable means of fertility restoration may result in zero yields. The remaining part of this book is, therefore, devoted to the consideration of the following: - The dynamics of mineral soils - Soil micro-organisms - Soil fertility and soil testing - Soil Nitrogen and phosphorus - Blending of fertilizer - Fertilizer types - Rates, methods and times of fertilizer application - General principles of soil management - Erosion and dissertation problems and control - Soil classification and mapping CHEMICAL DYNAMICS OF MINERAL SOILS The Concept of Plant Nutrition Nutrition is the process by which living organisms obtain food materials from their environment. The “food” which plants absorb from the soil and air are mainly raw materials. The supply and absorption of chemical compounds needed for growth and metabolism may also be defined as nutrition while the chemical compounds required by the organism termed nutrients. The knowledge of the nutrients required by plants, how plants take in the nutrients and how the soil stored and makes them available are very crucial in plant nutrition. The nutrients which plants use for growth and development are present in soils in the form of elements through their roots in the form of salts, which are dissolved in the soil water. Plants take in simple materials (water, carbon dioxide and mineral salts) and build them into more complicated substances, which can be used as food. From these they build up carbohydrates oil and protein. The process of building up of chemical substances from simpler substances is known as synthesis. All the water, carbohydrates and mineral salts absorbed by a plant from its surrounding are not themselves used in tissue respiration, growth, replacement and repair. They are used only for the synthesis of the complex food substances, which are in turn used for these living processes. Photosynthesis occurs only in the green parts of a plant that is mainly in the leaves and in the stems of herbaceous plants, which are green. In photosynthesising cells, carbohydrates are formed and it used to be thought that carbohydrates were the only end products of photosynthesis. Most plants store the sugar formed in their leaves during the day as starch. This is called transitory starch because during darkness it is turned into sugar and carried to other parts of the plant to be used or stored, Protein synthesis normally takes place in the green leaves of flowering plants, using N from nitrates absorbed from the soil. Phosphorus and sulphur are found in the molecules of some proteins and these are similarly obtained by absorption of the appropriate salts from the soil.

44

SOIL FERTILITY CONSERVATION AND MANAGEMENT

Availability of Soil Nutrient Elements for Plant Growth Soils are formed from various types of rocks that are composed of various types of minerals. The type of soil in any location depends on the type of rock from which if has been formed as well as the amount or extent of erosion (weathering) that has taken place. Thus, the soil usually resemble the rock immediately below except if the soil has been formed somewhere else and transported to it’s present location. In essence, the chemical elements contained in the rock minerals that form soil are usually contained in the inorganic framework of such soils. Optimum plant nutrition demands that the required nutrients should be present and made available to plants roots in the soil in the required quantities. Invariably, all the mineral elements known in chemistry are present in soils. However, only a few of them have been proved to be essential for plants growth. Such essential nutrients elements are so important that in their absence, the plant cannot complete its life cycle normally. This is because they are either components of plant tissue, some essential enzymes and co-factors or involved in the metabolic processes of the plant. These elements are usually grouped into two on the basis of the quantities required by plants. One group include those that are required by plants in large amounts and hence termed “MACRONUTRIENTS” or “MAJOR NUTRIENTS”. They can be further grouped as follows: a. Primary nutrients, which are those elements frequently applied by farmers in various forms as fertilizers t stimulate growth. Such are N, P, and K. b. Secondary nutrients, which are usually found abundant in soils and are not often limiting to plant growth. Such are C, H, O, Ca, S and Mg. The remaining essential elements include Fe, B, Mn, Zn, Mo and Cl which are termed “MICRONUTRIENTS” or “ MINOR NUTRIENTS” as they are needed in very small quantities and may be harmful or toxic to plants if present in large quantities in the soil. Importance of the essential Nutrient Elements i. C, H, and O are known to be major constituents of the plant tissue ii. N is an important component of Chlorophyll, which is the substance that enables plants to carry out photosynthesis (i.e. the conversion of sunlight to energy). It is also found on proteins that are needed for cell growth and tissue renewal. iii. P occurs in the protoplasm of plants iv. Fe activates a number of plant enzymes and is connected with chlorophyll production v. Mn is associated with the activation of range of plant enzymes vi. Zn is an activator of various plant enzymes vii. Cu is concerned with the activation of plant enzymes and its proportion to the other trace elements metals is of importance. viii. Mo is required by plants for N – reduction and therefore for protein synthesis. Plant Nutrient Uptake Plants feed mainly by taking nutrients from the soil through their roots. Occasionally, the leaves and other plant parts like the leaf stomata can also absorb the elements. The pineapple plants are noted for absorbing fertilizer nutrients from the leaf bases. Some major and minor elements are also sometimes given to crops by bases. Some major and minor elements are also sometimes given to crops by spraying the leaves with nutrients solutions. When the nutrient uptake is by the plant roots, the root hairs which are minute extensions of the outside cells of the plants roots is mainly concerned, the roots hair obtains most of its needs from the soil solution (i.e. soil water containing small quantities of nutrients in solution). This is done by diffusion, ion exchange and/or active ion transport. In the case of iron exchange, a positively charged 45

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

ion (a cation) such as an H+ ion can be exchanged for another cation such as K+ ion or NH4+ ion. Anions like SO4=, NO3- or PO4= can similarly be exchanged for OH- ion. Soils colloids (clay sized particles having small electrical surface charges) are of immense importance in plant nutrient uptake, soil clay and humus have excess negative charges, which enable them to attract swarms of adsorbed cations to these negative positions. Cations (NH4+, K+, Mg2+, and Ca 2+) are thus held in a readily exchangeable way in this manner by the soil clays and the negative positions on the organic colloids. The anions (e.g. NO3- and SO4-) needed by the plant are usually repelled by the negative positions since particles with the same charge repel each other. They are held by the positive positions, which also exist n clays and humus. Such positions are, however, in smaller quantity than the negative. Some other soil colloids like Fe and Al oxides also have positively charged sites giving them an anion capacity. The supply of nutrients ions comes from either of the following: i. Release into the soil solution from decomposing minerals. this is very slow and non existing at the rooting zone of most topical highly weathered soils ii. The decomposition (mineralization of organic matter. This is the main source of supply.

Factors Affecting the Availability of Soil Nutrients An available soil nutrient element is one that exists in a condition that is easily soluble in water and which can easily be taken up and assimilated by plants. The factors that can influence this condition are, hereafter discussed: (i) Weathering: Rock weathering leads to disintegration and decomposition of minerals contained in rocks. The ions are thus released from the minerals and dissolved in the soil water. The colloids (also produced by weathering) are capable of attracting the ions to give a “swarm” of adsorbed ions around each colloidal particle. Positively charged ions (cations) like Ca, Mg and K are normally attracted to negative charges on the colloids. Negatively charged ions (anions) like PO4-, Cl- and SO4= are normally attracted to the positive sites on the colloids. It is thus evident that the following 3 main forms of plant nutrients exist in soils (Fig. 12.): (a) Ions bound up within the mineral particles (b) Ions adsorbed on to the surface of colloids (c) Ions in solution within the soil water Ions that are in solution are the most readily available to plants. They are either absorbed by plants or lost in drainage water. For a soil to remain fertile supplying the right types and correct proportion of the nutrients to support plants’ growth) the gradual withdrawal of nutrients should be replenished by continued release of ions by weathering of the soil minerals. Thus, the rate and type of weathering act as in important control on the nutrient content of the soil.

46

SOIL FERTILITY CONSERVATION AND MANAGEMENT

Figure 12: The main forms of soil nutrient ions ii. Parent materials and Soil minerals The supply of nutrients depends on the character (type) of the mineral particles contained in the parent materials and/or rock that form the soil. In general, siliceous, sandy rocks and soils usually provide very limited amount of nutrients since the rate of their weathering is low and they contain very limited range (types) of nutrients. Conversely, clayey and calcareous parent materials usually provide a far wider range and greater quantity of nutrients. There are the following three main types of clay in soils: (a) Kaolinite: This is the simplest form and consists of sheets of oxygen atoms linked to sheets of silica and aluminium atoms. One sheet of silica is available for every sheet of Al. As such, kaolinites are termed 1: 1 clays. (b) Montmorillonite: This has a more complex structure. It consists of 2 sheets of silica to every sheet of Al atoms and each group of sheets bound together by oxygen atoms. They are termed 2:1 clays. (c) Interstratified clays – Muscovite and Illite): These are intermediate in form. They are mixtures of the first two types and occur as inter-layered structures. The 2:1 clays have the greatest number of surface charges as a result of their more complex structure and chemistry. They, thus have the greatest ability to retain and absorb nutrients on their surfaces. On the other hand, the 1:1 clays have very small surface charge and therefore are able to adsorb only a limited umber of nutrients ions. The interstratified clays are again intermediate in character.

47

KAOLINITE

ILLITE

MUSCOVITE

MONTMORILLONITE

HUMUS

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

Figure 13: Relative quantities f nutrients adsorbed by different soil colloids. iii. Organic properties: The nutrients removed from soils by plants are eventually returned to the soil through the organic residues formed when the plants die. The decomposition of such residues releases the nutrient elements that are further changed and recycled by soil organisms. The organic residues can also break down to form colloidal particles, which like clay colloids can hold ions by adsorption. Their ability to retain ions is known to be better than in the case of the 2:1 clays. As such, the organic matter content of soils greatly influences the retention and release of nutrients. Soil Organic Matter Soil Organic Matter consists of roots, plant residues, and soil living or dead organisms. While Mineral soils contain less than 20% organic matter, organic soils are known to contain more than 20%. Organic matter accumulation is usually favoured by high rainfall, low temperature, and native grass vegetation or poor drainage. The rate of organic matter decay principally controls organic matter contents of soils even though total plant production is minimal since decay proceeds so slowly due to low temperature. Organic matter has been termed the “life blood” of soils. It has a tremendous impact upon the chemical, physical and biological properties f the soil. Humus is a complex and resistant mixture of brown or dark brown amorphous and colloidal organic substance either modified from living tissues or synthesized by soil population. Mineral soils, which have an abundant supply of organic matter, have good tilth, which refers to the workability of the soil. Organic matter will improve the structural condition of both coarse and fine textured soils by offsetting the high cohesion ad plasticity of the clay. Sandy soils have very little cohesion and plasticity will be bound together by organic matter. A good supply of organic matter will also improve the water holding capacity of sandy soils. 48

SOIL FERTILITY CONSERVATION AND MANAGEMENT

Three macro plant nutrients, N, P and S are constituents of soil organic matter. Greater than 99% of the total soil – N, 33 – 67% of the total soil – P and about 75% of the total soil – S are found in soil organic matter. These nutrients become available through decay activities. As discussed earlier, colloidal organic matter possesses cation exchange properties that are similar to those of clay produces CO2, which forms carbonic acid in the soil. This acid increase the solubility f many soil compounds, thus increasing nutrient availability. The numbers and kinds of soil organisms are greatly influenced by the soil organic matter levels. Most soil organisms derive their energy from the Carbon compounds in organic matter. Nitrogen for protein formation and other nutrients are also obtained from soil organic matter. Most soil organisms prefer wellaerated soils. Organic matter enhances proper air and moisture relationships for many soil organisms through its effect on soil structure. The maintenance of organic matter is essential for no – fertilizer agriculture, low CEC soils and in poorly aggregated sandy soils. The traditional way to increase it in cultivated soils is the addition of undecomposed raw materials in the form of animal manures, compost or plant materials incorporated as green manure. The beneficial effects of such additions can be summarized as follows: (1) Supply of most of the N and s and half of the P taken up by unfertilised crops. The slow – release pattern of N and s mineralization offers a definite advantage over soluble fertilizers. (2) Supply of most of the CEC of acid, highly weathered soils. Rapid decreases in OM results in sharp reduction in the CEC (3) Formation of complexes with amorphous oxides prevents P – fixation charges. (4) Contribution to soil aggregation and thus improves physical properties and reduce susceptibility to erosion in sandy soils. (5) Modification of water retention properties, particularly in sandy soils. (6) Formation of complexes with micronutrients to prevent leaching Cation and Anion Exchange in Soils This is the adsorption of a cation or anion by a colloidal material and accompanying release of one of more ions held by the colloid. Suppose a colloid has one – half of it’s capacity filled with Ca2+ (ions), one quarter with K+ (ions) and another quarter with H+ (ions) but treated with a solution of strong KCl. Eventually, K + ions from the KCl, will replace virtually all the cations on the colloid, creating an entirely K – saturated colloid and chlorides of Ca and H in solution.

H

H

H

K

K Ca

(Colloid)

+ KCl solution

K K

K (Colloid)

Ca K

K

K K

Ca

K K K + 3Cacl2 + 3 HCl

Such replacements depend on: 1. Relative concentration or numbers of the ions (This is an application of the chemical law or mass action). 49

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

2. The number of charges on the ions (The greater the number of charges carried by an ion, the greater is its efficiency assuming other factors are the same). 3. The speed of movement (i.e. activity) of the different ions. This is principally a function of ionic size, though the degree of hydration is also important. Thus, Li, Na, K and Rb are listed in the ascending order of size and one expects their efficiency of replacement to be in the same order i.e. Li, Na, K and Rb. During hydration (reduction reaction), however, the Li ion associates with so many water molecules such that its speed is reduced. Thus, due to its large hydrated radius, Li can not get as close to the colloids as the other ions. Likewise, the Na ion is more highly hydrated than the K+ ion. As such the order of replacement is reversed to Rb, K, Na, Li. Considering some of the most common cations in soils, the replace – ability series is usually Al > Ca> Mg> K > Na. Inorganic colloids (i.e. clay) and organic colloids (principally humus) are, thus, of utmost importance in soils. As they are in colloidal state, they expose a relatively large amount of surface area with small electrical charges for adsorption of water and ions. Nutrients either set free in solution during weathering (called “foreign” nutrients) tend to be adsorbed on the humus and clay surfaces and are released when needed by plants. This adsorption and storage of nutrients in exchangeable form on colloidal surfaces is a major factor in preventing or minimizing nutrient loss by leaching. Investigations have, however, shown that humus (organic matter) is a more efficient colloid (Nutrient exchange site) than the various inorganic colloids (Kaolinite, Montmorillonite and Interstratified clay types). The ability of colloidal particles to attract ions on to their surface is due to 2 reasons: (a) Isomorphous replacement or substitutions of ions within, the particles (organic or inorganic) occur during their breakdown. Usually, ions of high valency (combining power0 are replaced by others of lower valency but of similar size e.g. Silica (Si4+) may be replaced by aluminum (Al3+) which may also be replaced by Magnesium (Mg2+). In such instances, the electrical neutrality of the mineral or organic particle is upset and a net negative charge produced. (b) Breakage during weathering occurs along weak structural surfaces and leaves “loose ends” with unsatisfied charges at the edges. In such cases, both –ve and +ve charges may be produced. Monovalent ions (e.g. K+, Na+) may become attached to one of the unsatisfied charges in order to neutralize it while divalent Ca2+ or Mg2+ may become attached to 2 negative charges. Such cations are said to be “adsorbed” and the charges to which they are held referred to as “exchange sites” from which the ions can be released and replaced by other ions under appropriate conditions particularly when a chemical imbalance exists between the surface of the colloid and the surrounding soil solution. There will always be a tendency for the ionic concentration in the 2 phases to adjust towards equilibrium e.g. too much of H+ in soil solution can cause the tendency for other ions to be displaced from the colloids until equilibrium is restored. The extent to which adsorption and release of ions is done depends greatly on the type of colloids. Anion exchange may also occur, but since the presence of positively charged exchange sites on the colloidal particles is relatively rare, the process is far less important. Exchange of ions may occur only between the soil water and soil colloids, but also happens between soil water and plant’s roots and between roots and colloids directly. Plant roots have the tendency to release H+ during these exchanges and thus acts as a major source of soil acidity. The ions released into the soil water by solution, dissociation and cation exchange can be affected in 3 ways: (a) Some are taken up by plants (b) Some are re–adsorbed on the colloids (c) Some can be washed downwards through the soil percolating (sinking) water usually referred to as gravitational water. 50

SOIL FERTILITY CONSERVATION AND MANAGEMENT

The washing down of nutrients (leaching) is most serious under high rainfall, free drainage and low soil pH (i.e. high soil acidity). As a result, soils ay become deficient in nutrients and very acidic. Thus, the character of the soil profile and in particular the nature of the soil horizon, clearly reflect the chemical processes operating in the soil. The study of cations exchange in soils includes the determination of the following: (a) Cation exchange capacity (b) Total exchangeable metallic cations (c) Exchangeable hydrogen MEASURES OF SOIL CHEMICAL DYNAMICS Cation Exchange Capacity Cation Exchange Capacity (CEC) is an expression of the number if cation adsorption sites per unit weight of soil. It is the sum total of exchangeable cations adsorbed by the soil colloids and is usually expressed in Milli equivalents per 100 grams of dry soil. The exchangeable bases include Ca, Mg, K and Na. Technically, a base or a cation is a proton acceptor like the hydroxyl ion while an acid is a proton donor. The exchangeable cations are all associated with some compounds like Ca2CO3, Mg2CO3, K2CO3 while H is commonly called an exchangeable acid. The exchangeable base act in the formation of the hydroxyl as follows: i. ii. iii. iv. v. vi. vii.

Add 30mls of 1N neutral (pH 7) ammonium acetate (NH4OAc) to 5g of soil sample contained in a shaking bottle and shake on a mechanical shaker for 2hrs. This is because the replacement of the various cations by NH4+ is required. Centrifuge at 2,000 revolutions per minute for 5 minutes and decant / pour the clear supernatant into a 100ml volumetric flask Add another 30ml of 1N Neutral NH4OAc and shake for 30minutes, centrifuge again and pour the supernatant into the former 100ml volumetric flask. Repeat the third step Make up to the 100ml mark of the volumetric flask with distilled water. Use a photometer to determine the concentration of K, Na and Ca contained in the supernatant. Mg can be determined with absorption spectrophotometer. The summation of cations measured in the last step represents the CEC of the soil.

Soil pH pH is the logarithm (to base 100) of reciprocal f hydrogen ion (H+) concentration or the negative logarithm (to base 10) of the H+ concentration. i.e. pH = - log10(H+) = log10 1/ (H+) Thus, the soil pH is a measure of the degree of acidity or alkalinity of the soil. Its measurement is usually done with a pH scale ranging from 1 – 14 and in which 1 to about 6.5 is regarded as an acidic range and about 7.5 to about 14 is the alkaline range. Solutions or media having a ph of 7 are said to be neutral i.e. they are neither acidic nor alkaline. 51

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

The pH values of soils can differ widely from values of about 3 to as high as 10, being very low in acid, sulphate and podzolic soils and being rather high in calcareous and alkali soils. In alkali soils in particular, very high pH values may occur as the soil solution contains weak acids (HCO3-) and strong bases (Na+ or K+). The H+ concentration of the soil solution has a pronounced influence on a number of soil constituents and especially on the soil minerals, soil microorganisms and plant roots. High H+ concentrations are known to favour the weathering of minerals resulting in a release of various ions such as K+, Mg2+, Ca2+, Mn2+ and Al3+. The solubility of salts including carbonates, phosphates and sulphates is higher in the lower pH range. The optimum soil pH for crop growth is related to soil texture. It is rather low in organic soils and for mineral soils; it rises with increasing clay content. In inorganic soils in particular, the pH should not be too high for these soils are by nature poor in a number. Plants themselves (and particularly plant roots) respond to soil pH. The ph of the sap of many plant species is in the range of about 5.0 – 5.5. This slightly acid pH is often used in solution cultures in preference to neutral or alkaline conditions. If pants respond favourably to higher pH values (as in the soil medium), it may be due to other secondary effects resulting from the ph increase. Such include the alleviation of Al or Mn toxicity or an increase in Molybdenum or Phosphorus availability. Agricultural soils tend to be acidic for the following reasons: (a) H+ is one of the major constituents of H2O, which is constant supply. (b) Plants roots also release H+ (c) The CO2 usually released by plans roots can react with H to form carbonic acid i.e. H2CO3 CO2 + H+OH The following methods are available for the determination of soil pH in the laboratory and on the field/ farm. The field methods are usually rapid ones and are less accurate compared with the laboratory methods. (i) The Field Method / Test: This involves pouring 50ml of BaSO4 to 10g of fresh soil contained in a test tube. Twenty millimetres of distilled water and 10ml of BDH indicator liquid are also added before they are thoroughly shaken and a clear supernatant allowed to develop the colour of the clear supernatant is then compared with the pH colour chart whose changes with pH are as follows: pH 3–4 5 6 7 8 9 10

Colour Red Orange Yellow Yellow/Green Green Blue/Green Blue

(ii) Laboratory Method: For more accurate measurement, soil samples are taken and processed in the laboratory. The soil samples should be air dried and sieved with a 2mm sieve. Twenty grams of the air dried soil is then stirred in 20ml of distilled water or 1M KCl solution or 0.01M CaCl2 solution in a 100ml beaker. Use a glass rod for stirring occasionally to last for 30minutes after which the glass electrode of a pH meter is inserted into the soil suspension to measure the pH.

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Sample Calculations on CEC and Soil pH 1. A soil has the following exchangeable cation values on the exchange complex. Exchangeable cations Al K Ca Mg Na H

Value (meq/100g) 8.3 1.0 4.0 3.0 0.3 2.4

Calculate: (a) CEC (b) Exchangeable acidity (c) % Base saturation (d) Total exchangeable metallic cations Solution / Answer: (a) CEC = sum of all exchangeable cation values = 19.00 me/100g (b) Exchangeable acidity = Al3+ + H+ = 10.7 me/100g soil (c) % Base saturation = (K + Ca + Mg + Na x 100) % = 43. 6% (d) % K saturation (K x (CEC

100) % = 5.26 1

(e) Total exchangeable metallic cations = CEC – H+) or Al+ + K+ + Ca+ + Mg+ + Na+ = 16.6me/100g 2. Calculate the soil pH is the H+ concentration of it’s suspension in water is 2.3 x 10-9 moldm-3 pH = - log (H+) = -log 2.3 x 10-9 = - (9+ log 2.3) = - (9 + 0.36) = + 9 – 0.36 = 8.6

Factors affecting soil pH measurement 1. The more dilute the soil suspension is, the higher will be the pH value as more H+ ions will be released from the adsorption sites. 2. In practice, therefore, one should specify the soil liquid ratio used e.g. 1.1 soils: H2O ratio. 3. Soil pH increase with increasing salt concentration. Thus, there is the seasonal variation in soil pH 4. The CO2 of the air can decrease pH value as a result of the formation of carbonic acid. 53

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The significance for pH measurement includes: 1. Soil pH indicates the amount of liming that is necessary to correct soil acidity i.e. to make very acidic soil suitable for growing specific crops. 2. Since crops have specific pH ranges they can tolerate, the determination of the soil pH is helpful in knowing the crops that can be economically grown in an area. 3. It indicates nutrient availability e.g. when pH is low, Al, Fe, Mn, Cu and Zn are soluble to the extent that they may become toxic to some plants. As pH rises and rain falls, the quantities decrease and may reach insufficiency level at pH 7 or above. 4. It indicates the type and amounts of micro organisms that are present in the soil e.g. bacteria and actinomyctes are very active in soils with medium to high pH values and their activity is highly reduced when the pH drops below 5.5 Fungi predominate at lower pH values but at higher ranges, they encounter strong competition from bacteria and/or actinomycetes. Alteration of Soil pH and Soil buffering If soil pH is unfavourable, serious inhibition to plant growth can be minimized by: (1) Growing of plants that will grow well with the existing soil pH. (2) Finding means of changing the pH of the soil to suit particular plants The near constancy of pH of any system (e.g. soil) to when acid or base is added is due to a buffering cation of the acid – base equilibrium. Buffers are known to be media that are capable of maintaining the pH within a narrow range when small quantities of acid or bases are added. Thus, a buffer solution can be prepared in the laboratory using a weak acid and the Na salt of the acid i.e. Acetic or Boric acid or Phosphoric acid and their Na salt. A typical example is indicated with the equations below: CH3COOH Acetic acid CH3COONa+ (Na acetate)

CH3COO- + H+ CH3COO- + Na+

The soil buffering media are clay, organic matter, carbonates, bicarbonates and phosphates which act as exchangeable sites i.e. they either adsorb H+ from solution or release H+ to solution in an attempt to maintain equilibrium between adsorbed ions and ions in solution. The buffer capacity of a soil therefore, gives and indication of the extent to which it’s acidity or alkalinity can be maintained. Thus, it shows the difficulty or altering its pH. For alkaline soil, buffer capacity is the amount of acid that is required to reduce the pH value of 100g soil from the initial value to 4.8 which has been set as the lower limit for optimum growth of crops. For an acidic soil, buffer capacity is the amount of a strong base that is needed to raise the pH from the initial value, which is regarded as the extreme upper limit of alkalinity a crop can still grow. The specific buffer capacity of a soil is the total buffer capacity divided by the change in ph value that is produced. The lime requirement of a soil is related to both pH and buffer or cation exchange capacity and it is the amount of CaCO3 or its equivalent that must be applied to soil in order to increase the pH to 7 or some other desired values. Determination of Lime Requirement To determine the lime requirement of a soil it is necessary to determine the change in soil pH resuming from known additions of lime. The following procedure can be used: 5g soil samples of known pH are weighed into 5 separate 100ml capacity beakers and 50ml of distilled water is added to each other. 54

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They are then separately treated with 0, 0.01, 0.02, 0.05 and 0.10g Ca (OH) 2 solution and 2 drops of chloroform added to each to prevent microbial growth. Stir thoroughly and cover for 4 days. After the 4th day, stir again and measure the pH of each sample. The curve of pH versus Ca (OH) 2 treatment is then plotted. From such curves, the amount of Ca (OH) 2 needed to raise the pH to 6.5 or 7 or any required pH value can then be extrapolated. The quantity/weight (W1) that is needed to raise the pH 1 of hectare of land to a depth of 15cm will be given by: W1 = Wh x 5 x 2 x 106 kg Ca (OH)2 100 Where Wh = the amount of Ca (OH) 2 needed for 5g of soil sample As CaCO3 is the most common lime used on the farm. One has to convert the value got to that of CaCO3 since Ca (OH)2 was just used because it is more readily soluble than CaCO3. The conversion ratio is 1.37 i.e. multiply what is obtained by 1.37 to get the amount of CaCO3 needed.

Method and Time of Application of Lime The major requirement of any method used is even distribution in order to allow through mixing with the soil since lime does not move to any appreciable extent in soil and acidity is due to the colloidal clay acids which lime needs to come in contact with. Lime spreaders are normally used in the application while the mixing with soil is by tillage operation. Lime can be applied anytime of the year depending on the convenience. The type of rotation, the system of farming and the type of lime used can however help in deciding the most appropriate time. Limes should be applied in advance of planting of legumes such that they will have to correct the acid condition, which leguminous crops cannot withstand. If the form of lime to be applied is Ca (OH) 2 or CaO, it should be applied some months before planting so that they will be changed to CaCO3, which is not injurious to plants. Carbonates can be applied anytime without danger of injury.

GENERAL PRINCIPLES OF SOIL MANAGEMENT Introduction Tropical Africa is currently experiencing a food crisis as human populations obviously have far outstripped food production and the average African continuously has less food to eat. The growing urban populations need cheap food as farmers simultaneously need higher prices as an incentive to produce. Most African nations use scarce earnings obtained from foreign exchange for food importations, which have tremendously increased in volume and in cost. The various national institutes set up purposely for food crops in the continent are facing infancy “teething” problems such as poor funding and understaffing. The trained professionals (largely expatriate) in key positions that conduct research and extent the results to farmers are very scarce. The basic reasons for the inability of the small-scale farmers to meet the growing urban food demand include the following: 55

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i. ii. iii. iv.

The preponderance of fragile and easily eroded soils that keep production very low Poor infrastructure facilities that make the movement of agricultural output to consumers continuously difficult The limited farm power – inefficient backbreaking labour with small hand tools. The rural youth fleeing to the cities in growing numbers and becoming consumers rather than producers of food.

In order to get out of the problem, efforts should be directed at: a. Adopting technology from national and international agricultural research centres and b. Making a solid commitment to rural development. Concept, background and relevance of soil conservation and management Soil is man’s most valuable possession. All the products of farming (livestock and crops) depend on it and its fertility. As such, the use of soil needs to be planned and properly managed to ensure that it continues to support life and give the maximum possible yield. The productive capacity of a good soil should be sustained over generations. The concept of soil conservation implies the prevention of loss, waste and damage of the soil. It is synonymous with soil preservation. The actual processes or skilful treatments that are employed in this regard are termed soil management practices. The fertility of a soil can be measured by its capacity to support the climax population of plants and animals above ground and the flora and fauna below ground. It is also implied by the nutrient supplying power of the soil in terms of amount and proportion. Conversely, soil productivity is a characteristic of the soil to adequately support plants (crops) growing on it. No mater how fertile, some soils may not be productive. This is because soil productivity is actually a reflection of the interaction of soil factors, fertility and the immediate environment. Thus, for a soil to be productive, it must of necessity be fertile. It does not follow, however, that a fertile soil must be productive. An unproductive but fertile soil can be rendered very productive through adequate soil management practices. Similarly, an unfertile soil with all capabilities of productivity can be helped through the improvement of fertility using soil testing as a guide to fertilizer use. Loss of soil fertility is a major problem of tropical farmers. One or a combination of the following often causes land or soil degradation: i. Soil Erosion ii. Chemical soil poisoning (Drug abuse) iii. Building and/ or mining iv. Soil salinization (Salt accumulation in soil) Land or soil degradation is the turning or changing or a once productive fertile soil into a sterile gravel pit. Tropical farmers evidently have the greatest risk of degrading their soils during land clearing especially under large scale production of crops. The least damaging land clearing technique to tropical soils is the MANUAL technique, which has been known to be least efficient. Considering all the known mechanical land clearing techniques/devices, the use of the SHEAR BLADE is the least disturbing / damaging on tropical soils since trees are cut at the soil surface level. Nevertheless, the method encourages the destroyer of the aggregate soil structure of the very fragile and shallow (about 15cm top soil) of tropical soils due to the heavy nature of all mechanical devices involved just like in the case of the other mechanical land 56

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clearing techniques – use of TREE PUSHER and ROOT RAKERS. These mechanical techniques can, however, be employed if the soil is sufficiently deep (up to 100cm top soil) as found in the temperate climate regions of the world for which the machines were designed. Over the years, the main soil conservation and management techniques/practices involved in the improvement and/or maintenance of soil fertility in many parts of the humid and sub – humid tropics (particularly in Africa) has been bush fallowing (interspersing the shifting, slash and burn cultivation). This is in contrast to the situation in the developed world where improvement of soil fertility has for long time depended on the application of fertilizers (organic and inorganic). The shifting cultivation system of farming is considered primitive but has certain merits. In the system, short (1 – 2years) cropping periods alternate with long (6 or more years) fallow periods. The fallow period is meant to restore soil fertility and get rid of notorious weeds, pests and diseases. A large area of the humid and sub – humid region have soils that are dominated by low activity clays (LAC) which are characterized by low effective cation exchange capacity, low available water and nutrient reserve. They are also highly susceptible to soil erosion. The fertility – restorative power of the bush fallow is due to the following: 1. The re-growth of deep-rooted trees and shrubs that recycle plant nutrients and build up soil organic matter. 2. During the fallow period, plant cove and litter protect the soil from the impact of high intensity of drops of rain and the roots help to bind the soils, increase water infiltration and reduce run off and soil erosion 3. Litter mulch and shading by the tree and shrub canopies also help to reduce soil temperature and to maintain soil moisture conditions that are favourable for the growth of beneficial soil micro – and macro – organisms. The shading also reduces weed infestation. Apart from restoring soil fertility, the bush fallow provides supplementary food for the farmer, animal feed, staking materials, firewood and herbal medicine. Wherever land is abundant, the bush fallow has been found to be a stable and efficient biological method for soil productivity restoration. This is because food crops usually yield well on newly cleared land after a long rest period under bush fallow. “Land – hunger” has, however, resulted in the shortening of the fallow periods as a result of rapid population growth and the attendant increase in the demand for land for which is immovable can hardly be created. Over exploitation to land that are dominated by highly weathered kaolinitic soils can easily lead to soil degradation, a rapid decline in crop yield and invasion by notorious weeds including the difficult to control grass species. It is therefore, evident that the maintenance of soil fertility under continuous land use is one of the most serious agricultural problems in the tropical rain forest regions of the world in the face of increasing population pressure on the soil. Several research efforts have indicated that: 1. There is leaching and wash of the topsoil once the vegetative cover is removed. 2. At the beginning of cultivation, all the nutrients are at their peak (in availability) but as cultivation continues, the nutrient elements tend to leach out. 3. The use of heavy equipments accelerates soil erosion, destroys soil structure and accelerates decline of soil organic matter. 4. Soil organic matter function as the principal source of nutrients and its decline affects both the yield and soil nutrients. It has a profound effect on the structure of soils. The deterioration of structure that follows intensive tillage is usually less rapid in soils that have a high organic matter contents. When organic matter is depleted, soils tend to become hard, compact and cloddy. Organic matter all favourably affects aeration, water holding capacity and permeability. 5. In the tropics, organic matter decomposes four times faster than under the temperate field conditions.

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6. Under continuous land use, addition of fertilizers (particularly ammonium sulphate) results in a great decline in total nitrogen and soil pH. Differential uptake of anions and cations coupled with secretion of carbon dioxide by plants affect the pH of the soil in which they grow. The activity of microorganisms and base removal by crops are, however, the major sources of soil acidity under the traditional system of cultivation. In the tropics, particularly in Ghana, report of soil pH decreases (7 to 5.3) after using ammonium sulphate for nine years has been given. 7. The use of inorganic fertilizer alone has not been effective as a farming practice, which employs both organic and inorganic manures with fallow. Thus, for intensive cultivation of tropical soil, a new technology for land preparation, clearing, cultivation, harvesting etc. has to be developed. A system of crop rotation that will keep the soil under cover for a greater part of the rainy season and whose residues are incorporated into the soil to maintain high organic matter, reduce soil erosion and leaching losses of nutrients should also be developed. To achieve these goals, the following farming techniques have been suggested and practiced: 1. Inter-planting of legume under continuous land use 2. Rotational systems including and excluding legumes, planted fallow and natural fallow 3. Tillage practices including minimum and zero tillage with heavy equipments 4. Mixed farming and 5. Multiple cropping including a combination of crops 6. Alley cropping No – Till Farming No tillage refers to a cropping system that eliminates all pre planting mechanical seed bed preparation except for the opening of a narrow (2 – 3 cm wide) strip or hole in the ground for seed placement to ensure adequate seed – soil contact. The entire – soil surface is covered by crop residue mulch or killed sod. Chemicals are used to control weeds and inorganic fertilizers are applied over the crop residue mulch without further cultivation. The no – till system can be further improved by incorporating or integrating into it other complementary and compatible practices such as the Alley cropping and live mulch systems. Merits of No – Till farming 1. Soil Amelioration – The compaction and degradative effect of heavy machinery can be reduced by adopting no – tillage system especially when used in combination with a cover seeded immediately after clearing. The no – tillage system with residue mulch and cover crop improves and restores soil conditions degraded by mechanized land clearing. 2. Soil Conservation – The no – tillage system prevents erosion through the protective effect of residue mulch. This is achievable without using the expensive and ineffective practices of graded channels, terraces, contour ridges and other engineering structures. 3. Moisture conservation and water use efficiency - A decrease in water run – off and surface evaporation and an increase in the available water holding and the retention capacity of untitled soil makes more water available for crop use in no tillage than in ploughed soils. During dry spells, crops suffer less from drought stress on untilled on mechanically tilled soils. 4. Soil temperature – Untilled soil has lower maximum and higher minimum soil temperatures compared with tilled soils. This favours metabolic reactions that aid optimum performance of various beneficial soil micro – and macro organisms. 5. Root growth and development – the total root mass in no – tillage soils are generally more than with conventional ploughing. Some roots in no – tillage can penetrate deep into the profile along the path made by the decomposed roots of the preceding crop and also though the channels made by earthworm. The lateral roots or inter – row spread is also greater in no – tillage than in ploughed soil. 58

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6. Earthworm activities – Earthworm activity is related to the amount of mulch material and to the soil temperature and moisture regimes. High earthworm activity contributes erroneously to the mixing of nutrient and organic matter in the soil. Earthworms are important in improving soil structure and porosity in the mineralization of soil organic matter. They are the best “implements” for ploughing tropical soils. They turn over the soil without causing erosion problems for which the mould board plough is so notorious. Earthworm casts are structurally stable to rain drops impact and contains more silt and clay than parent soil. They enhance mineralization of crop residues and make nutrients in organic matter more readily available. Casting activity of earthworm is higher in no – till mulched soil than in ploughed land. 7. Nutrient Status – Soil management with the no – till system has higher concentration of organic carbon, total nitrogen, available phosphorus and exchangeable calcium and potassium in the surface layer than ploughed soil. The most noticeable effect of the no – tillage system with crop residue is on organic matter. The rate of decline of soil organic matter is drastically lower/reduced with no – tillage than with conventional ploughing. 8. Fertilizer use efficiency – The efficiency of applied inorganic Nitrogen depends on C – N ratio of the residue mulch, previous land use and on the level of soil compaction. No tillage with a mulch material of a high C:N may exhibit chlorotic symptoms of nutrients deficiency for zero or low rate of N – application during the first or second seasons of adopting the no – till system. However, when the immobilization and release of nitrogen have reached a steady state, the fertilizer use efficiency is generally greater on untilled than mechanically tilled soil. 9. Savings in fuel and Labour – Fuel requirement decrease with no – tillage system due to elimination of ploughing, harrowing and other operations that have high energy requirement. In addition, there is a definite saving in respect of energy required for seedbed preparation. Labour is a serious constraint in traditional farming system. Use of herbicides for weed control with no tillage can drastically reduce labour requirements and also increase labour efficiency by reducing drudgery. 10. Grain yields – No tillage generally out-yields the conventional tillage system if the crop suffers from moisture, temperature or nutritional stress. The no tillage system therefore, maintains a stable yield. Soil loss, or run – off loss and grain yield ratio is always higher for ploughed than untilled soil. 11. Environmental considerations - In addition to the reduction of soil erosion and water run – off, no tillage also reduces nutrient losses from agricultural from agricultural lands and reduces the movement of chemicals from the land. Most herbicides used in no – till crop production system are as persistent and have less residual effect than some insecticides and pesticides commonly used. The Alley Cropping Food Production Method This is essentially an agro forestry system in which food crops are grown in alleys formed by rows of trees or shrubs like Leucaena and Gliricidia. Leguminous trees and shrubs are preferred because of their ability to fix atmospheric nitrogen. The tree or shrubs are cut back and plants are kept pruned during cropping to prevent shading and to reduce competition with field crop. When there are no crops, the trees and shrubs are allowed to grow freely to cover the land. Alley crops retain the basic features of bush fallow. Resource-poor farmers in the tropics can easily adopt it. Importance of the Trees and Shrubs in the Alley System 1. They provide green manure or mulch for companion food crops. In this way plant nutrients are recycled from deeper soil layers. 2. They provide pruning, which are used as mulches and shade during the fallow period to suppress weeds. 3. They provide favourable conditions for soil macro – and micro – organisms 59

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4. They provide a barrier to control soil erosion when planted along the contours of slopping land 5. They provide staking materials and firewood 6. They provide biologically fixed nitrogen to the companion crop. Farmers in many parts of the tropics have recognized the importance of some plant species in soil fertility regeneration in the traditional bush fallow. The major advantage of alley cropping over the traditional shifting cultivation and bush fallow system is that the cropping and fallow phases can take place concurrently on the same land. This allows the farmer to cropland for an extended period without returning the land to bush fallow. Criteria for selecting suitable trees and shrubs in alley cropping 1. Easy establishment 2. Rapid growth 3. Deep root system 4. Heavy foliage production 5. Quick regeneration after pruning 6. Easy to eradicate 7. Provision of useful by-products 8. Ability to fix atmosphere Nitrogen

SOIL EROSION, DESERTATION PROBLEMS AND CONTROL Soil erosion is a term commonly used to describe the process or processes by which the products of rock decay are removed, transported or carried away by certain forces. It is the wearing away of soil in the earth’s surface by water or wind. There are two main types of erosion - geological / natural erosion and accelerated / speed / artificial soil erosion. Erosion that takes place under natural conditions (i.e. when the land surface and native vegetation have not been disturbed by man’s activities) is known as natural or geological erosion. The erosion speeded up as a result of man’s activities is accelerated / speeded/ artificial erosion. Geological erosion is a relatively slow process under many conditions and soil formation may keep pace with the removal of the surface- soil. On the other hand, accelerated erosion is very rapid and invariably destructive when environmental factors favour it. Geological erosion can also be seen and described as a natural process that tends to bring the earth’s surface to a uniform level. Thus, whenever a part of the surface is elevated above the other (surrounding) portion, geological erosion immediately begins the work of levelling off the “high – land”. Such a process results initially in a very rough topography due to the cutting of gullies. The accelerated form of soil erosion is usually occasioned by mismanagement of land. Evidences of soil mismanagement occur in most countries. In the midst of a productive agricultural area, many farms can be observed to have depleted in organic matter content as evidenced by a lighter colour and lower productivity than surrounding soil. Typically, sandy soils whose forest covers have been cleared, cropped for some years until the virgin fertility was exhausted may be blown by the wind. The need for increased food production to feed the everincreasing world population and satisfy nations with production to feed the ever-increasing world population and satisfy nations with inadequate food supplies afford the major reason for giving careful attention to soil conservation.

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The factors that encourage – soil erosion are principally two – the physical and human factors. The physical factors include the following: i. The nature of the soil and soil surface (whether compacted or loose) ii. The nature of the land surface (physiography) iii. The climate iv. The vegetative cover. The human factors are principally indicative of the extent of human interference. Soil erosion types as caused by the two main agents (water and wind) on agricultural lands are discussed in the sections below. Soil Erosion Caused by Water This is the more common type of soil erosion. It occurs in all areas where agriculture (arable or pastoral) is possible (i.e. where rains falls and affect the topsoil that contains the bulk of the plant nutrients. The energy of falling rain drops is applied from above and it’s main function in soil erosion to detach soil particles from the soil mass. This can be described as the loosening or detachment of soil particles or groups of particles from the main body of the soil. By contrast the energy of flowing water is usually applied parallel to the soil surface and it functions to transport (remove) soil materials to new environments. Thus, water erosion can be classified into two types: (a) The rain drop type – (principally splash erosion) and (b) The running water or flowing water type including sheet, rill and gully erosion forms Rain drops and soil erosion Soil erosion by raindrops is usually termed splash erosion since soil particles fly in all directions (after being detached from the soil mass on impact when falling raindrops strike the ground surface or the thin films of water covering it. The amount of the detached soil is proportional to the detaching capacity of the drops and the detachability of the soil. In other words, the splashing or scattering of the very small soil particles causes the splash erosion by the impact of raindrops. On causal observation, this action seems very minor, but when the numerous raindrops striking a unit area of soil surface during a one hour rain and the force with which they strike are taken into consideration, it would be clear that the net effect in loosening and moving soil particles is tremendous. The effects of rain drops on agricultural soil involves the washing out of the more valuable parts of the soil, leaving the stone and sand fractions in the land, breaking down soil clods and crumbs and causing many of the aggregates to release their humus and other useful materials and thus render the soil unproductive. Sheet and Rill (micro – channel) Erosion Sheet and rill erosion types work hand in hand but the resultant effects is referred to as sheet erosion as distinguished from the more destructive gully erosion. Although erosion types designated, as the average observer may not notice sheet erosion, gullies attract immediate attention, disfiguring the landscape and giving the impression of land neglect and soil destruction. Sheet and rill erosion generally involve the removal of thin layer of soil from sloping lands. Strictly speaking, sheet erosion is the quite uniform removal of soil from the surface of an area in thin layer. Ideally, for it to occur, the soil surface should be smooth. This condition is, however, seldom the case (especially on cultivated soil) surfaces designated smooth usually contain small depressions in water, which can accumulate and may subsequently cut tiny channels (micro – channels) as it moves down a slope. When this occurs at numerous points, multitudes of very shallow trenches called rills are formed. Rills are often formed immediately surface or sheet flow begins. The surface soil is, however, removed rather uniformly as none of the rills grow to appreciable size or depth.

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Gully erosion Gully or channel involves the joining up of many micro channels (rills) to make large channel with greater discharge and greater erosive power as a result of which rill erosion is changed to gully erosion. It is the channel erosion that cuts so deeply into the soil such that the surface cannot be smoothened by ordinary tillage tools. Gullies do not only result in soil loss but also deposit eroded materials over more fertile soils at the foot of slopes. The rate and extent of gully development are closely related to the degree of run off, which is closely related to the size of the drainage basin and its hydrological properties. Rainfall, and soil characteristics, land slope and the extent or degree of human activities, the nature e.g. if the slope are also decisive. A change in any of these factors affects gully development e.g. if the slope angle decreases (e.g. when the fallen debris accumulate at the foot slope) or if vegetation re – establishes itself and establishes itself and improves soil stability or if rainfall decreases so markedly as to affect runoff capacity. Damages Caused by Erosion Occasioned by Water Erosion by water leads the way for the following five types of damage: 1. The loss of the water that causes the erosion. The water might have been useful in crop production had it entered the soil instead of running off over the surface. 2. The soil carried away frequently stops to be value in crop production remain soil is deficient of the surface or top or plough soil and is as such of very low productivity. 3. The soil carried away often causes much damage since a layer of infertile subsoil may be deposited on a productive soil area especially during gully formation. This usually reduces such soil’s crop producing power. 4. Farmlands may be cut into irregular pieces especially when gullies are formed. Too deep gullies are difficult to cross with farm implements. They thus, cause great inconvenience and loss of efficiency in cultivating, planting and harvesting crops. 5. The soil removed during erosion may be deposited in streams, harbours and reservoirs and thereby increase floods, impedes floods, impede navigation and reduce water storage capacity. Control of Erosion caused by water The four basic principles that guide many of the methods employed in soil erosion especially when the erosion is caused by water are: i. ii. iii. iv.

Protection of the soil surface against the impact of rain drops Preventing water from concentrating and moving down a slope in a narrow path Slowing down the movement of water on slopes Encouraging a higher percentage of the water to infiltrate the soil.

The above-enumerated principles of water erosion control are directed at the two basic processes that are usually involved in soil erosion, namely: (a) The loosening or detachment of soil particles or groups of particles from the main body of the soil and (b) The transportation or removal of the loosened / detached particles groups from their positions. The actual measures for the control of water erosion are: 1. Avoidance of concentration of water before if flows down a slope. This can be achieved by causing the absorption of excess water into the soil of the area that feeds the channel. 62

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Alternatively, run off can be diverted such that it runs around rather than into the drainage way. It is easy and less costly to prevent the concentration of water before if flows down a slope than it are to stop gully formation when the process is well established. Farming practices than can help in increasing absorption of rainwater are: (a) A good crop rotation including ample use of cover crops (b) Permanent pasture (c) Strip cropping (d) Contour cultivation and/or combined terracing (e) Minimum tillage 2. Use of thick growing and Sod crops to decrease erosion. Dense sods (produced by grass species), legumes that develop sod – like root formation at the topsoil and crops that rapidly cover the soil surface are effective in preventing the direct impact of raindrops on the soil. Stems of some other close – growing crops can help to reduce erosion by slowing down the movement of water (erosion power) over the soil surface. This also allow for more of the water to infiltrate the soil. Contour Cultivation A contour is an imaginary line that connects points of equal elevation on soil surface. Contour terraces are usually laid out on sloping lands at right angles to the direction of the slope and level throughout its course. They are very effective in water erosion control especially when rainfall is gentle and slopes are short and moderately sloping. Best results can be got if contouring is combined either with strip cropping or terracing. The capacity of furrow made by contour tillage is small and easily become filled with water. When such furrows overflow during storms, they result in considerable loss of soil. Lister furrows on the contour hold much water until it percolates into the soil apart from preventing erosion. Strip Cropping This is the planting of different growth habits in strips across sloping fields. Erosion from a strip of cultivated crop (maize or cowpea) can be arrested in another strip of hay that grows next down the slope. This is because the effect of different crops on erosion varies. A strip of another crop can then follow, then another strip of hay and then a cultivated crop. The width of the strips of each crop depends on the steepness of the slope, the amount and nature of the precipitation and the erodibility of the soil. The strips should be planted on the contour as nearly as possible. Strip cropping required less power, more convenient and efficient since work (planting, tillage, harvesting etc.) is done almost on level ground instead of up and down hill. This is more so if the land has long and fairly regular slopes as against the case of short and very irregular slopes. Terraces Terraces are channels (broad surfaced) constructed across at specific intervals on contour lines. They intercept run off and retard it for soil infiltration or direct it to an outlet at non – erosive speed. The bench, graded channel and broad base ones are known. Sod Waterways These are depressions (shallow channels) kept in sod. They help to reduce gully erosion as water moving down slope naturally drains into them. Sodding is quite useful when there is an alteration in the drainage way. It prevents soil washing prior the establishment of a grassed water way by seeding mulch 63

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held down by wire netting or brush can also be used to control erosion while a stand of grass is being developed. Soil erosion by wind Wind is air in motion. Soil erosion by wind, therefore occurs when soil particles are involved in the movement of air. It common on soils unprotected or only partially covered by vegetation and especially when land is being prepared for planting, before the crop reaches sufficient size to protect the soil and when land is being fallowed. Soil blowing can also occur on rangeland has been overgrazed. Though wind erosion is greatest in semi arid and arid regions (with dry land surface and sparse vegetation0 much damage to both crops and soil also occur in humid areas. Wind erosion may occur wherever soil vegetation and climatic conditions permit free wind action. The blowing is not confined to small particles alone. Larger particles can also be moved over the soil surface by rolling and short skips and do much drainage to a young crop or pile up into small ridges and great dunes. Smaller particles may be carries along somewhat above the soil surface or they may be swirled high in the high in the air and transported to long distances. The factors conducive to free wind action are: a. Loose, dry and fine grained soil b. Fairly smooth soil surface c. Vegetative cover that is sparse or totally absent d. Wind that is sufficient strong to initiate soil movement In any area, some of these factors may favour while some others may hinder wind erosion. The process of detaching soil particles is not important in wind erosion. The three stages that are important are initiation of soil particle movement, soil particle transportation and soil particle deposition. The first phase (i.e. initiation) is influenced by the turbulence of the wind. The minimum wind velocity required for the phase is the “threshold velocity” which principally depends on the size of the particle since the erosive power of wind is controlled by the load it carries. Repeated tillage of dry soil usually increases erodibility especially if it involves pulverization. In addition to roughness of the soil surface governed by clods or aggregates, ridges affect wind velocity and depressions formed by tillage implements or by other causes. Similarly, living or dead vegetative matter protects the soil surface from action of winds. Apart from reducing wind velocity at the soil surface by increasing the surface roughness, it also absorbs much of the force exerted by the wind and traps the soil particles. Other wind barriers or wind or windbreaks like fences, reduce wind velocity near the ground surface and thus check wind erosion. Types of erosion caused by wind The two major forms of soil blowing based on the movement of soil particles can be described as follows: (a) Saltation: Fine soils particles (about 0.1 – 0.5mm in diameter) can be rolled on the soil surface by direct wind pressure and then suddenly jump up almost vertically. Once in the air, the particles gain velocity and later descend in an almost straight lie. They may rebound again into the air or knock other particles into the air and come to rest themselves. Soils composed entirely of extremely fine particles (0.1mm in diameter) are usually very resistant to wind erosion. They can nevertheless be thrown into the air by the impact of particles moving in saltation. Once in the air, the particles movement is governed by wind actions. They may be carried very high and over long distances. (b) Surface creep: Relatively large particles (0.5 – 1.0mm in diameter) may be too heavy to be lifted by wind action but can be rolled or pushed along the soil surface by the impact of particles in saltation. Thus, wind erosion is principally due to the effect f wind on particles of suitable size to move in saltation. 64

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Principles Guiding the Controlling of Wind erosion 1. Build up soil particles into clusters or granules of too large a size to move in saltation. 2. Reduce the velocity of wind near the soil surface e.g. by ridging, increasing vegetative cover and/or developing a cloddy surface. 3. Provide strips of vegetative cover that is sufficient to catch and hold the particles moving in saltation. Management practices in the control of wind erosion a. Leaving soil areas in sod. Land in regions of low rainfall is more safely left in native grasses. b. Do not leave land to fallow in the late/dry season. Any cropping system that leaves the land unprotected by growing crops for an appreciable length of time opens the opportunity for soil blowing. Strips of a late season crop (e.g. sorghum) alternated with strips of fallow can help to reduce soil blowing when fallowing is necessary. c. Keep the soil surface rough by making furrows right angles to the prevailing wind and planting furrows instead of on the level ground d. Use of crop rotation, which involves a minimum of soil tillage. e. Strip cropping at right angles to the prevailing wind. The width of the strips should be determined by the nature of the soil, exposure to wind and similar factors. f. Stubble – mulch culture – crop residues can effectively protect soil against wind erosion by reducing wind velocity and catching soil particles moving in saltation. Strips of soluble left at frequent intervals across a field being fallowed form effective barriers. g. Stabilize blowing sand with trees and coarse grasses. h. Use of trees as windbreaks on organic soils. Rows of trees can be planted across the land area (known to be troubled by wind erosion) at right angles to the prevailing wind. i. Use of additional materials (e.g. fences) as supplementary windbreaks. The devices for wind erosion control (whether applied in arid or humid regions) are by and large either vegetative or purely mechanical and are stages of the broader problem of soil moisture control. Obviously, if the soil can be kept moist there is little danger of wind erosion. A vegetative cover also discourages soil blowing especially if the plant roots are well established. By roughing the soil surface, the wind velocity can be decreased and some of the moving particles trapped – stubble mulch has proved to be effective in this manner. Tillage to provide for a cloudy surface condition also is at right angles to the prevailing winds. Similarly, strip cropping and alternate strips of cropped and fallowed land should be perpendicular to the wind. Barriers such as tree shelterbelts are also effective in reducing velocities for short distances and for trapping drifting soil. Soil Desertation Desertation is the turning of productive soil to a non – productive soil when developed and managed by practiced that are least desirable for the harsh climatic environments. In the process, large land tracts where bush green forests once grew are turned barren and unproductive. Invariably, soil desertation could be traced to soil erosion, improper land clearing methods, soil surface exposure weather hazard, deforestation, grazing, poor farming techniques and management. When the natural vegetation cover on the soil is disturbed or removed or when bad farming and land use practices are adopted, soil erosion a precursor of soil desertation – results.

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Effects of Desertation 1. Effect on the microclimate – significant changes in air and soil temperature as well as relative humidity result from forest removal. The magnitude of change is greater for soil temperature than for air temperature. The maximum soil temperature is proportional to soil disturbance and inversely related to the ground cover. The relative humidity is generally lower on the cleared land and under forest canopy. Desertation increases the aridity of the microclimate since evapo-transpiration is aided by low relative humidity occasioned by higher temperatures. 2. Effect on soil physical properties – Desertation increases soil compaction, decrease total porosity, water retention and transmission properties. The most significant increase in bulk density and changes in other soil physical properties occur in upper few centimetres of the soil. 3. Effect on Hydrological characteristics – hydrological balance is drastically affected by deforestation due o an increase in water run – off and seepage flow and a decrease in soil water storage in the rooting zone. Desertation may increase the total water loss from less than 1% to as much as 30% of the precipitation received. This increases the hazard from erosion and the susceptibility of crops to drought stress even a few days after a heavy rain. The amount of water run off is higher from mechanically cleared land and is directly related to compaction and soil disturbance caused during clearing. 4. Effect on biological activity - Earthworms and some other soil microscopic and macroscopic organisms play a significant role in productivity of tropical soils. Such organisms are usually adversely affected by desertation once the microclimate is affected. A typical and notable effect is on earthworms which remain dormant or burrow deep into the soil when soil moisture is too low and soil temperature is super optimal. Mechanical clearing that causes soil compaction and removes the entire biomass from the soil surface results in a drastic reduction in earthworm activity. Dealing with Desertation problems i. Tree Planting: Forest tree planting helps to regenerate the soil through the decomposition of leaf litter, which decomposes and encourages undergrowths to cover the soil. The leaf litter also gives the soil more water and thus reduces run – off. Another way of tree planting is the establishment of orchards of fast maturing fruit trees or multi – purpose trees so that while they help to rejuvenate the soil, they also produce nutritious food items and other useful by – products. ii. Inter – cropping of arable crops with tree crops. This ensures that vegetative cover almost always protects the soil. This is most helpful in situations whereby erosion is likely to be a major problem. iii. Use of appropriate and modern farming practices and techniques like ploughing, ridging, planting on the contour, use of leguminous cover crops, grass and strip cropping, terrace farming and mulching. iv. Partially clear trees such that useful ones are retained when preparing farming sites. Such trees are to play some beneficial roles like nutrient recycling, providing protective cover, stabilizing the soil and provision of economic products. v. Mechanical and chemical bush clearing (slashing, pruning, burning) should be done with care to avoid soil degradation and the consequent erosion menace. vi. Lodging and forestry operations should be handled with care as they easily lead to erosion problems. vii. Avoid overgrazing by restricting farm animal while planted pastures must not be overstocked. Rotational grazing and good management of range eliminate the potential or erosion hazard.

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CHAPTER 5 SOIL BIOLOGY AND FERTILITY This is the study of the relationship between living organisms and soil characteristics, the contribution of soil – microorganisms to soil fertility and the influence of living organisms on soil development. The soil ecosystem consists of both the plant and animal materials that are either living or dead. These are the macroflora, the macrofauna and several other microscopic organisms. They are inter related by the transfer of energy or food web since each organism obtains its food directly or indirectly from neighbouring organism. The autotrophic ones (i.e. primary producers) derive energy from the sun by photosynthesis. The heterotrophic ones or consumers are ether herbivores or carnivores. Others are decomposers e.g. fungi and bacteria that can release enzymes to decompose the organic matter outside their own bodies absorbing them. During such decompositions, organic matter can be transformed into forms that can be re-used by the plants e.g. Nitrogen cycle. Soil Micro Organisms Microbiologists view the soil environment in the following ways: (1) It consists of different and numerous microscopic organisms (2) It is one of the most dynamic sites of biological interaction in nature. (3) It is the region in which occur many of the biochemical reactions concerned with the destruction or organic matter, in the weathering of rocks and in the nutrition of crops. Soil Biota

Fau na

F lo ra

M acro

M icro M icro N em ato d es

M acro

P ro to zo a

C arnivo res

M o les

A nts

B eetles

S p id er A u to tro p hic

H etero tro p hic

Fungi A lg ae

M am m als A nts

S p ring tails E arthw o rm s

Figure 14: Soil Biota

B acteria

A ctino m yctes

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

Soils contain five major groups of micro – organisms – Bacteria, antincmycetes, fungi, algae and protozoa. The soil ecosystem includes three microbial groups and the inorganic and organic constituents. The Bacteria These are usually the most abundant group of microorganism though the individual ones are very small. It accounts for almost half of total microbiological cell mass. Bacteria stand out in significance because of the rapid growth and vigorous decompositions of natural substrates. There are two types – the indigenous and the invaders. The indigenous populations may have resistant stages and endure for long periods without being active metabolically but later participate in the biological functions. The invaders do not participate at all in the biochemical activities. They are usually introduced into soils by rainfall, diseased tissues, animal manure etc. Bacteria can also be classified into: a. Aerobes, which must have access to oxygen for survival b. Anaerobes, which grow only in the absence of oxygen c. Facultative anaerobes that can develop or grow in the absence or presence of oxygen. On the basis of the shape and size of the microorganisms, the following groups are recognized: (a) Bacilli or rod shaped bacteria, which are the most numerous in soils. (b) Cocci or spherical shaped bacteria (c) Spirilla or spirals (d) Vibrio or Coma shaped bacteria The last two are not common in soils. Economic Significance of Bacteria Bacteria as a group participate vigorously in all the organic transactions that are vital if a soil is to support higher plants. They monopolize the three basic enzymatic transformations – Nitrification, sulphur oxidation and Nitrogen fixation. If theses were to fail, life for higher plants and for animals would be endangered. Their significance can be summarized as follows: 1. The fermentation process brought about by them in the recycling of nutrients. Elements in dead organic matter are returned to the air and soil for the use of plants again and again 2. They have the ability to fix atmospheric nitrogen for plants use 3. They cause many diseases such as tuberculosis, tetanus etc. 4. Some can be used to cure diseases e.g. curing of tobacco. 5. They decay food materials 6. They are involved in antitoxic development for resistance against infection Actinomycetes These represent a transitional group between the simple bacteria and more complex fungi. They produce slender branched filaments that develop into a mycelium. The individual hypahe (i.e. the filament) appears similar to that of the fungi, but not as broad as the fungi. They resemble the fungi in the following ways: 1. The mycelium has the extensive branching found in fungi 2. They have serial mycelium and conidia. 3. Their growth in liquid culture really results in the turbidity associated with bacteria On the other hand, the morphology and size of hyphae, conidia and the individual fragments are similar to that of bacteria.

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Activities and functions of Actinomycetes: Actinomycetes are heterotrophic feeders and their presence is conditioned by the presence of organic substances. They are believed to have a lesser biochemical importance than the bacteria and fungi. They however participate in the following ways: 1. Decomposition of very resistant components of plants and animals tissues. They perform poorly in competition with bacteria and fungi of only simple carbohydrates are present. 2. Formation of humus through the conversion of plants’ remains and leaf litter. 3. Transformation at high temperatures when other micro organism will be inactive 4. Causing of certain soil borne diseases of plants e.g. potato scab and sweet potato pox. 5. Causing of infection of human and farm animals 6. Taking part in micro antagonism and in regulating composition of the soil community. They can excrete antibiotics and produce enzymes that are responsible for lysis of fungi and bacteria. Fungi In most well aerated cultivated soil, fungi account for a large part of the total microbial population due to their large diameter and extensive network of their filaments. The organic matter status, hydrogen ion concentration, organic and inorganic fertilizer, the moisture regime, aeration, temperature, and position in the profile, season of the year and composition of the vegetation are factors controlling fungi population in soils. Organic matter serves as food and thus provides energy for fungi, acidic soils favour fungi multiplication, and fertilizers can either reduce or increase the pH. Too much water affects fungi because air is excluded while too high soil temperature may have adverse effect on fungi population. The position in the soil profile is important because as one moves down the profile, organic matter decreases and fungi population also decreases. Finally, members of the grass family (e.g. rice, maize) favours fungi growth. Functions and Activities 1. The fungus has not chlorophyll. Hence, it depends on sugars, organic acids, disaccharides, starch and other performed organic molecules. Some fungi also parasitize on higher plants. They thus participate in the microbiological balance in the soil. 2. Degradation of complex molecules 3. Utilization of proteinaceous substances that results in the formation of ammonium and other simple nitrogen compounds 4. Formation of hums 5. Pathogenicity – they can cause diseases. Some can cause diseases of humans especially where people are scantily dressed. Algae These are not as numerous as bacteria, actinomycetes or fungi. As such, they are not usually sufficiently appreciated as a group. Since they are photosynthetic, they require access to sunlight and moisture. They can be frequently notes in virgin or uncultivated land with the naked eye. Their presence can be demonstrated readily by the addition of small amounts of soil containing NO3, KPO4, and MgSO4, Ca and Fe salts and traces of other inorganic nutrients. The resultant growth is visible macroscopically as a green colour. Morphologically, algae may be unicellular or occur in short filaments but the soil strains are smaller and structurally less complex than their aquatic counterpart.

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Environmental Influences Soil organic matter content has no control on algal population since they have photosynthetic ability. Rather, need for sunlight and CO2 is imposed. Carbon dioxide and bicarbonates are usually produced in excess of the autotrophic demand. Light accessibility is hence a dominant factor governing the distribution of photoautotrophic microorganisms. Each strain also has an optimum pH and a range outside of which they fail to multiply. The Blue green algae develop in a neutral or alkaline soil (pH 7 – 10). They are often absent at pH values below 5 and uncommon below 6. Moisture is also a common limitation to growth and herbicides usually have devastating impact on algal species. They are also susceptible to attack by the other macro and microscopic organisms such as protozoa, nematodes, mites and earthworms. Significance of Algae: They do not contribute much to the many biochemical transformations necessary for soil fertility except in flooded soils planted to rice. This is due to much composition from bacteria, fungi, etc. Yet, the photosynthetic microflora is capable of exerting the following effects. 1. Generation of organic matter from inorganic substances i.e. they can convert CO2 into carbonaceous materials and thus increase the total quantity or organic carbon in soils. 2. With the colonization of barren surfaces, algae are able to corrode and weather rocks. A thick layer of algal cells is found covering the surfaces of rocks and the organic matter in these cells, on their death, supports the growth of bacteria and fungi that appear as secondary colonizers. 3. They can contribute to soil structure and erosion control. The surface blooms reduce erosion loses probably by binding together soil particles 4. Their photosynthetic processes liberate molecular oxygen, which benefits the growth of ricesubmerged roots. 5. They can utilize atmospheric molecular nitrogen as a nitrogen source for growth and results in enrichment of the soil with combined forms of Nitrogen. In Asia, rice has been produced over years with no addition of Nitrogen. 6. Some can parasitize cultivated plants like tea, citrus and cocoa trees. Protozoa These are the most abundant invertebrates found in s oils and are the simplest forms of animal life. They are primitive, unicellular organisms ranging in size from several micrometers up to 1 or more centimetres. The terrestrial species are microscopic and are smaller than their aquatic counterparts. Nutrition is varied. At the same extreme of nutritional independence are the photosynthetic protozoa, which are alga-like flagellates. Majority are, however, dependent on preformed organic matter either as saprophytic feeders (feeding on dead organic materials) or phagotrophic feeders that feed directly on microbial cells. Significance of Protozoa: Very little is known about their functions in soils. They have, however, been postulated to take part in the following functions in soils i. Regulation of the size of the bacteria community ii. Enabling different competing bacteria to co-exist in a soil which one – bacteria species might otherwise have eliminated other. iii. Participation in the decomposition of plants remains iv. Enhancement of certain bacterial transformation such as the utilization of nitrogen or the degradation of phosphorus containing organic materials v. Pathogenicity: for example, Entamoeaba histolytica is the causative agent of amoebic dysentery. 70

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The Soil Inhabiting Mammals These cause loosening of sub and surface soils, resulting in soil aeration and water movement improvement. They eat plant materials and their excreta later raise the soil nutrient status. The large and often long, winding tunnels dug by the rats also help in the draining of excess soil water. Slugs and Snails These are surface feeders usually acting in damp environments. They feed on drying vegetation, fallen leaves, and old grasses and can attack growing plants when food is scarce. Smaller organisms can work upon their excreta to release nutrients to soils. Earthworms These live in well aerated drained soils having a lot of vegetable matter. They are very sensitive to acidity, prefer clayey soils and rarely found in sandy soils. Their activities can be summarized as follows: (1) Improvement of aeration in soils by borrowing into the soil to create extensive channel, which allow easy water movement. (2) Their ability in mixing soil with vegetable matter also help in improving soil aeration (3) They convert plant materials into humus in order to improve the surface soil nutrient status. (4) They help to produce stable soil aggregates through the breakdown of their cats. This will in turn improve the soil water holding capacity. Nematodes or Eelworms These are non-segmented worms having cylindrical shape. These are 3 main groups: 1. Those that depend on decaying organic matter and soil organism. 2. Predators of other soil organisms – including other nematodes 3. Those that can be parasites on plants. Their known beneficial effect on soil is very little. They, however, increase soil nutrient status by their addition of excreta, which can be converted into humus by other soil organisms. Insects The Millipedes, centipedes and the termites are the most important insects in soils. Millipedes and centipedes feed on decaying plant remains. Their excreta is easily converted into humus and incorporated into the soil to increase its nutrient value. Termites, however, are the most important insects affecting soils and plant life. They also have the ability to make mounds of fine textured materials from the subsoil or the surface soils. When broken down, the mounds improve the texture of some otherwise sandy surface soils.

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SOIL FERTILITY Factors contributing to soil fertility The mere presence of plant needs for healthy growth in the form available for plants’ use make a soil fertile. The soil is, however, only productive when the various needs are in appropriate amounts or ratio. The factors that help in maintaining the level of these needs in soils are: 1. Humus, the decomposing plant and animals bodies that certain soil organisms can convert into Nitrates 2. Micro organism: they are helpful in the decomposition of plant and animal bodies into humus and other soil biochemical transformations 3. Air: oxygen is needed for respiration of growing roots of flowering plants and soil micro organisms 4. Water: All living things need water. Usually, plants roots absorb water from the soil. A lot that hold much water by capillary is therefore potentially fertile. 5. Soil texture: soils with good texture (e.g. loamy soil) usually have adequate amounts of water and air that are essential for plants growth. 6. Amount of mineral elements present in the soil: Plants need C, O, K, P, N, S, Ca Fe, Mg etc. for healthy growth. In most cases, these elements are absorbed from soils by the roots of plants. Soil Nutrient Losses Improper management of soil could result in nutrients’ loses which could occur as a result of erosion, fire effect, repeated cultivation of a piece of land, and removal of plant cover. 1. Fire action or fire effect: Late burning of bush especially in the savannah areas can cause death of many of the soil micro organisms and thus reduce soil fertility 2. Erosion: this can be wind or water that may carry surface soil to other areas. Since the surface soil is the richest layer, the fertility is reduced when erosion occurs. 3. Repeated cultivation of a piece of land. Crops usually remove mineral elements from the soil. The continuous use if a piece of land can therefore lead to depletion of the soil nutrients. 4. Removal of plant cover: exposing soil surface to direct rays of the sun can cause the death of certain soil micro organisms, rapid decomposition of humus an leaching or/ and erosion of the nutrients released from the humus. 5. Losses due to water percolation: In humid regions gravitational water migrates through the soil profile in most years and enters the water table. When precipitation percolates through the soil, significant amounts of plant nutrients are removed from the soil and their removal results eventually in the development of acid soils.

Renewal of soil Fertility 1. Planting of crops that can encourage the multiplication of micro organisms in soil. These are usually leguminous crops e.g. Groundnut, cowpea etc. 2. Bush fallowing in the shifting cultivation system of farming 3. Fertilizer application 4. Prevention of soil erosion e.g. use of cover crops, terracing etc. 5. Early burning 6. Crop Rotation.

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Soil Testing in Agriculture To ensure a more intensive crop production to feed the ever-increasing human world population, yields of genetically improved crop varieties should be further enhanced by optimum plant nutrition. In broad terms, soil testing is any chemical, physical or biological measurements made on a soil with a view to evaluating the fertility status of the soil. This can be done using several techniques viz: a. Nutrient deficiency symptoms b. Plant analysis c. Field trip d. Greenhouse trial e. Microbiological assay f. Rapid tissue or Sap analysis g. Chemical soil test. In a more restricted sense, soil testing is the rapid chemical analysis for assessing the fertility status of the soil. This is because the chemical soil tests are the most common means of assessing soil fertility. They are rapid reasonably relatively inexpensive and adapted to routine test procedures. Sixteen elements (C, H, O, N, P, K, S, Ca, Mg, Cu, Zn, B, Mo, Cl, Mn, and Fe) are known to be essential to be essential for plan growth. Three of them (N, P, K) are widely deficient in soils. Soil pH is also a common limitation to plant growths. Soil testing predominantly involves N, P, K and pH with secondary and micronutrient analysis varying widely on regional basis. The essential steps in the establishment of a sound soil-testing program are: a. Good sampling b. Accurate laboratory analysis c. Establishment of nutrient fixation curve d. Green house testing based on conclusion drawn from laboratory result e. Establishment of fertilizer experiment in farmers plots in each area of production to verify the data obtained in preceding steps. f. Correlation of chemical analysis with the percentage yield obtained in field experiments g. Establishment of optimum fertilizer dosage based on fertilizer response curve under field condition Soil Testing can, thus help to: 1. Recognize nutrient deficiency 2. Determine the amount of particular nutrient present in soil. This is intended to be related to the quantity of the nutrient available to the plant 3. Know the forms of a particular nutrient in soils 4. Indicate nutrient availability 5. Indicate the quantity of Fertilizer element to be applied to obtain optimum yields 6. Design application method. The disadvantages of soil testing include the facts that it may be expensive time consuming and labour intensive. Soil Nitrogen In its elemental form, nitrogen is a colourless, odourless and very inert gas. It is inexhaustibly supplied in the air. It is in the Free State and does not easily combine with other elements. Regardless of these facts, certain groups of soil organisms have the ability to utilize it in the building of their cells. The Nitrogen of the air is thereby changed to a fixed form in which it can be of subsequent use to higher plants. The process involved is termed atmospheric Nitrogen fixation and it is accomplished mainly by two groups of bacteria – symbiotic and non – symbiotic.

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Non-symbiotic Nitrogen fixation – there are certain groups of bacteria living in the soil independently or higher plants and having the ability to use atmospheric nitrogen in the synthesis of their body tissues. Since these bacteria do not grow in association (mutual relationship) with higher plants they are termed non – symbiotic. The two organisms that have been mostly studied in this group belong to the genus Azotobacter and the genus clostridium. Azotobacters are widely distributed in the soil of pH 6.0 or above and would not be very active at pH below 6.0 while Clostridium are much more acid tolerant than most members of the aerobic group and perhaps for that reason are more widespread. Symbiotic Nitrogen Fixation The most important bacteria, from the agricultural point of view, capable of utilizing the free N2 of the air are those that cause the formation of nodules on the roots of legumes. These organisms, when growing in the nodules of legume plants, derive their food and minerals fro the legumes, and they in turn supply the legumes with some of its nitrogen. This growing together for a mutual benefit is called symbiosis and hence the organisms are designated symbiotic Nitrogen – fixing bacteria. Legume plants form a symbiotic relationship with heterotrophic bacteria of the genus Rhizobium. The legume host plant is benefited by the nitrogen fixed in the nodules and non – Nitrogen – fixing associated plants ay also benefit from the fixed nitrogen when nodules disintegrate and decompose, since most nodules don not live longer than one year. Nitrogen Deficiency Deficiency of nitrogen is evidenced by a gradual loss of chlorophyll, which results in a light green to yellow color, and by a slow and stunted growth. An abundance of Nitrogen promoted rapid growth with a greater development of dark green leaves and the encouragement of above ground vegetative growth. This growth can however take place only in the presence of adequate amounts of available phosphorus, potassium and other essential elements. The organic matter is the major nitrogen reservoir. Generally speaking, the soil organic matter contains about five percent Nitrogen. Soil Phosphorus Phosphorus is an essential macro nutrient occurring in organic and inorganic forms in the soil. It is usually found in soil in combination with other ions such as Ca, Fe, Al and Fl. The total content is usually in the range of 0.02 to 0.15%. From the point of view of plant nutrition, soil phosphate can be categorized into the following: (a) Phosphate in soil solution (b) Phosphate in the soil labile pool (c) Phosphate in the non – labile fraction The first fraction if s the phosphate dissolved in the soil solution, the second fraction is the solid fraction held on the surface so that is in rapid equilibrium with soil solution phosphate. The third fraction is the insoluble portion, which can be released only when very slowly into the labile pool. Plants utilize a large amount of phosphorus in comparison to the total amount present in the soil. The presence of phosphorus in the soil is determined by the amount of soil solution. Decreasing amount of total phosphorus exists in the soil solution as the soil dries out because the volume of water in which phosphorus compounds can dissolve becomes smaller. Phosphorus tends to be unavailable in acid soils. It is present in seeds in larger amounts than in any other part of pants, although it is found extensively in the young growing parts. It is a constituent f every living cell. It is a constituent of phospholipids, nucleoproteins and Phytin, the latter being storage form of phosphorus in seeds. 74

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Phosphorus appears to hasten maturity more than most nutrients, as excess stimulates early maturation. Stunted plants characterize phosphorus deficiency as it about equally affects roots and top growth. If phosphorus is deficient, cell division in plants is retarded and growth is stunted. A dark green colour associated with a purplish coloration in the seedling stage of growth is also a symptom of phosphorus deficiency. Later, plants become yellow. The yellowing is associated with early maturity but is definitely a symptom of phosphorus starvation. Occasionally a pale or yellowish green colour develops when the lack of phosphorus inhibits the utilization of Nitrogen by the plant. Bronze or purple leaves are sometimes observed at the top of new shoots of phosphorus – starved apple trees. Soil analysis attempts to estimate the total amount of nutrients available to plant and to what extent these nutrients in the soil are available to the plant. This calls for the determination of cation exchange (CEC), soil pH, organic matter, micronutrients and macronutrients. In soil, high maximum availability of nutrients occurs at pH of 6 – 7. Phosphorus is not usually available in very acid soils while N, Potassium, Mg and Ca are relatively available at pH between 5.5 and 6.0. The CEC is the exchange of one cation for another on the surface of colloids. It is the source of metallic nutrients to the plant. Soil pH measures the soil reaction – the active acidity of the soil. It arises from clay minerals, soil humus, CO2 from atmosphere and moisture. It reflects the extent to which the CEC of the soil colloids is saturated by exchangeable cations. In alkaline or neutral soils, most of the CEC positions are occupied by cations. The knowledge of soil pH is necessary to determine the nutrient availability and micro organic activities in the soil. Soil Analytical Data Interpretation and Calibration The various analytical results of soil are not on their own sufficient as basis upon which recommendation of how soil should be used and for what purpose. It is most important for practical value that the analysis be interpreted and correlated with the crop responses on the field. Soil analysis is interpreted by correlating the laboratory experiment of soil nutrient status with the actual crop performance. This can be achieved by subjecting the samples whose nutrient levels have been determined to different rates of fertilizer application. Crop response to the applied nutrient is observed. From observation, opti9mum fertilizer level for economic yield of crop would be recommended. Soil is calibrated against the results obtained from soil testing. There are various methods of calibration but the most used is: very high; medium; low and very low based on the level of nutrients. A soil which has high test value will require little fertilization while the one which has very low test value would require high level of nutrient application for economic growth. Farmers need to know the amounts of effectively available nutrients in their soils before crops are grown so as to adapt fertilizer measures accordingly. Soil fertility evaluation is the process by which nutritional problems are diagnosed and fertilizer recommendations made. Soil testing is the major tool in this direction. It is a pre-requisite for determining the soil nutrient factors or fertilizer needs of various crops. The need for reliable soil test methods that will give precise information about the fertilizer quantity required for an optimum nutrient level for particular crops is however difficult to be met. For highly productive faring, in which the use of improved varieties herbicides pesticides and growth regulators have also contributed to improve efficiency of fertilizers, multipurpose extractants are needed. Several soiltesting methods are unsatisfactory because they are limited to the determination of a few nutrient elements. For example, the exchangeable - k level of a soil cannot be regarded as a good index of total availability of the cation in soils as it occurs in various forms (the main reserve being the non – exchangeable K) and several factors affect it’s transformation or release from the non – exchangeable to available forms. There are various “bottlenecks” of the conventional soil testing procedures to justify the need for a more detailed and precise method to ascertain the reliability of fertilizer recommendations. The first problem is the inability to differentiate between the amounts of effectively and potentially available nutrients. Effectively available nutrients are those present in the soil solution in a dissolved ionic form. On the other 75

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

hand, potentially available nutrients might not go into the soil solution in the course of the vegetation period by desorption or solution processes. When certain chemical extractants are used for extraction, both the effectively and the potentially available fractions are obtained together. It is possible that up that up to 80% of the amounts extracted is only potentially available. Changes of the effectively available nutrients can be due to nutrient removal by the crop, leaching, weathering etc. Furthermore, much time is usually wasted when the conventional soil testing procedures are employed since each of the conventional chemical extractants can only be used in extracting 1 or 2 nutrients elements. Other soil properties (clay content, lime requirement, cation exchange capacity etc.) also need to be separately determined before any meaningful recommendations can be made. For example, information obtained from conventional soil tests for P and K needs to be supplemented with results of P – sorption and K – fixation studies. An attempt to comprehensively characterize the nutrient status of a large number of soil samples will be cumbersome, costly and frustrating as H2O – soluble, exchangeable, HNO3 – extractants, fixed or difficulty exchangeable or mobile reserve, total and/or residual (structural or non – extractable) nutrients fractions as well as the release characteristics, retention and fixation and the Quantity - intensity ratios have to be determined. In practice, farmers that are supposed to utilize the fertilizer recommendations are delayed and sometimes frustrated. Thus, one of the major constraints in soil testing programs is the development of multipurpose extractants. In line with this, much effort has been directed towards finding acceptable simultaneous (or multipurpose) chemical extractants. The adoption of the Electro – ultrafiltration technique (Figure 15) is a relatively recent development in soil testing. It is based on the use of an electric field to separate nutrient fractions in soil suspensions. The introduction of varying voltage and temperature during the extraction process gives the method a considerable advantage in plant availability in one extraction run and to determine at the same time other soil properties such as K – selectivity of clay minerals, content of CaCO3, etc. the resulting data can help to characterize a soil comprehensively and ensure optimal plant nutritional management. The EUF – technique is essentially an alternative extraction procedure to the conventional chemical extraction methods and involves a combination of electro dialysis and ultrafiltation. It is more rapid than the conventional methods since in a single extraction time of 35 minutes. Anions and cations in the soil suspension to which an electrical potential is applied can be separated. By a step wise increase of voltage from 50V to 400V, the extraction power can be varied, while at certain time intervals (5 minutes each for 7 – fractional parts or at 10, 30 and 35 minutes for 3 – fractional parts), specific fractions are collected. The nutrient concentrations can then be determined by the use of the conventional equipment such as the emission spectrophotometer for K and Ca. The most spectacular advantage of the EUF – technique over the conventional methods is that separate determinations of other soil properties (clay content, sorption capacity, lime requirement, heavy metal toxicity, etc) are not required and from a singe extraction, the desorption and solubility rates of 10 – 20 nutrients that are important to plant growth can be determined. Desorption rates give an indication of effective and potential availability of the nutrients. Seven thousand soil samples per person (or 150 – 200 samples per day), can be analysed using 10 EUF cells combined with an auto – analyser. About fifteen to twenty thousand soil samples every year analysed at Tulln sugar factor in Austria where the EUF procedure had been introduced for routine analysis. Similarly, large samples are analysed each year in Hungary and Yugoslavia with the aim of improving fertilizer use in grape and sugar beet productions, respectively. Thus, when fully automated, the EUF – procedure is extremely time saving, precise and has high reproducibility.

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Figure 15: Schematic diagram of the electro-ultra filtration (EUF) equipment and representation of its extracted nutrient values at varied voltages (After Nemeth, 1979) FERTILIZERS Introduction Plants absorb mineral elements from the soil. If the use of land is continuous, there will come a time when the soil will be deficient in mineral salts and the soil, therefore, becomes barren. Substances, which are added to the soil to correct the deficiency, are known as fertilizers i.e. fertilizers supply those elements required in the nutrition of plants to the soil. There are 2 major types of fertilizer: (a) Natural (or organic) fertilizers/ manures (b) Artificial (or inorganic or commercial) fertilizers/manures. 77

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

The natural / organic fertilizers are of the following types: (a) Green manure This is the laying down and/or ploughing over of plant materials in soils. It involves whole season growing of the manure crop during which little or no income is obtained. The turning under of plant materials is difficult with crude implements (hoes and cutlasses). Practically, all species of plants can be used, though legumes (Vigna, Vicia, stylosanthes, Pueraria, Cajanus, Calopogonium, Centrosema, Glycine, etc.) are mostly used. The farmer’s choice to either turn them under or leave them on soil surface depends on the following considerations: i. Maturity of the green manure crop ii. Time of the year (i.e. season). iii. Time lapse between turnings over and growth of succeeding crop iv. Nature of the succeeding crop v. Soil type amount of soil water (b) Farm yard manure This is usually produced from many materials e.g. various animal droppings (usually cow dung), discarded straw bedding mixed with animal dug or urine. They are usually recognized by the condition and state of decomposition of the constituents (e.g. presence of yam, banana or plantain peels, bedding straw, food remnants), putrefying smell of cow dung, pig dung or poultry dung. (c) Compost manure This is a collection and compaction of plant and animal remains and household refuse in a pit or heap. It may be composed of freshly cutgrass or vegetables, crop wastes, peelings and ashes that may be bulked, compressed and left to rot to form a heap of humus. The state of decay has advanced further than in FYM and the constituents are in less recognizable state. Generally, nutrient content of organic manures is determined by: (i) Type of livestock, their feeding and their bedding (ii) Plant species and degree of rotting

Advantages of Organic Manure 1. Provision of energy to microorganisms in soil. They particularly promote bacteria activity 2. Soil structure and aeration improvement resulting from the cementation of sol particles together 3. Increase of water holding capacity 4. Micro organisms use organic manures to produce antibiotics and growth regulators 5. Regulation of soil temperature 6. The N – content becomes slowly available to plants. Hence, leaching and erosion losses of nitrates are reduced. 7. Provision of all essential nutrients in fairly balanced proportions Disadvantages of Organic Manure 1. Nutrient composition is unknown 2. When produced from plant materials with high C/N ratios, N – deficiency results. He release of nitrogen in them dependent on environment factors (climatic and edaphic factors)

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Artificial fertilizer This is the type whose chemical composition is known. It may be available in form of potassium, phosphate and nitrate salts. When the elements absent in the soil are known then one can look for the fertilizers to be used. One advantage of artificial fertilizer is that it cannot remain in the soil for a long time, because it is readily soluble and so easily leached away. They are also not easily available to the farmer. Fertilizers are available in the form of single element fertilizers, incomplete fertilizers and complete fertilizers. The single element fertilizers contain only one fertilizer element e.g. ammonium sulphate (N), urea (N), super phosphate (P2O5), muriate of Potash (K2O). Incomplete fertilizers contain 2 fertilizers elements e.g. Ammonium phosphate (N+P2O5). ‘Compound fertilizers” refers to fertilizer materials containing two or more plant nutrient elements. Of these, types containing all the three elements are designated “complete fertilizers”. Ammonium Phosphate which has an analysis of 11 – 45 – 0 is also called a compound 2 – element fertilizer, while an example of a complete fertilizer is compound 14 – 14 – 14. The composition of a fertilizer element in a fertilizer formulation is expressed in percent. If ammonium sulphate has an analysis of 21% N, it means that in every 100kg of ammonium sulphate, there is 21kg of available N. The analysis of most fertilizer materials available commercially is expressed by a numbering system showing the percentage of composition of each in the order N – P – K. these numbers are printed on the label of each fertilizer container. Each fertilizer materials have its residual effect in soils e.g. most nitrogenous fertilizers when applied to the soil have acid forming residual effect but the degree varies. Incomplete or 2 element fertilizers like mono-ammonium phosphate and di-ammonium residual effects: These conditions generally arise in the presence of soil water (from rainfall or irrigation water sources). The chemical compositions of the common fertilizers are: 1. Ammonium sulphate (NH4)2SO4 20 – 21% N, 24% S. 24% S. 2. Urea (OCNH2)2 3. Ammonium Chloride (NH4Cl) 25% 4. Ammonium Nitrate (NH4NO3) 35%N 5. Anhydrous Ammonia (NH3) 82%N 21% Ca. 1. 5 % N 6. Calcium Nitrate Ca(NO3) 2 7. Calcium cyanide CaCN2 20 – 22% N. 37%Ca 8. Calcium Ammonium Nitrate - 11.6%N, 5.4 % N, 8.7% Ca. CaNH4NO3 NH4NO3 9. Super phosphate single or ordinary - 20%P2O5, 18 – 21% Ca or solophos - 12%S . 10. Super phosphate Double or Triple – 45 – 50% P2O5, 12 – 14 % Ca Or Triphos Ca (H2PO4)2 H2O - 12% S 11. Potassium Chloride (muriate of potash) - KCl

- 60 % K2), 0.3% Ca,

12. Potassium sulphate K2SO4

- 53% K2O, 18%S

13. Mono Ammonium Phosphate

- 11% N, 45% P2O5, 1.4% Ca, 2.5% S 79

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NH4H2PO4 14. Diammonium Phosphate (NH4) 2 HPO4 15. 12 – 24 – 12 16. 14 – 14 – 14

-

21%N 53% P2O5

- 12%N, 24%P2O5, 12% K2O - 14%N, 14%P2O5, 14% K2O

P2O5 and K2O (P and K oxide forms) can be converted to P and K (elemental forms) by multiplying the oxide forms with 0.4364 and 0.8301, respectively. The elemental forms can similarly be converted to the oxide forms by multiplying with 2.2915 and 1.2047, respectively. Like the numbering system for expressing an analysis of fertilizer, fertilizer recommendations are given in kilograms of nutrients per hectare in the order N- P – K. If only nitrogen is needed, the rate is given in kilograms of nitrogen per hectare. When the recommended rate of fertilizer is given, one must be able to weigh the exact amount of the appropriate fertilizer material to be applied to each individual plot. The term “carrier” refers to the material or compound in which a given plant nutrient is found or supplied. “Analysis” or “Grade, is the minimum guarantee of the percentage of total nitrogen, available phosphoric acid and H2O soluble potassium. “Ratio” is the grade reduced to its simplest term. “Filler” is any inert material mixed with a fertilizer to make proper weight, to control moisture and keep fertilizers from caking and balling in the bag e.g. rice husks, ground peanut husk, vermiculite, etc. Methods of Fertilizer Application Fertilizers are applied to the soil sometimes on the plant itself in various ways, depending on – (a) The type of fertilizer used (b) The crop involved (c) Soil characteristics (Edaphic factors) (d) Climatic situation The following methods are available. (1) Broadcasting – this is the uniform distribution (or application) of the fertilizer material over the whole area before the crop is planted. Sparingly soluble fertilizers are subsequently ploughed or harrowed – in – so that it occurs within the plants’ root zone. The method is mostly employed for water – insoluble phosphates, for dense crops not planted in rows, for crops that occupy the whole volume of the soil (e.g. manure fruit trees) and when relatively large dressings (applications) or fertilizers are used on fairly, fertile soils. (2) Row Placement or side band placement: This is the placing of the fertilizer material a little below and a few centimetres to one or both sides of the seed or plant. It should be near enough to be used by the plant but not close enough to injure it. There are specially designed equipment for this purpose, though our local farmers usually imitate this manually. The method is recommended for row crops with relatively large spaces between the rows, on soils which there is a danger of phosphate and/or potash fixation and when relatively small quantities of fertilizer are used on soils. (3) This is the broadcasting of fertilizer on to a field containing a growing crop (e.g. grain crops, cotton and sugar cane). It is very good for the relatively soluble fertilizers (e.g. nitrates0 that can be easily washed into the soil and quickly made available to the plant. Such applications are often split e.g. fractions of the fertilizer dose can be applied during the growing season to give extra shots of the nutrients when needed. (4) Side Dressing – this is often the most efficient way of applying minor elements that are needed only in small quantities and may become fixed (unavailable) if applied to the soil surface and used only 80

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for very soluble fertilizers that can be easily washed into the plants rooting zone. The application is more localized than Top dressing. (5) Foliar application – this is often the most efficient way of applying minor elements that are needed only in small quantities and may become fixed (unavailable) if applied to the soil. Urea is sometimes applied in foliar sprays, usually in combination with insecticides or fungicides (except CaS and other Sulphur containing fungicides) in order to save concentrations of Urea as well as in the apparent rate of utilization and in the growth and fruiting responses obtained. Time of Fertilizer Application Timing of fertilizer application is usually connected with the method chosen and the fertilizer type being used. Fertilizers can be applied before planting (pre – planting), at planting and later after planting (post planting). Row, sideband placement broadcasting and application of P – containing (and other slow release) fertilizers are usually done before seeds or seedlings are planted or at the same time. Very soluble, quick- acting fertilizers (e.g. nitrates) can be applied during the growing season or dissolved in irrigation water. Trace elements can be applied as leaf sprays. In pineapples, solid elements can be placed in leaf axils. Split application – this is the application of the same fertilizer more than once to a crop. It is useful in case of very soluble fertilizers in high rainfall areas. Sample Calculations on Fertilizer Rates 1. How much of sulphate of ammonia (20% N) is needed for a top dressing of a trial area/ plot of 500 M2 if the fertilizer recommendation is 30 kg N/ha. 1ha. of land (10,000 m2) requires 30 kg N as implied in the fertilizer recommendation. Therefore, 1m2 requires 30 / 10,000 kg N For the trial plot, 500 m2 requires (

30

x

10,000

500) kg N 1

But 20 kg N is supplied in 100kg of sulphate of ammonia Thus, 1kg n is supplied in 100 kg of Sulphate 20

Thus, 30 x 500 kg N will be supplied in 100 x 10,000

20

30 x 500kg of sulphate of Ammonia 10,000

= 7.5 kg of Sulphate of Ammonia 2. Suppose you wish to prepare a 1,000kg. 5 – 20 using the available 33 – 0-0, 0 – 45 – 0 and 0 – 0 – 60 fertilizer types. How much of a filler will be needed? If “X” represents the amount of each single element fertilizer that will be mixed in the preparation, 81

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X = kg. of mix desired multiplied by % element desired % of the element in the carrier Therefore, for Nitrogen XN = 1000 x 5 = 151.16kg 33 For phosphorus, XP =

1000 x 20 = 444.44kg. 45

For Potassium, XK = 1000 x 20 = 333.33kg. 60 Amount of filler needed = 1000 – (151.16 + 444.44 + 333. 33) kg. = 71.07kg. Major Effects of Fertilizers on Plant Growth 1. In seasons when it may be necessary to delay the planting of certain crops due to unfavourable weather conditions, the application of fertilizers may speed up the growth processes of the plant. In essence, it offsets the unfavourable effects of the season. 2. Fertilizers stimulate early crop growth in general. 3. If dry weather prevails about mid – season, the fertilizer application may result in yield reduction. This is because the fertilized crop through increased growth and greater leaf development more rapidly exhausts the soil moisture. 4. Fertilized crops may be more drought resistant due to more deeply penetrating root systems 5. Fertilizers stimulate early crop growth, but as the season advances the difference may disappear and at harvest no increase in yield is found; hence, the early growth of a crop should not be taken as a measure of the effect of a fertilizer on yield. 6. Fertilizers may have little effect on the rate of growth on certain crops, but at harvest a decisive increase I yield may be noted.

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CHAPTER 6 SOIL - WATER – PLANT RELATIONS Introduction Water is the most common substance on earth that is necessary for all life. Its importance to crop plants can be summarized as follows: - It helps in dissolving mineral salts, which are necessary for plants growth. Without it serving as a solvent, plants roots can not absorb the minerals - It is an essential raw material in photosynthesis - It serves as a solvent to carbon dioxide - Hydrolysis/ breakdown of many food nutrients like starch, protein and fats require water. - It accelerates enzymes activities - It is concerned in the protoplasm of plant cells - It is involved in the process of transpiration which helps to cool plants The soil plays an important role in determining the amount of precipitation that runs off the land and the amount of precipitation that enters the soil for storage and future use. About 70% of precipitation is usually lost by evapo-transpiration to the atmosphere as vapour with the soil serving in water retention and storage. The remaining 30% is used in homes, industry and irrigated agriculture. Water is not easily destroyed. The earth presently has much water now as it did thousands of years ago. However it is unevenly distributed by rainfall, changes form, moves from place to place and can be polluted. Water is constantly evaporated from the seas, lakes, rivers and the soil (Figure 16). Its vapour being lighter than air (0.62:1.00) raises cools and condenses. Coalescence products droplets and the water descend again as rain, hail or snow. All plants and animals absorb water. In the case of the higher plants, by far the greater proportion of the water passes into the atmosphere by transpiration. Indeed, the role of forests as rain – makers can no be over emphasized. Some water is used in photosynthesis during which oxygen is released and the hydrogen retained for reduction of carbon dioxide. Plants and adsorptive and inhibition powers of clay particles utilize the ultimate products of photosynthesis. If humus is present, its colloidal particles will hold water in the same way as clay. Vapour losses. The extent to which rainfall can support crop production is determined, in part, by the balance between rainfall and evapo-transpiration. The total quantity of water required to produce a crop includes: (a) The amount retained in the plant (b) The amount lost by evaporation from the soil surface and (c) The amount lost by evaporation from plant surfaces i.e. Transportation.

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

Respiration Transpiration Water in soils seas, rivers lakes etc.

Decay Decay

Precipitation

Plants Transpiration and Excretion

Animals Evaporation

Figure 16: The Water Cycle The term “evapo transpiration “refers collectively to the amounts of water lost by evaporation in the production of a crop. Water deposited by dew, rainfall or irrigation and subsequently evaporated without entering the plant system is part of consumptive use. Differences exist in the amounts of water transpired by various crops since transpiration is simply the evaporation of moisture of water transpired by various crops. Since transpiration is simply the evaporation of moisture from plants surfaces, it is influenced by the same factors that affect the evaporation of water from any moist surface. Such factors are exposure to direct sunlight, air temperature, humidity, wind movement and atmospheric pressure. Various crops essentially require the same quantity of water when the cover is complete, green and the soil moisture tension is the same. The significant differences that exist in the different maturation periods or they grow at different seasons of the year. Plants growing in mediums having small quantities of nutrients appear to grow slower and transpire more water per gram of plant tissue produced than those growing in mediums having abundance of plant nutrient materials. As such, any management practices that increase the rate of plant growth will tend to result in more dry matter produced per kilogram of water used. For example, maize grain can be increased from 3,000 to 4,000kg/ha for each centimetre of water utilized by the addition of fertilizer. Similarly, in the humid regions, fertilized crops are more droughts tolerant because increased top growth result in increased root growth and penetration to the extent that the total water consumed may be greater and in used more efficiently. Factors influencing Evapo-transpiration It is obvious fro our discussion so far that evapo-transpiration or consumptive use of water can be influenced by the following factors: - Temperature - Irrigation practices - Length of growing season - Precipitation - Humidity - Wind movement - Intensity and duration of sunlight - Stage of plant development - Foliage type - Nature of leaves

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The size of a farm may also (to some extent) affect water usage since borders of furrows, size of stream and crop rotation may be adjusted differently to different land areas – large or small. Actual evapo-transpiration is therefore very variable. It is dependent on the inter relation of a number of variable factors. Apart from varying from place to place and at different times of the year (due to climate or weather) it can also change as a result of changes in soil moisture, plant cover and land management. Measurements of consumptive use of water The source of water to plant life (crop and /or natural vegetation) is an important factor that should be considered in the selection of a method for the determination of evapo-transpiration. The usual sources of water are: - Precipitation/rainfall - Irrigation - Groundwater - Atmospheric water other than rainfall - Flood water The measurement of actual evapo-transpiration under field conditions is very difficult, laborious and gives data to specific places and times of investigation. By and large the following methods for the determination are known: a. Tank and Lysimeter Experiments – In this method, tanks are placed in surroundings of natural growth of the same species since consumptive use of water is presumably the same of similar growth of outside the tank. The amount of water withdrawn is determined by differences in daily or weekly readings of the glass gage attached to the supply tank. b. Field experimental plots - Measurement by soil moisture studies in field’s plots are more dependable than measurements with tanks and lysimeter which does not always represent the natural conditions of the soil. John Widtsoe pioneered measurement of consumptive water use in field plots in 1902. He did his work on land having a water table of about 75 feet (22.9meters) below the surface such that crops obtained no ground water and only rainfall. The crop – season rainfall, draft on stored capillary soil moisture and irrigation water furnish all water for the crops. There should be no run off and very little or no deep percolation losses. Widstoe measured the water used by 14 crops during a 10 – year period (1902 to 1911). Yields obtained were then plotted against the total water used. With nearly all the crops yield increased rapidly with an increase of water used. With nearly all the crops, yield further increase in water use. The amount of water used at the point of break in the curve was taken as the consumptive use. It is worthy to note that yield is very essential in the determination of consumptive use. c. Soil moisture studies: this method is recommended for areas where soil is fairly uniform and the depth to ground water does not influence the soil moisture within the root zone. Soil moisture should be determined before and after irrigation with some measurements between irrigations in the major root zone. A great number of measurements should be taken in order to obtain the desired accuracy. Computation of hectare – centimetre of water extracted per day from the soil should then be done for each period. A plot of rate of water use against time produces a curve from which the seasonal water use can be got. d. Integrated Method: this method accepts that the consumptive use if the summation of the products of unit consumptive water used for each multiplied by its area, added to the unit consumptive use of native vegetation multiplied by its area added to water surface evaporation multiplied by water surface area added to evaporation from bare land multiplied by its area. Thus, it is necessary to know the unit consumptive use of water and the area of various classes of agricultural crops, native 85

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e. vegetation, bare land and water surfaces. Aerial map and field surveys of various types of native vegetative cover, bare land and water surfaces can be used in such determinations. f. Inflow-outflow for large areas – using this method, valley consumptive water use, U is equal to the water flows into the valley precipitation on the valley floor, P plus water in ground storage at the beginning of the year, Gs, minus water in ground storage at the end of the year, Ge minus yearly outflow, R. All volumes are measured in hectare – meters. Thus, U = (I + P) + (Gs – Ge) – R The differences between storage of capillary water at the beginning of the year and at the end of the year are assumed to be negligible. It is taken that stream measurements are made on bedrock controls and the sub – surface inflow is almost the same as sub-surface outflow. The quantity (Gs – Ge) is considered as a unit such that absolute evaluation of either Gs or Ge is not necessary – only the differences are needed. This quantity is the product of the differences in the average depth of water table during the year, measured in meters and multiplied by the specific yield of the soil and area of the valley floor. The quantity, P is obtained by multiplying the average annual precipitation in meters by the area of the valley floor in hectares. Unit consumptive use of the entire valley in hectare – meters per hectare is obtained by dividing the total consumptive use by the area of the valley floor. Potential Evapo-transpiration Potential evapo- transpiration can also be related to evaporation from a free water surface. Potential evapo – transpiration is the water lost when a green crop completely shades the ground and there is an optimum supply of soil moisture. It is the maximum rate of evapo – transpiration permitted by the amount of the solar radiation, which provides the energy needed to heat the overlying layer of the air and to vaporize the water. It can be determined by growing plants in sunken tanks filled with soil that is constantly supplied with moisture and measuring the water added, the outflow of drainage water and the water retained in the soil. This measurement is laborious and the results apply only to the site of the experiment. Conversely, an open water surface is a reproducible surface of known properties and evaporation from it depends entirely on the weather conditions tat influence evapo – transpiration losses from soil and crops and can be measured at any place using an evaporimeter. Limitations of Potential Evapo – transpiration It is expected that calculations of potential evapo – transpiration should be of value in estimating the water need of crops especially when grown under irrigation and optimum water supply is not limiting to evapo – transpiration. Such calculations can be very useful in planning irrigation projects (especially when perennial crops e.g. sugar can which more or less completely cover the ground for the greater part of its growing period) in involved. The value of the method for estimating the water need of annual and rained crops is limited for the following reasons: - It is only valid when water supply is not limiting. Once the roots have been used up the immediately or readily available soil moisture, soil factors, fertilizer treatment, crop management and crop type may greatly affect evapo-transpiration. - Moisture reserved in the soil at the beginning of the growing are not considered in the calculations of potential evapo-transpiration. - Annual crops differ greatly from a grass or any other vegetation that provides at all times a continuous cover of uniform density and colour. There is usually a sequence of changing conditions in each seasons growth of annual crop e.g. bare soil at planting, a time of increasing leaf area, a time of maximum vegetative cover over the soil, and a period of decreasing leaf area as senescence sets 86

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in. At the last stage (senescence) many crops change colour which alters transpiration rates by affecting the amount of radiation that is reflected.. Physiological activity of the leaf is also affected. IRRIGATION AND MANAGEMENT OF IRRIGATED SOILS Introduction Irrigation can generally be defined as the application of water to soil for the purpose of supplying the moisture essential for plant growth. It is an ancient agricultural practice which can also be for any number of the following other purposes: - Provision of crop insurance against short duration droughts - Cooling the soil and atmosphere and thereby ensuring more favourable environment for plant growth - Washing out or diluting salt in the soil - Reducing the hazard of soil caking - Softening tillage pans Nowadays, numerous worlds’ highly populated nations are supported by irrigation. The inability of several other nations to adopt irrigational measures is a major reason responsible for diet deficiencies in such countries. This is because moisture is usually the major constraint on a year round crop production scheme. An efficient management of natural water resources permits the storage of excess water drained from valley bottoms in storage tanks and irrigating the upper portion of the landscape with the stored water at source or irrigation water by creating appropriate technology. Farmers can also provide their source of irrigation water by creating irrigation wells. Apart from reducing one of the greatest hazards (inadequate water supply) in crop production, irrigation also help to distribute weed seeds over the farm. Yet, it adds more complexities to farming practice. Much experience is required to know when to do the artificial watering, how to do it and how much water should be applied to different crops grown under different soil conditions. The length of time (usually dry seasons) when artificial wetting is required as well as the ease and cost of getting the water also determine how efficient an irrigation scheme or program can be. Generally, irrigation water is supplied to supplement water available from the following four sources none of which should be ignored when irrigation water requirements are being estimated: - Precipitation / rainfall - Atmospheric water other than precipitation - Flood water - Ground water Any failure to consider all the four sources and the proportion of water that each supply to total plant needs may result in faulty design of an irrigation system. Precipitation Precipitation amount is supposed to be sufficient to replace moisture removed by plants from the rooting zone while its frequency should be enough to replenish soil moisture before plans start suffering from lack of moisture. The intensity of precipitation should also be low enough that water can be absorbed by the soil. It is rarely possible for precipitation in specific locations to fulfil the necessary requirements for maximum crop yield at all times; thus the increasing amounts of irrigation in arid and humid areas. Irrigation systems must be designed to provide for expected frequencies of drought periods.

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Atmospheric Water other than Precipitation The atmospheric conditions that prevail to make atmospheric water other than precipitation significant are dew formation, fog, and clouds and high humidity. These conditions reduce plant’s water needs by reducing the forces causing water to transpire from the plant. The water that evaporates from the ground and foliage normally reduces the amount of water which would have been withdrawn the soil by the plant. For this reason the contribution of atmospheric moisture in forms other than precipitation should not be overlooked when considering the need for additional water for agricultural production. Floodwater This is similar in some respects to irrigation water though not supplied by man. As floods pass over the land surface, water is absorbed by the soil and stored for subsequent use of plants. In some regions, agricultural production is mainly dependent upon floodwater.

Ground water This is the water beneath the soil surface where voids/pore spaces are substantially filled with water. The upward capillary movement of water from the water table can be a major source for plant growth. Ground water needs to be near but below the depth from which the major portion of plant’s water are absorbed. It can restrict plant growth if it is within the normal root zone. Similarly, if it is too near the surface, the land’s water needs are absorbed. It can restrict plant growth if it is within the normal root zone. Similarly, if it is too near the surface, the land’s ability to economically produce crops becomes produce most crops becomes almost zero. However, a water table within the lower portion of the zone may supply a considerable amount of water and thereby reduce the cost of irrigation more than it offsets the loss of productions. The optimum depth of the water table is that depth which gives the maximum economic return. Irrigation Times Water should no be applied when the crop is most needed. To save water, farmers should apply it at a time when the crop does not need it, given that the soil has capacity to store additional amount of water. Hence, the storage capacity of the soil must be considered in knowing the proper time to irrigate. By and large the following factors should be considered in the determination of the time and frequency of irrigation and the amount of water that should be applied: - Soil texture - Presence of Impermeable stratum or gravel - Accumulation of soluble salts in injurious quantities - Slope and evenness of soil surface - Behaviour of soil under irrigation - Type of crop and its water requirement - Season or time of the year - Area of land to be irrigated - Amount and availability of irrigation water The soil should be readily permeable to water and still be moisture retentive. Clayey soils absorb water slowly, are sticky and hard to cultivate when damp, crack and bake on drying and so difficult to manage. Some can be so slowly permeable that they become wet to a depth of only 30cm or less in a day. Sandy 88

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soils absorb water very rapidly and show little coherence when dry. Some can be so coarse textured that water passes below the reach of plant roots and little available moisture retained. Quite often, it is possible to judge the behaviour of the soil under irrigation by observing similar soils in nearby irrigated fields. If such is not available, water may be applied to a small “trial” area and observed. Factors determining the capacity, location and design of farm ditches 1. Depth and permeability of the soil 2. Duration of water delivery, whether the water is delivered in a continuous stream or for short periods with long intervals between 3. The method to be used in applying the water to the land 4. The area of the land to be irrigated 5. The water requirements of crops to be grown Factors determining the choice of methods of applying irrigation water 1. Seasonal rainfall 2. Slope and general nature of the soil surface 3. Supply of water and how it is delivered 4. Crop rotation 5. Permeability of the top soil and sub – soil to water Types of irrigation Irrigation water may be applied or distributed to land by three main methods. These methods are: a. Surface distribution b. Sub – surface irrigation c. Sprinkling distribution Nowadays, several improved forms of flooding have been developed. Surface irrigation can be done in the following ways. 1. Uncontrolled or “wild” flooding” – this is the earliest method of irrigation. It is useful on smooth land with regular and moderate slope. It is practiced largely where irrigation water is abundant and inexpensive. The skill and attention of the irrigator are of immense importance. If water is allowed to flow too rapidly, very little amount will infiltrate the soil while a major portion may run off. Yet, if kept on the surface for too long a tie, infiltration may be beyond the root zone. Thus, it is a very difficult task to apply water very efficiently by flooding methods. The size of stream used the depth of water as it flows over the surface and the rate of intake water into the soil control the efficiency. The water is normally introduced to the field in permanent supply ditches and distributed through ditches created across the field. The grade of the land, the texture and depth of the soil, stream size and the nature of the crop determine the spacing of ditches. 2. Flooding controlled with boarder, checks or basins. (a) Boarder – strip flooding – this is usually employed where land or labour is expensive. It involves the division of the farm into a number of strips (9 – 18 meters wide and 100 – 400 meters long) to cut across the direction of the slope which is the supposed direction of water movement and separated by low levees or boarders. Large siphon tubes are used to transfer water from the source (supply ditch) to the boarder area. The method is ideal on lands having a slope in one direction. For optimum utilization of the method, the location of the levees and the strips should be planed in such a way that different forage grain crops may be 89

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irrigated with the same boarders. Soil of wide variation in texture may be so irrigated though their physical properties should be known. Long boarder strips are required with impervious subsoil and compact loams while short and narrow strips should be made for open soils having highly permeable and gravely subsoil. A gate can be made in the supply ditch at the head of each strip for convenience in turning water into and out of the strip. (b) Check flooding – this involves the running of the large streams into level plot that are surrounded by leaves. It is suitable for very permeable soils that must be quickly covered with water when the prevention of excessive losses near the supply ditches is desired and achievable through deep percolations. The method can also be adopted for heavy clayey soils with low infiltration rate and which can not be sufficiently moistened during the watering on the surface to assure adequate penetration. Preparing the levees along contours with vertical intervals of 6 – 12 centimetres and connecting them with cross levees at convenient places may make the checks. In order not to obstruct farm machinery and ensure satisfactory crop growth on the levees, the centimetres high. These are known as contour checks and are usually formed by building longitudinal levees approximately parallel to the contours and connecting them at desirable places with levees at right angles. (c) Basin flooding – this is the check method that is adapted to irrigation of orchards. In some cases, a basin is made for each tree while in other (especially with favourable soil conditions and surface slope) two to five or more trees are included in one basin. From the supply ditch, the water is conveyed to the basin by flowing from one basin into the next one. Alternatively, small ditches are constructed in such a way that water turns directly from a ditch into each basin. (d) The Furrow method – in this method, furrows are made across the field, leading down the slope. Siphon tubes are usually used to transfer the water from the head ditch into the furrows. The use of furrows for irrigation ensures that only a part of the surface (a half to one – fifth) is affected. This reduces evaporation losses, minimizes puddling of heavy soils and makes it possible to cultivate the soil almost immediately after irrigation. It is adaptable to a great variation in slope. It is customary to run furrows down the steepest slope so as not to overflow the banks of the furrows. Allowing only very small streams to enter the furrows and careful control of erosion can successfully use furrows having sloes of the 10 – 15 meters per 100 meters. Spacing of furrows for irrigation of row crops (e.g. maize and cotton) is usually determined by proper spacing of the plant rows. One irrigation furrow should be provided for each row. In orchards, irrigation furrows may be spaced from 6.9 to 1.8 meters apart. 3. Sub Irrigation This is the application of water to soils directly under the surface provided the soil and topographic conditions are favourable. It is the irrigation by water movement upward from a free – water surface distance below the soil surface. The subsoil should be impervious at a depth of 1.8meters or more, surface soil should be highly permeable loam or sandy loam, the topography should be uniform while the slope should be moderate to favour sub – irrigation. Given these conditions, proper water control to prevent alkali accumulation or excess water logging can result in economical use of water, high crop yield and low labour cost in irrigation. Introduction of water through sub irrigation may cause serious salt accumulation in arid regions. It is best used where natural rainfall removes salts that may accumulate. Sub irrigation may be of natural occurrence 90

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in many poorly drained fields. Artificial ones are expensive, not always reliable, adapted for only special situations, increases salinity problems and is an efficient way of using irrigation water. Requirements for the Design of Surface Irrigation Systems The design of an irrigation system is very complex. It may not be easy to quantitatively analyse such systems since the overall economics of the concerned farming operations is also of immense importance. Yet, ten principal requirements are basic to such designs. The requirements are – i. ii. iii. iv. v. vi. vii. viii. ix. x.

The required water should be stored at the root zone of the soil Reasonably uniform application water Minimal soil erosion Minimal runoff of irrigation water from the field Beneficial use of runoff water Minimal labour requirement for the irrigation Minimal land use for ditches and other controls to distribute the water Fitting irrigation systems to field boundaries Adapting the irrigation system to soil topographic changes Use of machinery for land preparation, cultivating, furrowing, harvesting, etc. should be facilitated.

4. Sprinkler Irrigation – This is the method of water distribution to the soil surface I the form of a spray that is similar to the case of an ordinary rain. It used in humid regions as a supplemental method of irrigation. Sprinkler irrigation systems involved stationary overhead-perforated pipe installations with rotating sprinklers that are expensive to install but fairly inexpensive to operate. They can be used on all soil types and on lands of widely different topography and slopes and for many crops. They are very commonly used to water (irrigate) lawns and ideal where infiltration rates or topography prevents proper levelling of the land for surface water distributed. It is also advantageous that the rate of water application can be carefully controlled. The portable nature of most sprinkler systems makes them very suitable for use where irrigation water is used to supplement the natural rainfall. Efficiency of sprinkler irrigation The best method of irrigation in any circumstance is that which can most economically distribute the required water uniformly to the farmland. When considering where sprinkler irrigation can be used to great advantage, the normal requirement of uniform distribution of water is of utmost importance. Some of the conditions that favour the use of sprinkler irrigation are as follows: i. Soils that are too porous for good distribution by surface method ii. Shallow soils, the topography of which prevents proper levelling for surface irrigation methods iii. Land having steep slopes and easily erodable soils iv. If the source of irrigation water (e.g. stream) is too small to distribute water efficiently by surface irrigation v. Undulating land that is too costly to level sufficiently for good surface irrigation vi. When the labour available for irrigating the land is not experienced or unreliable in surface methods of irrigation. Good surface irrigation requires trained and reliable labour. vii. When there is need to bring land high production quickly. Sprinkler systems can be designed and installed quickly.

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Differences between the sprinkler Irrigation and the surface methods of irrigation i. It is easier to measure water being disturbed when using the sprinkler than when using the surface methods ii. Shallow soils, the topography of which prevents proper levelling for surface irrigation methods iii. Land having steep slopes and easily erodable soils iv. If the source of irrigation water (e.g. stream) is too small to distribute water efficiently by surface irrigation v. Undulating land that is too costly to level sufficiently for good surface irrigation vi. When the labour available for irrigating the land is not experienced or unreliable in surface methods of irrigation. Good surface irrigation required trained and reliable labour vii.

When there is need to bring land into high production quickly. Sprinkler systems can be designed and installed quickly.

Differences between the Sprinkler irrigation and the surface methods or irrigation - It is easier to measure water being distributed when using the sprinkler than when using the surface methods - There is less interference with cultivation and other farming operations when the sprinkler is used than when the surface methods are used. Furthermore, less land is taken out of production than with surface method. - Minimum additional capital investment is required for the sprinkler once the water has been pumped to the point of use - High water application efficiency can be obtained by the use of sprinkler - A common distribution time can be used for domestic and irrigation water if the same source is used. This can be employed as a cost saving device. - The sprinkler can be provided at a lower capital investment per hectare of land irrigated than for surface irrigation. This is more so for areas that require infrequent irrigation. - The use of the sprinkler irrigation is particularly attractive whenever water can be delivered to the field under force of gravity - The sprinkler is the best for frequent and small applications of water - Fertilizers and other soil amendments can be applied quickly, economically, easily and effectively through the sprinkler. The Relevance of the Quality of Water for Irrigation The nature of water for irrigation should be considered before an irrigation system is established. The water may contain too many soluble salts such that its use for irrigation may not be advisable especially on lands that already have high soluble content. Sodium salts can deflocculates the colloidal soil fraction and result in poor soil structure. High sodium salts content in irrigation water is, therefore, more objectionable than Ca and Mg salts. Boron may also be present in irrigation water in quantities that may be toxic to plants. In general, the estimation of the quality of irrigation water involves the following factors: i. The conductivity or total concentration of salts ii. The sodium Absorption ratio (SAR) iii. The concentration of Boron iv. The effect of the salts on the soil v. The effect of the salts on the crop to be planted and irrigated vi. Soil drainage vii. Soil texture viii. The kind of clay material contained in the soil.

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It is worthy to note that the determination of the quality of water to be used in irrigation schemes is principally based on experience. This is so because there is not method of interpretation that is absolutely accurate under all conditions. How to Reduce the Effect of High Content of Irrigation Water 1. Crop failures may result from continuous salt accumulation in soils especially if low salt tolerant crops are grown. Farmers should plant crops that are reasonably salt tolerant e.g. cotton 2. Seed germination is usually a problem in salty soils. The farmer should apply the irrigation water in such a way that the salt content of the seed zone is minimal in order to ensure maximum germination. Alternatively, a higher than normal water content can be maintained in the soil at germination in order to minimize the effect of the salt. 3. In order to control salt content in soils, practices that allow uneven distribution and downward movement of water should be avoided. This is because such practices (e.g. furrow method) can cause differences in the distribution of salt in the soil. PRINCIPLES AND PRACTICE OF LAND DRAINAGE Introduction Excess water may occur in soils due to inability of adhesion and cohesion forces to retain them in such soils. The excess water usually exists in large (non – capillary pore spaces and may move down the soil profile in response to the force of gravity and suction (pull) by the underlying soil pores. The control of such excess water may be a problem to the farmer. Plant roots tend to suffocate in poorly drained soils. Root tips are regions of rapid cell division and elongation. They have a high oxygen requirement. Roots of most crops do not penetrate water – saturated (poorly drained) soils due to oxygen deficiency. Nevertheless, paddy rice and some other plants have stems through which oxygen can diffuse from the atmosphere to the roots growing in water – saturated soils. Such plants are, therefore not dependent on soil oxygen for root respiration. As soil becomes more poorly drained, however, rooting depth decreases while organic matter content increases and the soil colour becomes darker in the surface soil layer. The properties of soil constitute an important and reliable guide for predicting soil drainage classes – well drained, moderately drained, somewhat poorly drained and poorly drained – at any time of the year. Welldrained soils can e found on the landscape wherever the soil is never saturated with water. On the other hand, poorly drained soils are found in low wet areas where soil is saturated all or most of the year. It is obvious that natural drainage greatly influences soil use. Well – rained soils are ideal for crop production. Poorly drained soils need artificial drainage fro production of most agricultural crops. Importance of drainage in irrigation schemes It is difficult to apply sufficient irrigation water for maximum crop growth without having some excess. In fact, some excess water is often needed to wash out soluble salts. For irrigated land to remain continuously productive, therefore, it is essential to provide artificial systems if such lands are not naturally well drained. If the excess water is not removed, the water level is raised and the danger of salt accumulation will be increased. The resulting water logged soil condition can also lead to restriction of root development and subsequent crop damage. Drainage systems The use of drainage systems to remove water from soil is highly important whenever the water table is close enough to the surface so that water saturated soil occurs within root zone of plants, foundations of engineering structures, and so on. The two main types of drainage systems are the open ditch drainage and the tile drainage. Each of the two types has advantages, which dictate their use in any given situation 93

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Drain Drainage This can be quickly and inexpensively made to remove gravitational water though it requires periodic cleaning. Drainage ditches are usually of two types. (Figure 17) some are only useful as outlets for tile to support water to nearby streams. They may be deep and narrow. Others may be relatively shallow and broad to permit surface or controlled removal of water from soils before it infiltrates the soil. Drainage ditches have the advantage of large carrying capacities apart from the fact that their cost per unit of water removed is relatively low. They can however be costly to maintain, may interfere with agricultural operations as they are inconvenient for machinery use and occupy valuable agricultural lands that should heave been available for cropping. They are also unsatisfactory for water removal around the walls of the basement of buildings. When combined with land forming or smoothing, surface drainage ditches function more efficiently as a means of rapid removal of surface water from soils (Figure 18). Depressions of ridges that prevent water movement to the drainage outlet are filled or levelled off using certain field levelling equipment. The resulting land formation permits excess water to move slowly over the soil surface to the outlet ditch and later to a natural drainage channel. Land forming or smoothing controls the movement of surface water to the outlet ditch which transports it to a nearby natural water way.

A

B

Figure 17: The two common types of drainage ditches. a. Deep and narrow outlets with vertical sides for tile drains from one side as in the case of the draining of organic soil areas. b. Shallow but broad sloping – sided ditch commonly used to transport drainage water from either tile or open drain systems to nearby streams. Tile Drains The use of drainage tiles is the most important means of under – draining soils. Each tile is usually 30.5cm or more in length and with a diameter that varies with the amount of water to be transported; several ones can be laid end to end at the bottom of a trench of sufficient slope.

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Figure 18: Land surface before and after land forming or smoothening. They are then covered with straw or surface soil to facilitate water entry through the joints. Papers or other porous materials are used to cover the joints in order to prevent soil particles entry. The tile drain functions to remove the excess water and discharge it quickly from the land. It is very effective when the soil pores are large, numerous and connected in a way that allows for the rapid downward movement of gravitational water. It has been recommended that the system should be placed about 90cm beneath the land surface. For sandy soils or poorly drained saline soils in arid regions, a depth of about 1.2 meters or more can be used. The minimal (shallowest) depth that can be employed safely on mineral soils (if the danger of breakage to tiles by heavy machinery is to be avoided) is, 76cm and it is usual on slowly permeable soils where the laterals are close together. It is important also to ensure that the interval between tile lines is reduced for very compact (finer) textured soils. Thus, for clayey soil, 15 – 22.0 meters is common between the laterals and can sometimes be as low as 6.1 meters. The maintenance cost of the tile drain system can be very low if properly installed. The outlet is the major point of focus and it needs to be well protected such that the end is not loosened and the whole system endangered by clogging with sediment. The last tile can be covered by a gate or by wire to allow the water to flow freely and prevents rodents from entering during dry weather. Other Variations of Tile Drains Some drainage tiles can be made of fired clays and laid side by side with small cracks between adjacent ones. When the surrounding soil is saturated with water, water seeps into the tile and the water eventually reaches an outlet where it is disposed. It is easy to install such drain in fields that have trenching machines. A “MOLE” drainage system is sometimes employed in mineral soils. This is un-walled cylindrical metal plug through the soil at the desired depth. Such leaves a compressed wall channel through which water can drain. They are quite inexpensive to install but usually short-lived and not as suitable as tile drains. Long plastic tubes with holes can also be used for draining organic soils areas where the low supporting capacity of the soil results in unequal settling and misalignment of the shorter tile sections.

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Advantages of Land Drainage 1. Land drainage promotes the conditions that are favourable to higher plants and soil organisms. Such conditions include: (a) Encouragement of granulation and consequent development of good soil structure, (b) Alleviation of the bad effects of the alternate expansion and contraction consequent upon freezing and thawing of soil water, and (c) Maintenance of sufficiently deep and effective root zone by lowering the water table at critical times and thereby ensuring that the quantity of nutrients absorbable by the plants is maintained at a high level. 2. Removal of excess water by land drainage lowers the specific heat of soil and thereby reducing the energy required to raise the temperature of the layer. Cooling effect results particularly t the surface layer where most of the evaporation occurs. The combined effects make the warming of the soil easier. 3. Good drainage enhances ready diffusion of oxygen to and carbon dioxide from plant roots. It is pertinent to note that the activity of aerobic soil organisms is dependent on soil aeration, which in turn influences the availability of nutrients such as nitrogen and sulphur. Toxicity from excess iron and manganese (in acid soils) can also be reduced if sufficient oxygen is available because the oxidized states of the elements are insoluble. The previous efforts towards self-sufficiency in food production in most developing countries failed either because of misconception of the quality of the soils or an underestimation of the roles of the soil in successful agricultural production or both. The next chapter, therefore, continues the emphasis on the importance of proper understanding self – sufficiency in food production. The soils in most parts of the tropics have marginal capacity, susceptible to degradation and hence need careful managemen for continuous and successful crop production. There is the need for the provision of detailed information on the soils and their suitability for the crops, which should be made available to farmers and other land users.

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REFERENCES Agboola, A. A. and Obatolu, C. R. 1990. The use of organic materials to improve the quality of organic matter. Tech. Humid Tropics, CRIN, Ibadan, Nigeria. 2nd African Soil Conf. Agboola, A.A. 1980. Role of soil testing in Agricultural production in Africa. Paper presented at the first OAU Inter- Africa soil congress, Accra, Ghana. 10th – 15th Nov. 1980. Ahn, Peter M. 1970. West African Soils, Oxford University Press, Ely House, London W.I. Akinrinde, E.A. 1987. Assessment of the Electroultrfiltration of Available soil Nutrients. Unpublished Ph.D Thesis. Univ. of Ibadan. Nigeria. Akinrinde, E.A. and Adeoye, G.O 1995. Soils: Nature and Properties, Afolabi Press Ltd. Nig. Allayway, W. H. 1986. Soil Plant –animal and human interrelationships in trace element nutrition. In Trace Elements in Human and Animal Nutrition. Ed. Mertz pp. 465 – 488. Academic Press, Orlando, San Diego, New York, Austin, London, Montreal, Sydney, Tokyo, Toronto. Allen V. Barker and D. J. Pilbeam. 2006. Handbook of Plant Nutrition. C.R. C Press Anderson, T. F., and Rasmussen, H. N. 1996. The mycorrhizal species of Rhizoctonia. pp. 379-390 In Rhizoctonia species: Taxonomy, Molecular Biology, Ecology, Pathology and Disease Control (B Sneh et al., eds) Kluwer Academic, Dordrecht. Andreini, M., van de Giessen, N., van Edig, A., Fosu, M. and Andah, W. 2000. Volta Basin water balance. ZEF Discussion papers on Development Policy No 21. Center for Development Research, Bonn, Germany. Appiah, M. R., Ofori-Frimpong, K. and Afrifa, A. A. 2000. Evaluation of fertilizer application on some peasant cocoa farms in Ghana. Ghana Journal Agric. Sci. 33: 183 - 190. Aweto, A. O. 1981. Organic build-up in fallow soil in some parts of South-western Nigeria and its effect on soil properties. Journal of Bio-geog. Vol. 8, 64-67. Babalola, O., Bartholomew, W. V, Ogunwale, J. A and Obigbesan G. O. 1978. The developmet , conservation and production potentials of the soil resources on Nigeria. Journal of Environmental Management 7, 9 – 28. Barker, A. V. 1999. Plant Nutrients: Deficiency Symptoms. In: Laboratory, Problem Set, and Examination Manual. University of Massachusetts, Amherst, Mass. Blandari. A. L; K. N. Sharma, M. L. Kapur, D. S, Rana. 1989. Supplementation of Nitrogen through green manuring for maize growing. Journal Indian Society Soil Science 37: 483 – 486. Bonsu, M., Ofosu, K.Y. and Kwakye, P.K. 1996. Soil management action plan for Ghana. A Consultancy Report prepared for the World Bank, Washington, DC. Brady, N. C. 1969. The nature and properties of soils, Nineth Edition. Macmillan, New York, 9th or any later edition.

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Brady, N. C. and R. R. Weil. 1996. Soil Organic Matter. Ch. 12 in The Nature and Properties of Soils. Prentice Hall, NY. Braimoh, A. K. and Vlek, P. L. G. 2004. Impacts of land cover change on soil properties in northern Ghana. Land Degradation and Development, 15, 57-64. Briggs, D.J. 1977. Sources and Methods in Geography: Soils Butterworhs, London – Boston. Sydney – Wellington – Durban – Toronto. Cooke, R. C and Rayner, A. D. M. 1984 Ecology of Saprotrophic Fungi. Longman, London http://helios.bto.ed.ac.uk/bto/microbes/armill.htm#Top Deacon, J. W. 1997. Modern Mycology. Blackwell Scientific, Oxford. DellaPenna D 2001. Nutritional genomics: manipulating plant micronutrients to improve human health science 285, 375 – 379. Dorna, J. W., Sarranotnio, M. and Allebig, M. 1996. Soil health and sustainability. Advances in Agronomy. 56: 22-45. Easterbrook, G. 1997. The Forgotten Benefactor of Humanity. The Atlantic Monthly. 227(1):75-82. Enzmann, J., H. Mutscher., K. J. Michalski. 1983. The role of mineral fertilization and problems of an optimal nutrient supply to tropical crops from the view point of increasing plant production. Evans, C. E. 1987. Soil Test Calibration. In Soil Testing: Sampling, Correlation, Calibration, and Interpretation. SSSA Spec. Publ. 21. Soil Science Society of America. Madison, WI. F.A.O. 1992. Conversion and rehabilitation of African Land F.A.O. A document presented at F.A.O Regional Conference AIC/90/410/57001, Rome. F.A.O. 1994. Soil management for sustainable Agriculture and Environmental Protection in the tropics. Land and Water Development. Division Food and Agricultural Organization of United Nation. Foth, H.D and Turk, L.M. 1972. Fundamentals of soil science, 5ht Edition, Wiley Eastern Private Limited, New Delhi. Frossard, E., Bucher, M., Machler, F., Mozafar, A. and Hurell, R. 2000. Review. Potential for increasing the content and bioavailability of Fe, Zn and Ca in plants for human nutrition. J. Sci Fd. Agric. 80, 861 – 879. Gary, Wilson. 2004. Agriculture and Nutrition, Wise Traditions, p.13-17 Gerken, A, Suglo, J. V. and Braun, M. 2001. Crop protection policy in Ghana. Pesticide Policy Project produced by Plant Protection and Regulatory Services Directorate of Ministry of Food and Agriculture, Accra.

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Gerken, A., Suglo, J.-V. and Braun, M. 2001. Crop protection policy in Ghana. Ministry of Food and Agriculture. Accra. Gerner, H., Asante, G. H., Owusu-Bennoa, E. O. and Marfo, K. 1995. Ghana privatization scheme, IFDCAfrica, Lomé. Goto, F., Yoshihara, T., Shiegmoto, N., Toki, S., and Takaiwa, F. 1999. Iron fortification of rice seed by the soybean ferritin gene. Nat, Biotechnol, 177 282 – 286. Goto, F., Yoshohara, T., and Saiki, H. 2000. Iron accumulation and enhanced growth in transgenic lettuce plants expressing the iron binding protein ferritin. Theor. Appl. Genet. 100, 658 – 664. Graham, R. D., and Welch, R. M. 1996. Breeding for Staple-food crops with high micronutrients Working Paper 3, International Food Policy Research Institute, Washington, DC. Pp. 1-72. Graham, R. D., and Welch, R. M. 2000. Plant food micronutrient composition and human nutrition. Commun. Soil Sci. Plant Anal. 31, 1627 – 1640. Graham, R. D., Welch, R. M., and Bouis, He. 2001. Addressing micronutrients malnutrition through enhancing the nutritional quality of staple foods: principles, perspectives and knowledge gaps. Adv. Agron. 70, 77 – 142. Grunes, D. L., and Allaway, W. H. 1985. Nutritional quality of plants in relation to fertilizer use. In fertilizer Technology and Use. Ed. O.P Engeslad pp. 589 – 619. Soil Science Society of America, Madison, WI. Grusak, M. A., Penna, D. D. 1999. Improving the nutrient composition of plants to enhance human nutrition and health Annu. Rev. Plant Physiol. Plant Mol. Biol.1999.50:133–61. Havlin J. L. 1999. Soil Fertility and Fertilizers, J.D. Beaton, S.L. Tisdale, W.L. Nelson. 1998. Soil Fertility and Fertilizers. 6th Edition. Prentice Hall Helmut Kohnke 1988. Soil physics. Tata Mc Graw – Hill Publishing Company Ltd. Bombay – New Delhi. Holland B, Welch AA, Unwin ID, Buss DH, Paul AA and Southgate DAT 1991 McCance and Widdowson’s The Composition of Foods, The Royal Society of Chemistry, Letchworth, UK 462 pp House, W. A., and Welch, R. M. 1989. Bioavailability of and interactions between zinc and selenium in rats fed wheat grain intrinsically labeled with Zn and Se. J. Nutr. 119, 916 – 921. IITA, 1982. First International Symposium on land Clearing and Development in the Tropics held at the International Institute of tropical Agriculture, Ibadan, Nigeria, Nov 23 – 26, 1982. Institute of Statistical, Social and Economic Research (ISSER), 2003. The state of the Ghanaian economy in 2002. Accra. Irvine F. R 1969. West Africa Crops. O.U.P, Ely House London W.1. 99

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Jackson, M.L. 1958. Soil chemical Analysis. Prontice – Hall, Inc. Englewood Cliffs, N.J. Kishore, G., and Shewmaker, C.1999. Biotechnology: enhancing human nutrition in developing and developed worlds. Proc Natl. Acad. Sci USA. 96, 5968 – 5972. Kohnke, H., and D. P. Franzmeir. 1995. Soil science simplified. Fourth Edition. Waveland Press, Prospect Heights, Illinois. Lal, R and Kang, B. T. 1982. Management of organic matter in soils of the tropics and sub-tropics. In: Transaction of the 12th International Congress of Soil Science, New Delhi, India, Vol. IV, 152-178. Levy, P. 1984. Carbon. p 224-233. In The Periodic Table. Schocken Books, New York. Lombin, L. G., Adepetu, J. A. and Ayotade, K. A. 1991. Complementary use of organic manure and inorganic fertilizer in arable crop production. In: Lombin et al., (eds.) Organic fertilizer in Nigeria agriculture: Present and Future. Proc. National Organic Fertilizer Seminar held at Kaduna, Nigeria, 26-27 March, 1991, pp. 146-162. Lucca, P., Wunn, J., Hurrell, R. F., and Potrykus, I. 2000. Development of iron in rice grains. Their. Appl. Genet. 102, 392 – 397. Mann C. 1997. Reseeding the green revolution. Science 277, 1038-1043 Marschner, H. 1995. Mineral Nutrition of Higher Plants. Second Edition. Academic Press. New York. Mengel, K., and Kirkby, E. A. 1987. Principles of plant nutrition. IPI. Berne, Switzerland. 4th. Edition. Nemeth, K. 1979. The availability of nutrients in the soil as determined by Electro – ultrafiltration (EUF). Advances in Agronomy 31 : 155 – 188. Obigbesan, G. O. 1978. Nutritional problems in root crop production in a tropical country – Nigeria. Journal Beitrage trop. Landwirtsh. Veterinary Med. 16: 289-297. Ogunkunle A.O. 1995. Nigerian soils and their capacity for crop production GREEN. 11th edition July 1995. pp. 7 – 12. Russell, E. W. l961. Soil conditions and plant growth. Ninth Edition. John Wiley, New York, or Tenth Edition, 1973. Webster, C.C and Wilson, P.N 1966. Agriculture in the Tropics. 4th edition. The English Language Book Society (ELBS) and Longman.

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APPENDIX 1

2

102

3

103

4

104

5 (A)

105

5 (B)

106

6

107

7 (A)

108

7 (B)

109

8

110

9 (A)

111

9 (B)

112

Subject Index A Aggregated structure, 38 Air, 1, 38, 44, 49, 52, 53, 64, 66, 68, 69, 72, 73, 74, 83-86 Alluvium, 8 Anaerobic bacteria, 36 Actinomycetes, 69 Algae, 70 Ammonium phosphate, 80 Sulphate, 58, 77, 79 Anion, 46, 49, 50, 58, 76 Exchange, 49, 50 B Bedrock, 19, 25, 26, 86 B – Horizon, 26 Bouyoucos Method, 34 Bulk density, 34, 35, 40, 41, 66 Bacteria, 1, 8, 12, 19, 23, 24 Indigenous, 68 Invaders, 68 Basin Flooding, 90 Boarder – strip flooding, 89 Broadcasting, 80, 81 C Capillary water, 13, 86 Carbonation, 9 C – Horizon, 26 Color, 3, 5, 74, 75 Colluvium, 8 Crust, 3, 7, 8, 11, 19, 37 Weathering, 2, 3 Carrier, 80, 81 Cation, 2-6, 9, 11, 39, 46, 49, 50, 51, 53, 54, 57, 58, 75, 76 Cation Exchange Capacity, 50, 54, 57, 75, 76 (CEC), Check flooding, 90 Compost manure, 78 Contour, 58, 60, 63, 66, 90 D Drainage systems, 93 Ditches, 89, 90, 94 Ditch drainage, 93 E Essential Nutrient, 1, 45, 78 Electro – ultrafiltration technique, 76

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

F Feldspars, 4, 5, 9, 19, 27, 40, 41 Feel or Field method, 32, 52 Farmyard manure, 78 Fertilizers, 1, 45, 49, 57, 58, 69, 75, 77-82, 92 Complete, 79 Incomplete, 79 Organic, 69, 77, 78 Filler, 69, 77, 78 Field method/test, 52 Floodwater, 85, 87, 88 Foliar application, 81 Furrow application, 90, 93 Furrow method, 90, 93 G Geology, 2, 3 Geomorphology, 3 Gravity, 8, 13, 40, 92, 93 Green manure, 49, 59, 78 Ground water, 36, 85, 87, 88 H Hydrolysis, 9, 83 Hygroscopic water, 13-14 I Igneous rock, 3, 6, 10 Inorganic particles, 11 Inosilicates, 10 Isomorphous, 5, 50 Isomorphous substitution, 5 Interstratified clays, 41 Inorganic colloids, 50 K Kaolinite, 39, 47, 48, 50 L Leaf litter layer, 24 Limestone, 3, 6, 8, 27 Living organisms, 1, 11, 19, 44, 67 Laboratory method, 52 Lime, 8, 18, 30, 54, 55 Spreader, 51 M Massive structure, 38 Matric (capillary), 13 114

SUBJECT INDEX

Metamorphic rock, 3-7, 10 Micro organisms, 14, 19, 46, 54, 67, 72, 78 Mineral salts, 1, 12, 14, 17, 43, 44, 77, 83 Munsell Colour charts, 28 Macrofauna, 67 Macronutrients, 45, 75 Manual technique, 56 Micronutrient, 45, 49, 73, 75 Monovalent, 50 Montmorillonite, 47-48, 50 N Nesosilicates, 4 Nematodes, 67, 70-71 No – tillage, 58-59 O Oxidation, 9, 68 Organic colloids, 46, 50 P Parent materials, 3, 8, 19, 24, 47 Rocks, 8 Particle density, 35, 40 Percolation water, 13 Phyllosilicates, 4 - 5 Physical weathering, 8 Pipette method, 33-34 Porosity, 35, 41, 59, 66 Photosynthesis, 36, 44 - 45, 67, 82-83 Potential evapo – transpiration, 86 Protozoa, 8, 68, 70 R Rocks, 2-11, 19, 45-47, 73-75 Runoff water, 19, 91 Row placement, 80 S Sedimentary rock, 3, 10 Sedentary, 8 Sedimentation techniques, 33 Single grain structure, 38 Soil aggregates, 34, 35, 36, 38, 71 Consistency, 37 Fertility evaluation, 2, 75 Genesis, 8, 43 Profile, 8, 12, 13, 17, 21-27, 51, 69, 72, 93 Science, 2, 14, 43 115

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

Structure, 28, 35, 37, 38, 39, 41, 49, 57, 59, 70, 70, 78, 92, 96 Temperature, 41, 42, 57-59, 66, 69, 78 Texture, 28, 30, 35, 40, 52, 72, 88, 92 Stokes’ law, 33 Structural water, 13, 14 Subsoil, 24, 27, 38, 43, 62, 71, 90 Synthesis, 1, 8, 44 - 45, 67, 74 Saltation, 64 - 65 Shear blade, 56 Side dressing, 80 Slugs and snails, 71 Sod waterways, 63 Soil analysis, 75 Classification, 44 Colloids, 37-38, 45-48, 50-51, 75 Desertation, 60, 65-66 Erosion, 8, 19, 27, 37, 43-45, 49, 56-66, 70, 72, 78, 90-91 Ecosystem, 67, 68 Moisture studies, 84 - 85 Nitrogen, 44, 73 Nutrient losses, 72 Organic matter, 16, 33, 48 - 49, 57, 59, 70, 74 pH, 51-54, 58, 66, 73, 85 Phosphorus, 74 Testing, 44, 56, 73, 75-76 Sprinkler irrigation, 91-92 Strip cropping, 63, 65-66 Surface creep, 64 Symbiotic nitrogen fixation, 74 T Tectosilicates, 4 - 5 Tetrahedra, 4 -5 Topography, 19, 60, 90, 91-92 Topsoil, 12, 24, 27, 41, 57, 61, 63 Tank and Lysimeter Experiments, 85 Terraces, 58, 63 Tile drains, 94 - 95 Top dressing, 81 Tree planting, 66 V Void, 35, 39, 40 - 41, 88 Void ratio, 39, 41 W Weathering, 2-5, 7-12, 19, 41, 45-47, 50, 52, 67, 76 Wild flooding, 89 Wind, 3, 8, 19, 60-61, 64 - 65, 71-72, 84 116