Mineralogy Chemistry and Physics of Tropical Soils With Variable Charge Colloids

The Mineralogy, Chemistry, and Physics of Tropical Soils with Variable Charge Clays

Other Titles in This Series

Azolla as a Green Manure: Use and Management in Crop Production, Thomas A. Lumpkin and Donald L. Plucknett Irrigated Rice Production Systems: Design Procedures, jaw-Kai Wang and Ross E. Hagan Managing Pastures and Cattle Under Coconuts, Donald L. Plucknett Small-Scale Processing and Storage of Tropical Root Crops, edited by Donald L. Plucknett

Also of Interest

Tomatoes in the Tropics, Ruben L. Villareal Rice in the Tropics: A Guide to the Development of National Programs, Robert F. Chandler, ]r. Administering Agricultural Development in Asia: A Comparative Analysis of Four National Programs, Richard W. Gable and ]. Fred Springer World Soybean Research Conference II: Proceedings, edited by Frederick T. Corbin World Soybean Research Conference II: Abstracts, edited by Frederick T. Corbin

Westview Tropical Agriculture Series Donald L. Plucknett, Series Editor

The Mineralogy, Chemistry, and Physics of Tropical Soils with Variable Charge Clays Gore Uehara and Gavin Gillman

Because modern soil science was developed largely in temperate countries where soil colloids are primarily of the permanent charge type, the transfer of agroproduction technology from temperate zones to the tropics-where about two-thirds of the soils consist predominantly of variable colloids-has been only marginally successful. Drs. Uehara and Gillman explain the basic difference between the variable and permanent charge systems, first examining the ideal variable system and then searching for similar behavior in soil colloids. The theoretical analysis of variable charge colloids is followed by practical examples of soil behavior in the field. The authors use the surface charge characteristics of variable charge colloids to explain nutrient retention and buffering capacities of soils, which in turn are the basis for recommendations on fertilizer and lime applications. In the final chapter, they present analytical methods for characterizing soils with variable charge colloids. Goro Uehara is soil scientist and professor of soil science at the University of Hawaii at Manoa. He holds a Ph.D. from Michigan State University and was chairman of the Executive Committee of the Consortium on Soils of the Tropics. Gavin Gillman is senior research scientist in soils chemistry withCSIRO (Commonwealth Scientific and Industrial Research Organization) in Australia. He has a Ph.D. from the University of Queensland and was a visiting professor at the University of Hawaii in 1978-1979.

The Mineralogy, Chemistry, and Physics of Tropical Soils with Variable Charge Clays Goro Uehara and Gavin Gillman

Westview Press / Boulder, Colorado

Westview Tropical Agriculture Series, No.4

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, withou t permission in writing from the publisher. Copyright

© 1981

by Westview Press, Inc.

Published in 1981 in the United States of America by Westview Press, Inc. 5500 Central Avenue Boulder, Colorado 80301 Frederick A. Praeger, Publisher

Library of Congress Cataloging in Publication Data Uehara, Goro. The mineralogy, chemistry, and physics of tropical soils with variable charge clays. (Westview tropical agriculture series; 4) Includes bibliographies and index. 1. Clay soils-Tropics. 2. Soils-Tropics. I. Gillman, Gavin P., joint author. II. Title. III. Series. S592.367.U37 631.4'913 80-14293 ISBN 0-89158-484-6

Printed and bound in the United States of America.

Contents List of Figures

xi

List of Tables

xv

Prefa ce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XVll

1. Introduction

1

2. Mineralogy..............

7

Clay Mineralogy and Soil Behavior Mineral Properties Weathering and Clay Mineralogy Mineralogy and Soil Classification Silicate Mineralogy Classes Mixed Mineralogies and Non-crystalline Materials Specific Surface of Non-crystalline Materials Cation Exchange Characteristics of Soil Minerals

3. Chemistry

7 8 9 15 19 20 25 29

31

Origin of Charge on Soil Colloid Surfaces Gouy-Chapman Theory Stern Theory vii

31 33 37

viii

Contents

Other Theories Ion Exchange Application of Theory to Variable Charge Soils Application of Theories Soils Containing Mixtures of Permanent and Variable Charge Colloids Application of Theory to Soil Systems Implications for Soil Management Phosphorus

4. Physics Rheology The Atterberg Constants Clay Activity Rheology of Soils with Non-erystalline Clay Transport Phenomena Soil Water Solute Transport Soil Temperature SoilAir

5. Extent and Distribution Accessory Characteristics Soil Interpretation

6. Analytical Methods Cation Exchange Capacity Anion Exchange Capacity Exchangeable Cations ; Charge Characteristics Measurement by Compulsive Exchange Surface Net Charge and pH o Sign of Net Charge

39 40 .43 50 53 57 66 74

97 97 98 99 102 103 105 109 116 125

129 133 135

137 137 140 140 141 143 145

Contents

Measurement of Permanent Charge in a Mixed System Measurement of Point of Zero Net Charge Specific Surface

ix

146 148 148

References

153

Index

159

Figures 2.1 2.2

The effect of climate on clay mineral occurrence ..... 11 Relationship between specific surface of mineral and the stage of soil weathering 12 2.3 Minerals commonly found in soils 15 2.4 Classification of mineralogy according to Soil Taxonomy 16 2.5 Relationship between soil composition and soil classification 17 2.6 Relationship between inorganic soil constituents and soil orders 19 2.7 Separation of the silicate mineralogy classes into permanent and variable charge types 20 2.8 X-ray diffractograms of crushed basaltic scoria and clay separated from crushed basaltic scoria 24 2.9 Electron micrograph of iron and aluminum oxides cemented in non-crystalline matrix 25 2.10 Electron micrograph of non-crystalline silica precipitated from a water extract of montmorillon~e 26 2.11 Electron micrographs of non-erystalline material stretched over holes in the carbon substrate of the grid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29 3.1 Charging of a hydroxylated surface by protonation and deprotonation 33 3.2 Distribution of electric charges in double layer according to Gouy-Chapman theory 34 xi

xu

3.3 3.4 3.5 3.6 3.7

3.8

3.9

3.10

3.11 3.12

3.13

3.14

3.15 3.16

Figures

Variation in surface charge with electrolyte concentration for pH above and below pH o 37 Distribution of electric charges in double layer according to Stern theory 38 Displacement of aquo and hydroxy group from a metal oxide surface by phosphate 44 Variation of net surface charge with pH and electrolyte concentration for several oxides 45 Examples from the published literature of the variation of net surface charge with pH and electrolyte concentration for variable charge soils 45 The effect of applied phosphorus on pH o of an oxisol sample suspended in NaCI and CaCl2 solutions 47 Variation in net surface charge with (ionic strengthr't for selected highly weathered soils from tropical Queensland 49 Variation in net surface charge with surface potential for some highly weathered soils 51 from Brazil at four electrolyte concentrations Charge reversal in the diffuse layer brought about by adsorption of SO~ in the Stern layer 52 Dependence of total net surface charge in mixed systems on the point of zero net charge (PZNC), pH, magnitude of the permanent charge (ap ) , pH o , and electrolyte concentration 55 Relationship between buffering capacity and pH in a variable charge system at two electrolyte concentrations 67 Changes in net surface charge that might be observed when a variable charge soil is fertilized with an indifferent electrolyte such as KCI and where n2 is greater than n 1 . . . • . • • . • • . . . • . . . . . . 69 The effect of phosphorus application on the CEC of three soils with variable charge minerals 71 Effect of incubation time, application rate, and particle size of crushed basaltic scoria on the CEC of an oxisol 73

Figures

xiii

3.17 Relationship between crop yield and phosphorus applied for a low P-fixing soil and a high Psfixi ng soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 3.18 Phosphorus sorption curves for four clayey soils with different clay mineralogies 77 3.19 Effect of phosphorus in the soil solution on the relative yields of five root crops 78 3.20 Effect of organic matter content in reducing phosphorus sorption in high sesquioxide (R 20 3 ) soils 82 3.21 Phosphate sorption on kaolinite as a function of pH 83 3.2~ The relationship between crop yield and phosphorus application when phosphorus is concentrated in bands of varying widths 85 3.23 The relationship between nutrient uptake ratio UrlUb and the fraction of roots XrlXb immersed in a nutrient solution 87 3.24 Effect of phosphate application on the phosphorus sorption curve of a hydrandept 93 3.25 Phosphorus sorption hysteresis as indicated by the difference in the adsorption and desorption curves .. 94 4.1 Plasticity chart showing location of four different types of soil materials 99 101 4.2 Categorization of soil clay according to activity Water desorption curve for a eutrustox and a 4.3 torrox 106 4.4 Biomodal pore size distribution of aggregated soils from a eutrustox and torrox 108 Cumulative Ca leached from a gibbsihumox as a 4.5 function of leachate volume and P and Si application rate 112 4.6 Cumulative Mg leached from a gibbsihumox as a function of leachate volume and P and Si application rate 113 4.7 Cumulative K leached from a gibbsihumox as a function of leachate volume and P and Si application rate tt3 4.8 Distribution and recovery of Ca applied to soil in leachate, plant tissue, and soil J J4

xiv

Figures

4.9

Surface soil temperatures as a function of time of day for bare soil, soil mulched with 20 T/ha straw, irrigated soil with full potato canopy, and air temperature 4.10 Relationships between heat capacity, thermal conductivity, thermal diffusivity, and water content 4.11 Temperature attenuation and phase shift in the diurnal temperature wave as a function of soil depth 4.12 Sugar yields over 40-year period on some soils in Hawaii 5.1 Scheme for classifying soil materials into permanent, mixed, and variable charge categories 6.1 Variation of net surface charge with pH and electrolyte concentration 6.2 Variation of net surface charge with pH, nature of electrolyte, and electrolyte concentration

121

122

124 126 129 144 146

"

Tables 2.1 2.2 2.3 2.4

2.5

2.6

3.1

3.2 3.3 3.4 3.5 3.6

Selected properties of minerals. . . . . . . . . . . . . . . . . .. 8 Specific surface of common clay minerals in ern? /g . " 9 Occurrence of histosols, oxisols, andisols (andepts), and vertisols in the world 18 The non-crystalline silica and alumina content of clay minerals and soils as determined by the differential dissolutions method of Hashimoto and Jackson 22 Specific surface, cation exchange capacity, surface charge density, and abrasion pH of crushed basaltic scoria 23 Effect of dehydration and rehydration on the cation exchange capacity of soils containing non-crystalline materials 27 pH o obtained by potentiometric titration, and PZNC obtained by adsorption of K+ and Cl" at O.002M for a range of north Australian soils ..... 56 Soil characterization data from Puerto Rico and California 58 Chemical and physical data for a typic hydrandept. 61 Chemical and physical data for a typic eutrandept 61 Selected soil characterization data from Thailand and Brazil 64 Increase over control in exchangeable calcium, magnesium, and potassium when finely ground basalt scoria was applied at 3 rates and incubated at 25°C in a moist condition for 12 months 74 xv

xvi

3.7

Tables

Calculation of yield from banded fertilizer Ur and corresponding banded application rate Rr, for various band widths, using yield Ub from 89 broadcast application rate R 4.1 Activity (plasticity index/% clay-sized particles) of common soil minerals 100 4.2 Effect of drying on Atterberg constants in humic andosols (dystrandepts, hydrandepts) 104 4.3 Change in Atterberg limits after drying at various temperatures and time periods 105 4.4 Comparison of 85% total porosity with measured 107 "field saturated" soil water content 4.5 Effect of P level on CEC and extractable bases in an oxisol 115 4.6 Soil temperature regimes 117 4.7 Measured and estimated soil temperature (OC) at 20 em depth. San Ramon (Peru). Latitude 11° 07' south, longitude 75° 20' west, elevation 800 m ... 117 4.8 Measured and estimated soil temperature (oC) at 20 ern depth. La Molina (Peru). Latitude 12° 05' south, longitude 76° 57' west, elevation 238 m.... 118 4.9 Measured and estimated soil temperature (OC) at 20 ern depth. Huancayo (Peru). Latitude 12° 00' south, longitude 75° 05' west, elevation 3200 m... 118 4.10 Air and irrigated soil temperature atWaipio site (Island of Oahu, Hawaii). Tropeptic eutrustox, clayey, kaolinitic, isohyperthermic. Latitude 21 ° north, longitude 157° west, elevation 150 m 120 4.11 Soil temperatures at a nominal 7.5 em depth in the plant line for sunny as opposed to cloudy periods at Wahiawa, Oahu, beneath pineapple planted in November 1953 121 5.1 Distribution of the major soils of the world 130 5.2 Estimated world distribution of soils with variable, mixed, and permanent charge minerals 134

Preface In wnnng this book one question was constantly kept in mind: Why do soils with variable charge clays behave the way they do? This question forced us to reject an early temptation to write a descriptive book on the characteristics of tropical soils. Instead, the question compelled us to seek analytical models that would enable us to explain and even predict the behavior of soils with variable charge clays. We view soil behavior as the manner in which soils respond to human disturbance and manipulation. Formulation of sound soil-management practices depends on an understanding of the cause-and-effect relationship between soil response and soil manipulation. We are trying to understand why different soils behave differently. To say that different soils behave differently is to say that similar soils behave similarly. To us this is an extremely important statement. If similar soils behave similarly, it is only necessary to classify soils so that soils that behave similarly are grouped together. In a hierarchical or multicategorical classification system, the generalizations one can make about an object increase as one goes up the taxonomic ladder. Conversely, behavioral specificity increases as one goes down the ladder. Objects are separated at the highest category on the basis of important properties. In soils these properties should be fundamental causes of soil behavior. This book makes the assumption that the surface charge characteristics of soil clay are such a property. The degree to which soils within a class behave similarly v,.,u

XVlZl

Preface

must be apparent from the classification systems. We have used a classification system developed by the u.s. Department of Agriculture known as soil taxonomy. The fact that it is necessary to name the soil classification system used in this book points to a major deficiency in soil science. We belong to a discipline that still lacks an internationally approved classification system to identify the objects described. A system is crucial here because this book is intended for an international audience. It would have been easier for us to correlate soils from different parts of the tropics if different countries and regions used identical instead of different classification systems. Not only did the classification systems differ, the differentiating criteria for separating soil classes and the analytical methods for measuring these criteria differed. However, we have known for some time that analytical procedures for characterizing variable charge soil materials were not available. This is another reason why it has not been possible to adequately organize and condense what is known about the behavior of tropical soils with variable charge clays. We have tried not only to identify and apply models to explain the behavior of soils with variable charge days, but also to provide analytical methods to characterize them on the basis of these models. G. U. G. G.

1 Introduction

The title implies that there is something unique and different about tropical soils. If this were not the case, there would be no need for this book because a work with this title would have undoubtedly appeared many years ago if abrupt, visible changes occurred at the boundaries of the tropics. But no such changes are apparent, for the difference between soils in the tropics and elsewhere is not readily discernible by the eye. Indeed there are those who would argue that there is no soil-related problem in the tropics that cannot be explained by experience in the temperate regions. The argument almost always drifts into a discussion about the validity and meaning of the term tropical soils. Tropical soils is a common name used to identify any soil that occurs in the tropics. Like most common names it lacks precision, but it is more readily understood by a larger audience than are the scientific names. The term is frequently used to convey an image of a red, infertile, acid soil; in the past such a soil was called a latosol or lateritic soil. It is largely correct to say that latosols or lateritic soils are tropical soils, but the reverse is not true because there are soils in the tropics that by no stretch of the imagination can be called latosolic or lateritic. For geologically brief periods, the soils on young landscapes or recently deposited pyroclastic or river sediments are rich in nutrients and support a vigorous plant and animal community if water, sunlight, and temperature conditions are not limiting. But the nutrient supply is not limitless, and with time and the effect of warm and humid conditions, soluble nutrients arc

2

Introduction

washed into the rivers and eventually into the oceans. What remains on the landscape is an insoluble residue of highly weathered material that has been stripped of its capacity to retain and supply life-sustaining nutrients. These are the materials that make up the impoverished, extensive, underutilized, and potentially arable lands of the tropics. Unless there is a reversal in current population trends, the nations that own these lands will eventually have to learn to use them. Are the rules that govern soil management different for the highly weathered soils of the tropics? Many think not, because they feel that rules based on fundamental principles apply everywhere. The question would be academic were it not for the fact that tropical agriculture has depended not so much on knowledge generated in the tropics as on experience gained in the technically advanced countries of the temperate regions. The success attained by utilizing this experience has not been high, but there is a continuing temptation to accelerate the process in the hope that soil-related problems can be cured by larger doses of borrowed technology. What is implicit in the title of this work is made explicit in the text, namely, that two fundamentally different soil systems exist in the world-one is concentrated in the temperate regions, and the other in the tropics. Soil science as we know it was largely developed in the- temperate regions, and it takes into account only one of the two soil systems. The two mineralchemical systems found in the clay fraction of soils that form the basis for the distinction are (a) the constant surface charge and (b) the constant surface potential. Montmorillonite, a member of the clay mineral group called smectite, is a typical example of a constant surface-charge mineral. The surface charge on montmorillonite arises from an isomorphous substitution of an ion of lower valence in a position normally occupied by an ion of higher valence. Since ion substitution occurs in the interior of a crystal, the defect and therefore the corresponding charge is permanent and constant. Montmorillonite, like all minerals with constant surface charge, is commonly called a permanent charge mineral. In most permanent charge minerals, the surface charge is net negative and is counterbalanced by cations in the soil solution. These cations

Introduction

3

are an important source of plant nutrients, so the capacity to retain and supply cations is used as one index of a soil's vigor and productivity. Permanent charge clay minerals are common in soils of the temperate regions and are noted for their high cation-retention capacity. In the highly weathered soils of the tropics, minerals with permanent charge have been either severely altered or completely weathered out, so that the surface charge arises from adsorption of potential determining ions. The most important potential determining ions are the hydrogen and hydroxyl ions. Their adsorption on mineral surfaces depends on the soil solution pH, and for this reason minerals that behave in this manner are often called pH-dependent or variable charge minerals. Hematite, the oxide of iron that imparts a red color to many tropical soils, is a typical example of a constant surface potential, variable charge, pH-dependent charge mineral. One reason for the impoverished state of highly weathered soils is their natural tendency to achieve a soil pH corresponding to net zero surface charge. This condition means that a number of essential nutrient elements quickly become deficient through leaching. Although it is now known that virtually all soils consist of a mixture of permanent and variable charge minerals, the nature of surface charge was not generally known until about 1940. In the 1930s and 1940s Sante Mattson published papers (1931, 1932, 1940 [with Wiklander]) on the laws of colloidal behavior in which he correctly assumed the variable surface-charge nature of his synthetic samples. His ideas are valid even today, but soon after Mattson completed his work, Kelly (1944) published a critical and devastating review of Mattson's work. When Kelly published his review, soil scientists were just beginning to appreciate Linus Pauling's work on the nature of the chemical bond and Sir Lawrence Bragg's crystallographic analysis of silicate minerals. Overwhelming evidence was accumulating to indicate that the colloidal soil fraction was crystalline and that certain chemical and physical soil behavior could be linked to isomorphous ion substitution in the crystal lattices of clay minerals. When World War II ended, clay mineralogical research attained unprecedented popularity. New techniques for studying clay minerals, in particular X-ray diffraction

4

Introduction

methods, became readily available and supported Kelly's assessment of Mattson's work. At least two factors contributed to the swing toward the permanent charge concept. First, virtually all of the mineralogical research was conducted on European and North American soils, and the researchers found ample evidence to show that the dominant minerals in the clay fraction were of the permanent charge type. Second, in order to identify and study the mineral species, all organic matter, x-amorphous minerals, free carbonates, and the oxides and hydrous oxides of iron and aluminum were systematically removed from the clay fraction. The variable charge components were treated as contaminants and were largely ignored. It is only fair to state that their existence was never denied, but they were not intensely studied as were the permanent charge minerals. Mattson's work would have received greater acclaim if the soil scientists had given equal time to tropical soils, but the permanent charge concept was so appealing and the information from the tropics so sparse that the acceptance of Kelly's assessment of the situation was assured. Soil Taxonomy, which presents a comprehensive system of soil classification designed to accommodate all soils of the world, was published in 1975 after more than 25 years of development by the Soil- Conservation Service of the U.S. Department of Agriculture. For the first time, soil information assembled according to criteria set forth in Soil Taxonomy and in the earlier Soil Classification, 7th Approximation (1960) enabled the soil scientists to study tropical soils in a systematic and scholarly manner. From these studies emerged a picture of soils with a characteristic mark, namely, soils with low-activity clays. These soils were largely confined to the tropics, but they also extended into the more humid and warmer temperate regions that had been protected from the rejuvenating action of Pleistocene glaciation. New names were coined, and old ones exhumed, to describe the highly weathered soil materials that are so abundant in the tropics. Names such as constant surface potential, pH-dependent, and variable charge were employed to describe the unique chemical features of those soil materials, but the term low activity was introduced to reflect the behavior of soils. It was

Introduction

5

clear that soils with low-activity clays behaved and responded differently to management and manipulation by man. One purpose of developing the soil taxonomy was to organize existing knowledge so that soil information could be used to predict the behavior of soils, identify their best use, estimate their productivity, and permit the assembled information to be transferred to other similar soils. Mohr (1930), a Dutch scientist working in Indonesia when that country was still under Dutch rule, developed his own classification system and categorized soils as being either juvenile, mature, or senile. In terms of soil behavior, Mohr's senile soils correspond to the soil taxonomy's low-activity clay soils. We now know that the island of Java can sustain a dense population because of the youth and vigor of its soils, and that one difficulty that results from transplanting some of the Javanese to Sumatra is related to Sumatra's senile, low-activity clay soils. The health and wealth of the world's peoples are roughly correlated to the richness of the land they cultivate. A poor farmer can become rich on fertile lands, and a rich farmer can make infertile lands productive, but a poor farmer on infertile land has little chance of extricating himself from poverty's grip. In the tropics there is an abundance of poor people living on impoverished soils with low-activity clays. But no new principles are needed to tell a farmer that a crop growing in impoverished soil will respond to fertilizers, that a drought-stricken crop needs water, or that a compacted soil can be loosened by tillage. But new principles are needed to help farmers reduce the risk of failure when they are advised to implement practices unfamiliar to them. The poor nations and the subsistence farmers who live in them have a narrow margin for error. For this reason the most exacting principles are needed to forecast the promised results with great accuracy. This book is about tropical soils that are dominated by variable charge minerals, so it differs from other works that implicitly assume that the colloidal fraction of soils consists of permanent charge minerals. Because there are two soil systems, the transfer of temperate region experience and technology succeeds in some cases and fails in others. Failures cannot be eliminated, but repeated errors that are predictable. and therefore avoidable, can and must be minimized.

2 Mineralogy Clay Mineralogy and Soil Behavior Soil mineralogy is important only insofar as it affects soil behavior, and soil behavior depends to a large extent on mineralogy and texture. In localities where the mineralogy of the clay fraction varies little, one can make generalizations about soil behavior on the basis of texture alone. Soil properties such as water-holding capacity, permeability, cation exchange capacity, and rheology can be estimated from a knowledge of particle size distribution. The integration of these and other relationships into a soil management scheme is one of the first lessons learned by a student of soil science. If a student learns this lesson well and continues to live and work in an environment of unchanging soil mineralogy, another important lesson will be forgotten, namely, that the conceptual relationships between texture and soil behavior need to be modified, sometimes drastically, when one enters an area with a different soil mineralogy. If such a person were shown the data for a soil that had a clay content of 80% and a pH of 5.5, that person might picture a heavy, acid soil that was low in permeability, high in cation exchange capacity, and in dire need of lime. But the data might actually represent soil that had a characteristically good permeability and a low cation-exchange capacity. Persistence would lead to the discovery that such a clay soil possesses a low cation-exchange capacity, but it has a high buffering capacity and would require an inordinate amount of lime to raise its pH from the initial value of 5.5 to neutrality. Moreover a soil scientist might learn Ilt tcrcat cost in time, money, and effort that even crops thllt normally respond to lime might not do so on this type of soil. 7

Mineralogy

8

Soils that behave in this manner are rare in the high and middle latitudes, but they are not uncommon in the tropics. Their chemistry, fertility, and physics, and therefore their management, are altered by their mineralogy. Mineral Properties Soils derive most of their properties from the minerals in them, and some of those properties are listed in Table 2.l. Although minerals impart properties to soils, it is frequently the bulk properties that are of interest to soil scientists. For example, the bulk density of a soil differs considerably from the particle density of the minerals in the soil. The bulk density P{3 is related to the particle density Pp through the expression P{3

= Pp (l

- P)

(2.1)

where P is the porosity. Other bulk properties that vary with porosity are elasticity, electrical conductivity, volumetric heat capacity, hardness, and thermal conductivity. Fine-grained minerals in bulk have additional properties, such as water and air permeability and water retention properties, not normally considered features of minerals. But the bulk factor (1 - P) is not entirely independent of mineralogy. Experience tells us that porosity is generally higher in clay soils than in sandy soils, and also that minerals that

Table 2.1.

Selected Properties of Minerals

1.

chemical compos it ion

11. odor

2.

color

12.

3.

crysta11 inity

13. size

4.

density

14. sol ubil ity

5.

elasticity

15.

6.

electrical conductivity

16. surface charge density

7.

heat capacity

17.

surface tension

8.

hardness

18.

taste

9.

luster

19. transparency

10. magnetism

20.

shape

specific surface

therma 1 conduc t 1v tty

Mineralogy

9

occur in the coarse fraction rarely occur in the fine fraction and vice versa. Thus in Equation 2.1, the porosity would be lower if the mineral were quartz or feldspar and higher if the mineral were kaolinite or montmorillonite. For this reason, size is included as a mineral property in Table 2.1. The term clay mineral implies a group of minerals that normally occurs in the clay fraction. Particle size is of utmost importance in soils because it is related to another property, specific surface. Particle size is related to specific surface through the expression S

=

KIQ

(2.2)

where S is specific surface, K is a shape and density factor, and Q is the size factor. For disk-shaped particles for which disk thickness is one-fifth of the radius r, S

=

4.6

X

1O-21r cm2/g

(2.3)

The fact that most soil clays have specific surfaces greater than 50 x 104 cm 2/g suggests that clay particles are generally less than 2J.Lm in diameter. If a significant proportion of tropical soils differs from soils of the temperate regions, one should look for the difference in the specific surface of clays. An examination of the specific surfaces of common clay minerals provided in Table 2.2 provides a clue as to what may be the cause for the difference. Weathering and Clay Mineralogy Although all four clay minerals occur in all parts of the world, it is safe to say that kaolinitic soils occur more frequently in

Table 2.2.

Specific Surface of Common Clay minerals in cm 2/g

Montmori 11 onite

600 - 800 x 10 4

Vermiculite

4 400 - 800 x 10 4 60 - 200 x 10

Illite (mica) Kaolinite

10 -

50

x 104

Mineralogy

10

the tropics than elsewhere. The reason for the greater abundance of kaolinite in the tropics lies in the stage of weathering of the soils. If, for example, the weathering of the mineral feldspar is examined, it is found that this mineral can progressively weather to mica, kaolinite, and gibbsite in the following manner (Garrels and Christ, 1965). 3KAISi 308 + 2H+ + 12H ZO; KAI3Si301Q(0H)Z + 6H4Si04 + 2K+

t K - feldspar

t

t

K - mica

Dissolved silica

(2.5)

2KAI3Si301O(OH)Z + 2H+ + 3HzO; 3H4AlzSiz09 + 2K+

t

(2.4)

t Kaolinite

K - mica

(2.6)

H4AIZSiZ09 + 5HzO ; Alz03 • 3H zO + 2H4Si04

t Kaolinite

t

t

Gibbsite Dissolved silica

The weathering process can be short-circuited in the following manner (Garrels and Christ, 1965). (2.7)

2KAlSi308 + 2H+ + 9HzO; H4AlzSiz09 + 4H4Si04 + 2K+

t K - feldspar

t Kaolinite

t Dissolved silica

2KA13Si301O(OH)Z + 2H+ + 18HzO ; 3Alz03· 3HzO + 2K+ + 6H4Si04

t . K - mica

t Gibbsite

(2.8)

t Dissolved silica

Warm and humid conditions in the tropics facilitate a rapid removal of potassium and dissolved silica so that feldspar and mica quickly turn into kaolinite and gibbsite. It is no accident that most of the world's bauxite deposits occur in the tropics. Similar equations can be written for the transformation of rock-forming minerals to montmorillonite and other common clay minerals. What mineral is synthesized from rock weathering depends on the concentration and composition of the equilibrium solution. For a given rock, the solution concentration and composition will vary with temperature and water supply, and therefore the climate. The local variations in a microclimate

Mineralogy

11

contribute to a diversity in the soil minerals. Differences in drainage, for example, can result in marked mineralogical differences among neighboring soils. The length of time conditions are suitable for chemical change is also important, and it ranges from 12 months each year near the equator to only a few months each year near the poles. If the gradual and systematic global changes in temperature, moisture, and duration are superimposed on random local variations in the microclimate, a crude but instructive picture of the world's mineral distribution emerges. This distribution is shown in Figure 2.1. If one examines the soils on and near the equator, the highly weathered soils of Ecuador, Colombia, Brazil, Zaire, Sumatra, and Borneo will be found. Notable exceptions in this area are the fertile soils on the rejuvenated landscape of the Andes and the rift zone of Africa. As one moves away from the equator and approaches the dry edges of the tropics and subtropics, montmorillonitic soils, formerly called tropical black earths, cold zone illite, chlorite temperate zone mixed layers, vermiculites tropical zone with contrasting seasons smectite humid tropical zone and equatorial zone

.--- .. _--- .. - ....••.......... _.•....... kaolinite, gibbsite tropical zone with contrasting seasons smectite . temperate zone mixed layers, vermiculites co zone °llite, chlorit Figure 2.1. The effect of climate on clay mineral occurrence. Millot (1979).

Mineralogy

12

occur more frequently. These soils have also been called tirs in Morocco, regurs in India, and rendzinas elsewhere. They are now called vertisols (Soil Taxonomy, 1975), and they cover extensive areas of Mexico and Texas in North America; Morocco, Chad, and Sudan in North Africa; and the Indian subcontinent. In the Southern Hemisphere they occur in Uruguay, Botswana, and Australia. One consequence of rock weathering and soil formation is the synthesis of extremely fine grained clay minerals at the expense of coarser-grained rock-forming minerals. This change causes the specific surface, or the surface area per unit mass, of soil to increase markedly with weathering. There is probably no other single property more important to soil behavior than specific surface. Since specific surface increases with weathering, one might reason that a maximum specific surface is attained in the most highly weathered soils of the tropics. It turns out that this presumption is not correct. If one follows a feldspar crystal as it weathers first to mica or montmorillonite and then to kaolinite or gibbsite, one sees marked .initial increase in specific surface followed by a gradual reduction to low specific-surface values in the final stages of weathering as shown in Figure 2.2.

T

-.u.. u

w

lL

en

rock forming minerals feldspar

YOUNG

mica montmorillonite

kaolinite gibbsite

gl bbslte nodules Iron stones

MATURE STAGE

Of WEATHER ING

SENILE ---t

Figure 2.2. Relationship between specific surface of mineral and the stage of soil weathering.

Mineralogy

13

The differences in surface area alone are not sufficient to account for the differences between soils of the tropics and temperate regions. If one examines the coordination number of aluminum in feldspar, mica or montmorillonite, and kaolinite or gibbsite, one discovers that the aluminum is entirely 4-coordinated in feldspar, 4- and 6-coordinated in mica and montmorillonite, and entirely o-coordinared in kaolinite and gibbsite. In feldspar, trivalent aluminum substitutes for tetravalent silicon, and this substitution results in a net negative surface charge, which is satisfied by potassium or sodium. The charge on the surface of a feldspar crystal is negative and constant. Under proper conditions feldspars dissolve to reprecipitate as mica or montmorillonite, and in mica and montmorillonite some of the aluminum substitutes for silicon so that, like feldspar, the surface charge is net negative. This negative charge is counterbalanced by potassium or sodium. When weathering brings about the kaolinite and gibbsite stage, all aluminum is 6-coordinated, and the permanent surface-charge density, so crucial to the retention of nutrient cations, abruptly disappears. The feature that renders tropical soils different from soils of the temperate regions is the product of the specific surface S and surface charge density a. This product is the cation exchange capacity (CEC) of a soil and can be written as CEC

= Sa

(2.9)

The CEC is a measure of a soil's capacity to retain and supply cationic nutrients such as potassium, calcium, and magnesium to plants, and it is often used as an index of soil fertility. An immature soil rich in feldspar has a low CEC because feldspars occur as coarse particles. Coarse minerals have low specific surfaces and, therefore, low CECs. The weathering of feldspars and other weatherable minerals results in the release of soluble ions that serve as plant nutrients and as a source for the synthesis of clay minerals. Mica (illite) and montmorillonite have relatively high CECs because they possess moderate to high specific surfaces and surface charge densities. In kaolinite and gibbsite, the specific surfaces are low to moderate, but the surface charge attributable to isomorphous substitution is low

14

Mineralogy

or zero, so that the CEC that is the result of the permanent surface charge is negligible. The feature most frequently associated with highly weathered tropical soils is a low CEC of the clay fraction. A prolonged weathering of soils under warm and humid conditions results in the dissolution of permanent charge minerals and the subsequent precipitation of insoluble residues that are incapable of providing adequate nutrients to plants on a sustained basis. Chemistry and fertility are not the only two qualities of soils; the specific surface and surface charge densities affect the physics of soils as well. They affect such properties as the capacity to store and transmit water, gases, heat, and solutes, and they also affect rheology and therefore the bearing capacity, tillability, and trafficability of soils. Swelling, cracking, and crusting of soils are also related to specific surface and surface charge density. In soils dominated by permanent charge minerals, specific surface and surface charge density are not subject to human manipulation. Thus the chemistry, physics, and fertility of those soils are managed by manipulation of the concentration and composition of the soil solution. On the other hand, in soils dominated by variable charge minerals, the surface charge density is often unintentionally altered by farming practices. The exploitive practices of the shifting cultivator or the sedentary subsistence farmer generally result in a lowering of the surface charge. If the subsistence farmer can lower the surface charge, can the process be reversed? The title of this book implies that it can. It is not only the farmer who unknowingly alters the surface charge characteristic of a variable-charge soil mineral but the laboratory technicians as well. The chemical and physical methods used to measure the physical and chemical parameters for soil management recommendations can alter the very parameters being measured. The most obvious and desirable way to circumvent the laboratory analytical problem would be to develop methods appropriate to the variable charge system. Several methods are outlined in Chapter 6, but in general, few standard procedures for charac-

Mineralogy

15

terizing variable charge samples are available. This unfortunate situation means that the laboratory technicians must use the same procedures to characterize two different systems. A user of the laboratory data must know the soil's mineralogy and have the skill to interpret the data properly, particularly when the mineralogy is of the variable charge type. To do this one must be able to distinguish between permanent and variable charge minerals. Mineralogy and Soil Classification Virtually every soil mineral can be placed in one of the categories listed in Figure 2.3. All major categories, with the exception of the silicates, contain minerals or materials of the variable SOIL CONSIITUENTS I

WATER

I

AIR

SOLID

I

I

INORGANIC

I

ORGANIC

..----1_-----, I

i

CRYSTALLINE

NON-CRYSTALLINE

Hydrous oxides of iron, aluminum and

silicon

Alumino-silicates

(1) plant opal (2) hydrated gels

all ophane (2) imagol ite (3) glass

oxides and hydrous oxides of: 1. Iron 2. Aluminum 3. Manganese 4. Silicon 5.

Titanium

(1)

slightly soluble salts

silicates Nesosilicates Sorosil i cates Cyc Iosili cates Inosilicates 5. Phy110silicates 6. Tektosili cates 1. 2. 3. 4.

1. 2. 3. 4.

Carbona tes Sulfates Sulfides Phosphates

Figure 2.3. Minerals commonly found in soils.

soluble salts

16

Mineralogy

charge type. Organic matter, non-crystalline materials, the oxides and hydrous oxides, and the slightly soluble carbonates and sulfates can all be treated as variable-charge soil constituents. Even among the silicates, the important permanent-charge minerals are largely confined to the phyllosilicates, and even that group is not entirely made up of minerals with permanent charge. For example, kaolinite and halloysite are phyllosilicates that have a variable surface charge. In Soil Taxonomy (1975) mineralogy appears in the family category of the classification scheme, and excluding mixed mineralogies, the soil taxonomy recognizes 15 mineralogical classes. These classes are placed under broad categories in Figure 2.4. Nine of the 15 classes fall under the silicates, which indicates the importance of silicate minerals in soils. None of the classes appears under organic matter or non-crystalline materials. SOIL CONSTITUENT

I

I

I

I

INORGANIC

I

CRYSTALLINE

OXIDES AND HYDRDUS OX IDES Ferritic Gibbsitic Oxidic Siliceous

i

SOLID

WATER

I

SILICATES Chloritic Glauconitic Halloysitic Illitic Kaol initic Micaceous Montmorillonitic Sepenti ni ti c Vermiculitic

AIR

I

ORGANIC

I

NON-CRYSTALLINE

SLIGHTLY SOLUBLE SALTS

SOLUBLE SALTS

Carboni tic Gypsic

Figure 2.4. Classification of mineralogy according to Soil Taxonomy.

17

Mineralogy

The behavior of soils that consist predominantly of organic matter or non-crystalline materials is so intimately related to the soil composition that for them the soil taxonomy replaces mineralogy with other indicators of soil behavior. In organic soils, the differences in soil behavior arise in part from differences in the degree of decomposition of the organic residue. These differences in the degree of decomposition are accommodated at the suborder level in the soil taxonomy. For soils dominated by non-crystalline materials, the differences among soils are accounted for by particle-sized modifiers specifically designed for these soils. In fact mineralogy plays such an important role in forming the character of a soil that the key features employed to differentiate soils at the highest level depend on mineralogy. For this reason a strong correlation exists between mineralogy and the soil orders of the soil taxonomy, as is illustrated in Figure 2.5. Figure 2.5 consists of a tetrahedron whose apexes represent pure end-members of soils containing organic matter, oxides,

Organic matter

Oxides and

6\._.- -I-

-vXISO

s-

.---- ---.

Non- crystalline material

hydrous oxides

Smectites

Figure 2.5. Relationship between soil composition and soil classification.

Mineralogy

18

non-crystalline material, and smectite. The four soil orders of the soil taxonomy that most closely correspond to the endmembers are histosol, oxisol, andisol (andept), and vertisol. (Andisol has been proposed as a new order for soils now classified as andepts by Dr. Guy D. Smith, the major author of Soil Taxonomy.) Three of the four end-members are dominated by minerals with a variable surface charge. This of course does not imply that 75% of all soils are dominated by variable charge minerals. Soils that approach the four end-members in composition are not abundant in nature-the histosols, oxisols, andisols (andepts), and vertisols make up only 13% of the world's soil resources, as shown in Table 2.3, and it is quite possible that the occurrence of oxisols has been overestimated. Although the soil orders nearest the apexes of the soil composition tetrahedron are not abundant, they deserve careful study because they help to explain the behavior of the soils that are in the intermediate positions. If only the inorganic soils are considered, four more soil orders can be placed in intermediate positions within a triangle representing the inorganic soils, as illustrated in Figure 2.6. The ultisols fall near the oxisols, the spodosols near the andisols, and the mollisols near the vertisols. The inceptisols are young soils that are not yet developed enough so that it is clear where they belong, and therefore they are placed in the center. There are three soil orders that do not fit into this scheme, alfisols, aridisols, and entisols. The aridisols do not fit because

Table 2.3. Occurrence of Histosols, OXisols, Andisols (Andepts) and Vertisols in the world Soil associations dominated by Histosols Oxisols (Ferralsols) Andisols (Andepts) Verti sol s Total Source:

Dudal, 1976

in 1,000 ha 240,200 1,068,450 100,640 311,460 1,720,750

in percent 1.82 8.11

0.76 2.36 13.05

Mineralogy

19 (variable charge clay minerals)

oXIDEs..-AND HYDROUS OXIDES

....,~NON-CRYSTALLINE

MATERIALS

SMECTITES

(permanent charge clay minerals) Figure 2.6. Relationship between inorganic soil constituents and soil orders.

they are classified on the basis of aridity, and the entisols do not fit because they lack evidence of pedogenic development. Alfisols, however, are a different matter because they, and to a lesser extent the ultisols, in some cases fall near the oxisols and at other times near the mollisols and vertisols in mineral composition. Thus alfisols in the tropics frequently contain variable charge clays, much like oxisols, whereas those in the temperate regions contain permanent charge clays. Considerable effort has been expended to rectify these problems, and it is likely that when a revised edition of Soil Taxonomy is issued, they will be resolved. Silicate Mineralogy Classes In Figure 2.7 the silicate mineralogy classes are subdivided

into permanent and variable charge types. With the exception of the serpentinitic class, which permits some inosilicates, all other classes fall in the category of phyllosilicates, which are given prominence in the soil taxonomy because they possess large specific surfaces and impart more key properties to soils on a unit weight basis than do the other silicates. Of the nine mineralogy classes in Figure 2.7, five are clearly

20

Mineralogy

SILICATES

Permanent Charge Chloritic Glauconitic Illitic Micaceous Montmorillonitic

Variable Charge Chloritic (aluminum interlayered) Ha 11 oysiti c Kao1i niti c Serpentinitic

Vermicul itic Figure 2.7. Separation of the silicate mineralogy classes into permanent and variable charge types.

of the permanent charge type, and three are clearly variable charge in character. The chloritic mineralogy class is an exception. Under acid conditions, minerals such as vermiculite and montmorillonite can be transformed into a chloritelike mineral by incorporating solubilized aluminum in the interlayer spaces. In this manner montmorillonite and vermiculite lose their permanent charge and extensive surface in the crystal interior. Montmorillonite and vermiculite that are altered in this way are sometimes called aluminum-interlayered chlorites and approach kaolinite in property. They are very common in naturally acid soils, in and out of the tropics. Rock-forming chlorites, on the other hand, behave for the most part as permanent charge minerals. Mixed Mineralogies and Non-crystalline Materials Mixed mineralogies are the rule rather than the exception. To avoid an overuse of the mixed mineralogy class, the soil taxonomy places strict boundaries on this class. Thus a clayey soil

Mineralogy

21

with 55% kaolinite and 9% montmorillonite is kaolinitic in mineralogy. Similarly a clayey soil with 55% kaolinite and 10% free sesquioxides is also kaolinitic in mineralogy. Although it would be impossible to find a montmorillonitic oxisol or a ferritic vertisol, kaolinitic vertisols and oxisols do exist. In both cases kaolinite would be the dominant mineral, but the accessory minerals would be sesquioxides for the oxisol and montmorillonite for the vertisol. A third component, often overlooked because it is non-crystalline and therefore not detectable by X-ray diffraction analysis, plays an important role in soil behavior. In oxisols with kaolinitic mineralogies, the noncrystalline or amorphous fraction is silica-poor and sesquioxiderich, which results in a low silica-sesquioxide ratio. In vertisols the silica-sesquioxide ratio of the amorphous fraction is high. Before X-ray diffraction methods came into use in soil science, a high value was placed on the silica-sesquioxide and silica-alumina ratios of soil colloids. Mattson (1927, 1928) was able to explain the properties and behavior of his synthetic mixtures on the basis of their silica-sesquioxide ratios. He showed, as others before him had shown, that the cation exchange capacity of his synthetic soils increased as the silicasesquioxide ratio increased. Mattson's generalizations still apply to the non-crystalline fraction of soils, but that fraction was largely forgotten when soil colloids were demonstrated by X-ray studies to be largely crystalline. Interest in the non-crystalline fraction was revived when Hashimoto and Jackson (1958) published a method to measure that component in soils and sediments. To this day the noncrystalline fraction remains the least-understood soil component. Table 2.4 shows examples of the quantity of non-crystalline silica and alumina extracted from soils and "pure" minerals by the Hashimoto and Jackson method. It is clear from Table 2.4 that soil clays contain significant amounts of non-crystalline materials, and even "pure" montmorillonite used as a standard contains 6.9% amorphous silica. The ability of the Hashimoto and Jackson method to differentially remove non-crystalline materials from soils without attacking the crystalline solid has been questioned, but Jones and Uehara (1973) were able to detect non-crystalline, gel-like

22

Mineralogy

Table 2.4

The non-crystalline silica and alumina content of clay minerals and soils as determined by the differential dissolutions method of Hashimoto and Jackson

Mineral or Soil All ophane * All ophane * Ha 11 oysi te * Ha l Ioys t te Kaolinite* Kaol inite * Kaolinite Montmorillonite Oxisol (oxic horizon) Oxisol (oxic horizon) Andept (Eutrandept) Andept (Hydrandept)

* Source:

Si0 2

A1203

Si0 2/ A1203

%

%

mo 1e ratio 1. 31 1.40 1. 34 1.80 2.96 8.02 4.22 39.12 1. 17 1. 31 2.09 0.94

29.4 28.8 17.2 9.8 3.5 3.3 2.6 6.9 8.2 9.2 13.1 7.3

Hashimoto and Jackson

38.3 36.5 21. 7 9.1 2.0 0.7 4.9 0.3 11.8 11.8 10.5 12.9

(1958)

coatings on mineral surfaces by electron microscopy. These non-crystalline materials may very well be the most active of all soil components and influence soil behavior to a greater extent than has been thought. Gillman (1980) examined the properties of partially weathered basaltic scoria and measured high specific-surface and cationexchange capacities in relatively coarse materials, as shown in Table 2.5. A high specific surface is possible in coarse materials if the particles are cemented aggregates of smectites or zeolites. An X-ray diffraction analysis of the crushed scoria identified large quantities of feldspar, olivine, and other minerals common to basaltic rock but revealed no smectite or zeolite. However, the clay fraction contained amorphous materials in addition to crystalline rock-forming minerals, as is indicated by the broad X-ray band B in Figure 2.8. Amorphous materials are difficult to detect and generally go unnoticed when they occur in quantities below 10% by weight. When amorphous materials coat crystalline particles, the particles take on many of the features of the coating. When coated particles come in contact with each other, the amorphous coatings coalesce to form aggregates as shown in Figure 2.9. The

Mineralogy

23 Table 2.5

Particle Size % < 250 microns

Specific surface, cation exchange capacity, surface charge density and abrasion pH of crushed basaltic scoria

Specific Surface M2/ gm

Surface Cation Exchange Cha rge D~ns i ty Capacity me/cm mellOO gm

Abrasi on pH

12.3

1.1 x 10-5 1.6xlO- 5

8.2

16.3

z.r

8.5

14

84

9.5

75

78

98

78

v

io?

7.8

cements that bind particles together in soil aggregates, surface crusts, and duripans are most likely non-crystalline materials. Some non-crystalline materials dehydrate irreversibly into aggregates of silt, sand, and gravel size as in the hydrandepts (Soil Taxonomy, 1975, p. 235). In such soils the weathering produces a gelatinous mixture of non-crystalline, silica-poor aluminosilicates, free sesquioxides, and organic matter. The soil is extremely porous, with bulk densities between 0.2-0.5 g/crrr' , and is highly hydrated. The cement that binds the particles in laterite is most probably of this nature. In arid regions, the cement that binds the particles in surface crusts that form after irrigation and dehydration is most probably silica. Uehara and Jones (1974) showed in a discussion of crust cementation that the water soluble residue from a montmorillonite suspension is amorphous silica, which appears as strings of spherical beads of coalesced silica bodies (Figure 2.10). Their findings are consistent with the high amorphous-silica content associated with montmorillonite that was reported by Hashimoto and Jackson (1958). Unlike the amorphous or non-erystalline materials, which are rich in sesquioxides and which dry irreversibly and form stable aggregates, amorphous silica appears to shift reversibly from a viscous state when moist to an elastic state when dry. For this reason arid-region crusts, which form in irrigated furrows, soften and harden with wetting and drying. The degree to which the amorphous materials change reversibly between the viscous and elastic states may very well depend on the silica-sesquioxide

Ii

j

I

r~ I I

II

r

U d II

11111111111111111111111111111111

64 60 56 52 48 44 40 36 32 28 24 20 DEGREES2e

16

12

8

4

Figure 2.8. X-ray diffractograms of (A) crushed basaltic scoria and (B) clay separated from crushed basaltic scoria.

~

Mineralogy

25

Figure 2.9. Electron micrograph of iron and aluminum oxides cemented in non-crystalline matrix (arrow B). Shrinkage crack (arrow A) has occurred in the non-crystalline material from heat from the electron beam. The particle located above "C" is an anatase crystal (Jones and Uehara, 1973).

ratio of the amorphous mass and on the concentration and composition of the ambient electrolyte solution and pHall of which affect the sign and magnitude of the variable surface charge. Specific Surface of Non-crystalline Materials Although fine-grained solids may be characterized by a welldefined specific surface value, non-crystalline substances-which can flow, coalesce, and form contact angles with other materialsvary in specific surface with deformation or dehydration. Amorphous materials in hydrandepts, which irreversibly dehydrate into aggregates of silt, sand, and gravel size, lose specific surface with dehydration. Kanehiro and Sherman (1956), for example, demonstrated that the cation exchange capacity of hydrandepts decreased measurably with dehydration (Table 2.6). Since the cation exchange capacity is the product of specific surface and surface charge density, a reduction in specific surface must be accompanied by a reduction in cation exchange capacity. A reduction in the surface charge density can also lower the cation

26

, 1#

I

Figure 2.10. Electron micrograph of non-crystalline silica precipitated from a water extract of montmorillonite (Uehara and Jones, 1974).

Table 2.6 Effect of dehydration and rehydration on the cation exchange capacity of soils containing non-crystalline materials

Soi1 Great Group

Depth (em)

Field Soil water Cation Exchanqe Capacity (me/100 Qm) Sun-dried Sun-dried Content "tresli oven-dried 100 days 100 days and Sample 7 days % rehydrated 7 months

Loss due to sun-drying

Loss after rehydration %

%

-

Hydrandept ( low silica-sesquioxide ratio)

0 30 60 80

-

30 60 80 95

130.1 274.5 305.7 217.0

82.2 81.2 109.9 126.7

48.0 48.1 45.4 42.9

36.8 36.2 37.1 32.1

39.9 38.1 35.0 31.8

55.2 55.4 68.2 74.7

51.4 53.1 68.1 74.9

Eutrandept (high silicasesquioxide ratio)

0 19 40 70

-

19 40 70 100

8.7 40.8 49.3 13.4

39.7 98.9 99.5 68.6

28.9 64.0 78.2 62.9

30.4 67.7 72.9 57.9

40.7 85.8 85.8 67.0

23.4 31.5 26.7 15.6

-013.2 13.8 2.3

Source:

Kanehiro and Sherman, 1956

N 'J

28

Mineralogy

exchange capacity, but Balasubramanian and Kanehiro (1978) reported results that clearly show that the surface charge density at pH 7 increases with dehydration. Kanehiro and Sherman's (1956) work also shows that some of the cation exchange capacity lost through dehydration can be recovered by rehydration. They showed that the greatest percentage loss in cation exchange capacity occurs in soils with low silica-sesquioxide ratios and the greatest recovery of cation exchange capacity lost through dehydration occurs in younger, less-weathered soils with high silica-sesquioxide ratios. The large reduction in cation exchange capacities caused by soil drying prompted Kanehiro and Sherman to look for seasonal fluctuations in the CEC. They measured the cation exchange capacities of vertisols, mollisols, ultisols, and oxisols and noted a consistent drop in CECs during the dry season. The reduction ranged from 9.5% to 17.0% in vertisols and mollisols and 17.0% to 28.2% in ultisols and oxisols. Since the cation exchange capacity is generally considered to be a stable constant in mineral soils, it would be preferable to attribute variations in the CEC to laboratory error or to changes in the soil's organic matter content. The fact remains that in soils high in noncrystalline materials, these changes in cation exchange capacities can be reproduced in the laboratory. The most plausible explanation appears to lie in a lowering of specific surface caused by the shrinkage of, and cementation by, non-crystalline substances. If one accepts this premise, one must also accept the fact that seasonal changes in specific surface ranging from 9% to 28% must occur. This means that in a vertisol with 50% clay and a moist specific surface of 300 m 2/g, a change in specific surface of 30 m 2/g between the wet and dry season is possible. In a clayey oxisol with a specific surface of 80 m 2/g, a 15 m 2/g change is possible. These are not insignificant changes, and it is not easy to picture the specific surface of a soil fluctuating by 10% or 20%. Figure 2.11 is an electron photomicrograph of a film of noncrystalline substance stretched across a hole in a carbon substrate. Although stretching may account for some changes in surface area, it is more likely that cementation and changes in the porosity of the non-crystalline materials account for most :

Mineralogy

29

Figure 2.11. Electron micrographs of non-crystalline material stretched over holes in the carbon substrate of the grid. "Au is a material of low silica-sesquioxide ratio from a Hydrandept and "8" is material of high silica-sesquioxide ratio from a Eutrandept (Jones and Uehara, 1973).

of the surface area changes. High sesquioxide substances lose their porosity irreversibly, whereas high silica materials appear to regain their porosity and therefore their cation exchange capacity with rehydration, as is suggested by the data in Table 2.6. Cation Exchange Characteristics of Soil Minerals From the preceding discussion it is possible to categorize minerals into three groups on the basis of their surface characteristics. Expressing the cation exchange capacity CEC as the

Mineralogy

30

product of specific surface S and surface charge density categories can be described as

0,

these

CEC = Scoc

(2.10)

CEC = Scov

(2.11)

CEC = Svov

(2.12)

where the subscripts c and v indicate a constant or a variable. The typical soil contains all three mineral types but is generally dominated by one type. The vertisols, for example, can be adequately handled by Equation 2.10; the oxisols, by Equation 2.11; and the andepts (andisols), by Equation 2.12. Considerable effort is still needed to develop models that can describe mixed systems, but a simple case of a mixed permanent and variable charge system is described in the next chapter.

3 Chemistry Origin of Charge on Soil Colloid Surfaces The surfaces of soil colloidal materials are electrically charged, whether the colloid be of mineral or of organic origin; in other words, the surface has an excess or a deficit of electrons. It is the purpose of this section to describe the origin of this charge and the factors that influence its formation. The minerals discussed in the previous section can be separated into two general types with respect to the origin of their surface charge. They are (a) the constant surface-charge and (b) the constant surface-potential clay minerals (Overbeek, 1952). The separation is not a rigid one because a single soil mineral can exhibit both types and, in addition, soil colloids usually consist of an intimate admixture of various minerals. But the broadest treatment of surface charge origin is best presented by describing the end-members and then discussing the surface charge characteristics of actual soil colloids. (a) Constant Surface-Charge Minerals

A perfectly formed crystal lattice would possess no excess charge at the surface because all atoms in the crystal would be electrically balanced. Imperfections in the lattice structure, however, cause an excess of positive or negative charge, which is then compensated for by the accumulation of oppositely charged ions (counterions) at the crystal surfaces. Such an imperfection, for instance, might be the substitution of a trivalent aluminum atom in a silicate sheet, which would lead to an excess of negative charge at the particle surface. The substi31

32

Chemistry

tution of trivalent aluminum for divalent magnesium would lead to an excess of positive charge at the surface. As this defect occurs in the interior of the crystal lattice, the resulting charge imbalance is permanent and cannot be influenced by external factors such as the pH of the ambient solution; we therefore have a constant surface-charge colloid. (b) Constant Surface-Potential Minerals

In this general type, surface charge is created by the adsorption of ions onto the surface, the net charge being determined by that ion which is adsorbed in excess. The charging process requires the presence of these ions, called potential-determining ions, in the ambient solution in quantities sufficient for adsorption. An example will explain this more clearly. When suspended in an aqueous solution, the iron oxide hematite has a hydroxylated surface layer that is capable of sorbing and desorbing protons (H+) and hydroxyl ions (OH-). The sorption is governed by the'activity of H+ and OH- in solution (i.e., by the pH). An acid medium will cause an excess of H+ to be adsorbed, whereas an alkaline medium will result- in the adsorption of excess OH- as shown in Figure 3.1.\The net adsorption determines the electrical potential of the oxide surface, so that H+ and OH- are the potential-determining ions. For any given pH, the surface potential is fixed, and a constant potential colloid exists. In the past, such minerals have often been called pH-dependent charge minerals, because the surface charge is dependent upon solution pH, but as will be seen later, in this constant potential system, the magnitude of the surface charge is influenced by other conditions, so that the more general term, variable charge minerals, is preferred. Though the above discussion has centered around inorganic minerals, it should be remembered that organic matter is a very important constituent of soil colloidal material. The functional groups of organic matter such as carboxyl and amino are able to sorb and desorb H+ and OH- in a manner similar to the mineral oxides, so organic matter is a good example of a constant potential or variable charge system.

Chemistry

33

3+

,,1/

0

Fe

/! "OH

"1/ Fe

/1"

3-

0

,,1/

oH

"1/ Fe

Fe

3H+ • / !"OH '\.1/ Fe /I"OH

0

30H~

/1,\ + o 0 3HOH

'\.1/ Fe

/1"0

Figure 3.1. Charging of a hydroxylated surface by protonation and deprotonation.

Gouy-Chapman Theory Regardless of the origin of the charge on a colloid surface, the laws of electrical neutrality dictate that an equal amount of charge of the opposite sign will accumulate in the liquid phase near the charged surface. Thus positively charged cations would be attracted to a negatively charged surface. Diffusion forces, however, tend to draw them back toward the equilibrium solution, where their concentration would be diminished. Thus an "atmospheric" distribution of cations in a "diffuse layer" is established in which their concentration increases toward the surface from a value equal to that of an equilibrium solution to a higher value principally determined by the magnitude of the surface charge. At the same time, negatively charged anions will be repelled by electrostatic forces, with diffusion forces acting in the opposite direction so that there is a deficit of anions close to the surface. The charged surface and the redistribution counterions constitute the electrical double layer, which can be depicted as in Figure 3.2. Based on such a model, a simple theory developed by Gouy (1910), and independently by Chapman (1913), related the density of charge on the surface to the electrical potential across the double layer. The formulas will not be derived here as they

34

Chemistry

--

+

+

-

-.. - + + '"e

.em Q)

+

+

+

'"m• - + + z - + -- + + Q)

+

+

-

-

-

+

+

-

+

+

+ + ;+ ·

+

.s:

·· ··: ···· · ··

-

:·

·: ··· ··

+:·

-

+

+1

- +

:::) In

~

+ +

+

Q)

: .-

+

+

-

-

-

+

- +

-

+

-

+

+

+

+

-

+

-

Bulk Liquid

Diffuse Layer

Figure 3.2. Distribution of electric charges in Double Layer according to Gouy-Chapman theory.

have been adequately dealt with in other texts (see, e.g., van Olphen, 1963). The Gouy-Chapman equation is Uo

where

= n = Uo

= k = T = z = e = 00 = e

- (2nekT)V" -

7r

. h~ 2kT

SIll

0.

~0

(3.1)

surface charge density (esu/cm") counterion concentration in the equilib. solution (ion/ern") dielectric constant (esu 2/dyne ern") Boltzmann constant (ergs/degree) absolute temperature counterion valence charge of an electron (esu) surface potential (esu)

This expression shows that the charge on the particle surface is dependent upon the following:

Chemistry

1. 2. 3. 4. 5.

35

potential difference across the electrical double layer electrolyte concentration valence of the counterion dielectric constant of the medium temperature

We will now explore the consequences of applying this equation to constant charge and constant potential systems. (a) Constant Surface Charge

As was described in Chapter 2, surface charge is controlled in this system by lattice defects in the interior of the crystal, so that double layer potential, concentration of electrolyte, and counterion valence are not able to influence the sign or magnitude of the surface charge. Equation 3.1 therefore takes the form

( 2 n1T€k T ) ~

. ze smh 2kT 0 0 = constant

so that if the electrolyte concentration or valence is increased, or if the dielectric constant of the medium is increased, the electrical double-layer potential difference must be concomitantly reduced. This is accomplished by a reduction in the distance that the double layer extends into the equilibrium solution; i.e., the "thickness" of the double layer is reduced or, as commonly expressed, the double layer is "compressed." (b) Constant Surface Potential

In this system, the surface potential is controlled by the adsorption of potential determining ions, which in turn depends on the activity of those ions in the equilibrium solution. Thus, assuming a certain activity in solution, Equation 3.1 now takes the form °0

=

T)~ ( 2n€k 1T

. ze smh 2kT (constant)

It is assumed that the concentration of the potential determining ions has a negligible effect on the value of n. Thus if the electrolyte concentration or valence, or the dielectric constant,

36

Chemistry

is increased, the effect is to increase the charge on the particle surface. To return to the example of the iron oxide, hematite, where H+ and OH- are the potential determining ions, the potential is governed by the H+ or OH- activity in solution, i.e., by the pH. The Nernst equation 00

= kT e

In aI;!+ aH+

= 2.303kT (pH o - pH) e

(3.2)

relates surface potential 0 0 to pH, where pH o is the pH at which the surface potential is zero. Thus by combining Equations 3.1 and 3.2, we have ao

r2nekT] liz sinh . = [7f 1.15 z(pHo -pH)

(3.3)

This is an important equation, for it shows that at some point on the pH scale, ao can be zero. There is, therefore, a pH at which the oxide surface has zero net charge. At present there is confusion in the literature as to the name to be given to this pH value, and we prefer to simply designate it as pH o . Equation 3.3 can be simplified if sinh 1.15 z(pH o - pH) is less than one, because at these values sinh x "" x. Then [2nekTJ liz ze . [87fne 2 Z2 ] liz ao = [7f 2kT 0 0 and If we let K. = ekT '

L

a - etc 0 - eK. • 2.303kT (pH o - pH) o - 47f 0 - 47f e

(3.4)

These conditions are met if ~

2kT 0 0

_ ze • 2.303kT _ _ _.,;;: 2kT e (pH o pH) - 1.15 z(pH o pH) """ 1

-

i.e., if the soil solution pH is within ± 0.5 units of pH o when z = 2, or within ± 1 unit of pH o when z = 1. Equation 3.4 indicates that over a small range of pH near plio, the surface charge is linearly related to .../iz- and to z and

Chemistry

37

-1,0

+0·5

+1·0

+1·5

+ Figure 3.3. Variation in surface charge with electrolyte concentration for pH above and below pH o.

thus can be illustrated graphically as in Figure 3.3, where beyond the pH limits specified above, a sinh function would be followed. Such an illustration clearly shows the dependence of variable charge not only on pH, but also on electrolyte concentration and valence. The dependence of surface charge on the dielectric constant should also be remembered. Stern Theory The Gouy-Chapman equation has only limited use because even with moderate surface potentials (e.g., 250 mY), absurdly high values are predicted for the amount of counterions adsorbed into the diffuse layer. This is because the Gouy-Chapman equation does not take into account the actual size of the counterions, which are treated as point charges. The Stern theory (Figure 3.4) attempts to correct this deficiency by allowing the ions to approach the surface to within a certain minimum distance (a few angstroms). Thus there is a counterion-free, uncharged space (of width d) between the surface and the first layer of adsorbed counterions, and the electrical potential drops linearly from 0 0 to 0d . The remainder of the double layer consists of a diffuse layer of counterions, as described in the Gouy-Chapman

Chemistry

38

-G)

-. .. u

0

:::)

en ."

--

G)

.c

-'

G)

u

en

•

z

>-

0

c

0

G)

.

G)

.

m

m

+

-

+

+~

+- + +

+

+

+

+

+

Diffuse Layer

+

+

+ + + + + + + + + + + +

··· ···· .··

·.··

~+ ·.

+ +

+

-

+

+

+

+

+

·· +~ + ·-

+

+

+

Bulk Liquid

Figure 3.4. Distribution of electric charges in Double Layer according to Stern theory.

theory, in which the potential decays exponentially from 0d to zero. The charge in the Stern (or compact) layer was derived by Stern (1924) as Nze N

1+ ;

(3.5)

exp [-(ze0 d + 0s)/kT]

where N = no. of adsorption sites available per crn' of surface NA = Avogadro's number M = molecular weight of the solvent 0s = specific adsorption potential At this point it should be noted that Stern introduced the concept that some ions might be adsorbed into the compact layer by forces other than those that are purely electrostatic and that. the energy required for this is accounted for by 0s ' the specific adsorption potential. In the absence of specific adsorption,

39

Chemistry

05 = zero. The charge in the diffuse layer

a2

f2nekT] Y:z • h ze sin 2kT 0d

= [1r

(3.6)

and electrical neutrality demand that (3.7)

Experimentally there are too many unknown parameters to solve these equations until a further expression for a molecular condenser (with which the compact layer is likened) is introduced. The equation is (3.8)

where e' is the dielectric constant in the compact layer, and d is the thickness of that layer. Of course, e' and d are not determinable quantities, but, as shown by van Raij and Peech (1972), appropriate ratios of e'/d can be introduced in the computations. Other Theories There have been a number of further modifications to double layer theory in attempts to more fully explain experimental observations. Perhaps the most notable is that of Grahame (1947) who, working with the mercury electrode, postulated that anions are specifically adsorbed into the Stern layer when they lose some of their water of hydration, whereas the hydrated cations are only electrostatically attracted to the surface. On the other hand Bolt (1955) modified the Gouy-Chapman theory to allow for the effect of ion size, the coulombic interaction energy between ions, and the polarization energies of ions in the electric field without resorting to a specific adsorption mechanism. However, in order to use the double layer theory just outlined to explain soil colloid phenomena, there is little point in. proceeding any further than the Stern modification.

Chemistry

40

Ion Exchange The discussion so far has indicated that when a charged particle is placed in an electrolyte solution, there is a redistribution of counterions in a layer (or perhaps in a number of layers) near that surface. Equations have been presented to show the relationship between the surface charge and the surface potential. We will now turn our attention to the counterions in the electrical double layers and examine some of the factors that influence their adsorption and repulsion according to the Gouy-Chapman theory. (a) Exchange Density and Exchange Capacity As illustrated in Figures 3.2 and 3.4, the surface charge is balanced by an accumulation of ions of opposite charge and a deficit (or negative adsorption) of ions of like charge. The excess of ions of opposite charge over those of like charge are called exchangeable ions, because any ion can be replaced by an ion with the same charge by altering the chemical composition of the equilibrium electrolyte solution. In clay systems the sign of the surface charge is almost always negative, and the accumulated ions are exchangeable cations. The maximum number of exchangeable cations present in the electrical double layer per unit weight of exchanger under a given set of solution conditions is the cation exchange capacity of the system. Conversely, if the surface is positively charged, any excess anions held electrostatically in the double layer are exchangeable anions, and their maximum number per unit weight of exchanger under a given set of solution conditions constitutes the anion exchange capacity. It is possible to compute die surplus and deficit of ions for the diffuse layer; only the final equations will be given here. The deficit of charge is U_

=

rl,ne11'k TJIh

ze0 0 [exp 2kT - 1]

and recalling Equation 3.1, the total charge is

_ rznekT]1h . h ze 0 [11' Sill 2kT o-

Uo -

(3.9)

Chemistry

41

Therefore the excess of charge (the exchange density) is

°+=°0-°_

(

=

n2€~ T) Y2 /I

.

ze0 0

ze0 0

[2 sinh 2kT - exp 2kT + 1]

(3.10)

Note that the term exchange density is used here to denote charge per unit area. Finally, the deficit of charge expressed as a fraction of the total double-layer charge is I

0_ 00

= - [exp

ze -ze 2kT 0 0 -1] I[exp 2kT 0 0

-

ze

exp 2kT 0 0 ] (3.11)

Although these equations appear clumsy, they are not difficult to handle in practice, and they do highlight some important points. Computations involving these equations will be found in Chapter 6. Equation 3.11 shows that the fraction of total surface charge balanced by charge deficit depends only upon the double layer potential. Thus in a constant potential system, this ratio will not change as the electrolyte concentration is varied. In a constant charge system, however, changes in electrolyte concentration alter the double layer potential, so that as the concentration increases, so does the proportion of charge deficit, which means that the actual number of oppositely charged ions in the double layer decreases. In other words, the cation exchange capacity of a constant (negative) charge system is dependent to some degree on the solution electrolyte concentration. We know of course that in a constant potential (or variable charge) system, the exchange capacity will be even more dependent upon electrolyte concentration because of the greater dependence of total double-layer charge density on concentration. By substituting 0 0 = 2.303(kT/e)(pH o - pH) in Equation 3.10, the exchange density of an oxide surface for which H+ and OH- are the potential determining ions is 0+

€k T ) Y2 [2smh1.15z(pH . ) = ( n 211' o-pH

-

exp1.15z(pH o-pH)+1]

(3.12)

Chemistry

42

where o, = is the cation exchange density when pH > pH o and the anion exchange density when pH < pH o . In practice, the negative adsorption term is usually neglected because it is a small value, and the equation for the exchange density of an oxide system is reduced to

a,

T )Y2 smh . [1.15 z(pH - pH)} = ( 2nek 1T o

(3.13)

which is of course equal to the total net surface charge. The negative adsorption term has been used, however (e.g., Schofield 1949), to estimate the surface area of colloidal systems. We feel that it is important to restate here that when using the double layer formulas, o is expressed as a density value, i.e., as units of charge per unit area. In soil science, exchange capacity is normally expressed as milliequivalents per unit mass of clay, since experimental determinations are performed on known weights of material. We therefore need to relate the exchange capacity to the density through a specific surface term. Thus in the case of a negatively charged surface, cation exchange capacity = specific surface x cation exchange density, or CEC(meq/g) = S(cm 2/g) . u+ (meq/cm")

(3.14)

(b) Exchange Equilibria

The Gouy-Chapman theory was used to explain exchange capacity concepts, because some factors neglected in that theory partly compensate for each other in the range of surface charge density normally encountered in soil clays. But as pointed out by Bolt (1955), when we begin to consider the ratios of two or more ions adsorbed on a surface from a mixed electrolyte, some corrections to the simple theory are required. For instance, if a particular ion is adsorbed in excess of the amount predicted by simple theory, it can be postulated that forces other than those of purely electrostatic origin are acting on that ion, and a specific adsorption energy term can be inserted, as was discussed in the treatment of the Stern theory. It could be argued that this amounts to little more than curvefitting. The Bolt modifications, which allow for secondary

Chemistry

43

energy terms to account for ionic polarization, the radius of the hydrated ion, and short-range repulsion of ions, allowed that author to successfully describe the distribution of sodium and calcium around an illite particle without having to invoke any specific adsorption mechanism. Bolt (1955) suggested that when experimental data on ionic equilibrium are being examined, they should first be "reduced" for the effect of ionic valence and size. If the ionic ratio differs greatly from unity, then a specific adsorption for one of the ions is indicated. (c) Specific Adsorption ofAnions

Anions such as silicate, phosphate, and fluoride are able to adsorb onto oxide surfaces when the surface charge is zero, or even negative. Obviously more than an electrostatic attraction of the anion to the surface (as is seen for anions such as chloride or nitrate) is involved, and recent studies (e.g., Rajan, 1976, and Parfitt et a1., 1975) have confirmed that, at least for phosphate, anion coordination with the surface metal ion occurs. It appears that H20 and OH- ions bonded to the metal ions are replaced by the anion, as is illustrated in Figure 3.5. In this proposed mechanism (Rajan, 1976) the replacement of a water molecule by phosphate results in a diminution of positive charge, i.e., an increase in net negative charge, whereas the replacement of hydroxyl by phosphate does not alter the charge. This will be more fully discussed later in this chapter. Application of Theory to Variable Charge Soils Iron, silicon, aluminum, and to a lesser extent manganese and titanium oxides are important constituents of the clay size fraction of many soils in tropical areas. As explained earlier, oxide surfaces are hydroxylated and, in the absence of specifically adsorbed ions, derive their charge from the adsorption of the potential determining ions H+ and OH-. The adsorption of OH- is equivalent to the deprotonation of a surface hydroxy1. If the oxides' surface charge characteristics can be described according to the simple models presented earlier in this chapter, then the graphical representation of their surface net charge as a function of pH and electrolyte concentration should have the .

Chemistry

44

0

+1

I M

I

M-OH2

~>~PO

I 0 I M-OH

I 0 I

M

+ H2P04- c

•

I 0

I

M

M

+)OH

~>OH

I

I +1

I

M-OH

I

+ H2P04- c

•

+)H2P04 M

+OH-

I

0 I

0 I

M

M

t)o~

~1

M

M-OH I

0 I

(a)

M

M

I

+2HOH

+)OH2

M

M

I

I

(b)

Figure 3.5. Displacement of (a) aquo and (b) hydroxy group from a metal oxide surface by phosphate.

form illustrated in Figure 3.3. This is indeed the case, and examples for three oxides are given in Figure 3.6. Such evidence has prompted several workers to apply the theory to soil systems, and Figure 3.7 illustrates some of the remarkable results that have been achieved-we say remarkable in view of the heterogeneity of soil colloid composition. Further reflection, however, leads us to believe that even though a particular soil is composed of a variety of mineral species, it is

Chemistry

45

c 5

7

9

11

7

9

8

10

4

6

8

10

pH Figure 3.6. Variation of net surface charge with pH and electrolyte concentration for several oxides. (a) Hematite (Parks and de Bruyn 1962). (b) Alumina [Yopps and Fuerstenau 1964). (c) Goethite (Atkinson et al 1967).

.

ClIO)

0

.s:.

u

cCII+

3

a

3

b

7

c

d

pH Figure 3.7. Examples from the published literature of the variation of net surface charge with pH and electrolyte concentration for variable charge soils. (a) Acrohumox Ap (van Raij and Peech 1972), (b) Hydrandept Ai (Keng and Uehara 1973), (c) Acrohumox Ai (Gillman 1974), (d) Tropohumult Ap (Gallez et aI1976).

possible that the surfaces of particles are relatively uniform as a consequence of coverage by homogeneous amorphous coating. Amorphous gel coatings have been identified in aluminosilicate systems, in highly weathered, oxidic soils, and on quartz surfaces (J ones and Uehara, 1973). At this point, it is appropriate to present again Equation 3.3 .

Chemistry

46

discuss the various parameters that affect the sign and magnitude of charge on a variable-charge soil colloid: to

00

= (

. 1.15 z(pH - pH) 2n€kT )Y2 sinh 1T o

(a) pHo

This is the pH value where equal amounts of H+ and OHhave been adsorbed onto the hydroxylated surface so that the net surface charge from this source is zero. It is a most important parameter in a variable charge system because it determines the sign of the net surface charge. Thus if the actual pH of the system is less than pHo (i.e., is more acid than pH o), then the surface is net positively charged, and conversely, the surface is net negatively charged when pH > pH o. Iron and aluminum oxides have relatively high pHo values, usually between pH 7 and pH 9 depending on their composition and degree of crystallinity, so that soils whose colloidal fraction is dominated by these oxides usually exhibit an anion exchange capacity. Silica and organic matter, on the other hand, have low pHo values and therefore increase the cation exchange capacity of variable charge soils. For such constituents to influence the overall soil charge characteristics, they do not have to be present in large amounts. For instance, coarse-textured siliceous soils containing less than 5% iron and aluminum oxide have been shown to have pHo values as high as 6, presumably because the oxides coat the siliceous particles and therefore constitute a large proportion of the surface (Gillman and Bell, 1976). The surface horizons of variable charge soils have lower values of pHo than the subsurface horizons because of a higher organic matter content. This is fortunate because it permits the soil to retain cations where they are most needed. In subsurface horizons within the reach of plant roots a measurable anion exchange capacity can have the beneficial effect of causing the retention of nutrient anions such as nitrate and sulphate. The value pHo corresponds to a point of maximum chemical stability, and at this pH the surface potential is zero, as is shown by the Nernst equation (3.2): 0 0 = 2.3(kT/e) (pH o - pH), Thus highly leached siliceous or organic soils with low pHo tend

Chemistry

47

to be more acid than highly leached oxidic soils with high pH o . This tendency for the soil pH to drift toward pH o was termed "isoelectric weathering" by Mattson (1932). It should be possible to increase the cation retention capacity of a soil by lowering the value of pH o . This could be accomplished by introducing onto the colloid surface an anion that would impart more negative charge to the surface, and this effect has been demonstrated for synthetic oxide systems (e.g., Hingston et al., 1972). More recently Wann and Uehara (1978) demonstrated that increasing amounts of phosphate applied to an oxisol caused pH o to decrease in a linear manner with a concomitant increase in surface net negative charge (Figure 3.8). (b) Soil pH

As shown in the preceding section, soil pH determines the magnitude of net charge because of its relationship to pH o . For a given counterion valence and electrolyte concentration, the value of (pH o - pH) determines the sign and magnitude of the net surface charge. Thus raising the pH of a soil by liming increases the soil's cation exchange capacity. It is usually diffi-

4.5

o

~

Q.

r •

o.ws*

4.0

3.5

o

500

ppm P

1000

1500

Figure 3.8. The effect of applied phosphorus on pH o of an Oxisol sample suspended in NaCI and CaCI 2 solutions (Wann and Uehara, 1978).

48

Chemistry

cult, however, to raise the pH of a variable charge soil above pH 6.5 if it has a high buffering capacity associated with a high specific surface. A liming material such as calcium carbonate introduces hydroxyl ions into the system by hydrolysis of the carbonate ion, and in many constant-charge soils this causes increases in pH up to and even surpassing the neutral point. But, as was shown earlier, a hydroxylated variable-charge surface will release protons to neutralize OH-, thus buffering the soil against pH change. Consequently, when one attempts to make (pH o - pH) more negative, it is sometimes easier to reduce the value of pH o than to raise pH. This will be dealt with in the discussion of management practices later in this chapter. (c) n- The Electrolyte Concentration Equation 3.3 predicts that the net surface charge is directly proportional to the square root of the electrolyte concentration. This can be seen by observing the effect of increasing n at a given pH in Figures 3.3, 3.6, and 3.7. In an analysis of 18 surface and subsurface samples from variable charge soils from tropical Australia, it has been found that the relationship between net charge (determined by potentiometric titration) and was linear-representative examples are given in Figure 3.9. This relationship indicates that fertilizing a variable charge soil causes the cation (and anion) exchange capacity to increase for at least as long as the electrolyte concentration remains high. However, the addition of an electrolyte to a variable-charge soil sample simultaneously decreases the surface potential and increases the surface charge. The lowering of the surface potential. is indicated by a change in solution pH. When the surface charge is negative, pH decreases with electrolyte concentration and increases when the surface charge is positive. If there is no change in pH when an indifferent electrolyte is added to a suspension that is relatively free of salt, one may assume that the net surface charge is zero. Equation 3.3 predicts this behavior and is used to formulate soil management practices and laboratory procedures.

vn

(d) z-The Valence of the Counterion The theory indicates that the surface charge and surface

Chemistry

49

Acrohumox

--6 0)

o -';o -5 GI E

-.. e ~

0)

-4

.c u

~

u

.E.. :)

-

-2

:

Acrohumox

en ~

c

-1

: : 0·1

Eutrustox

0·2

(lf2

0·3

Figure 3.9. Variation in net surface charge with (ionic strength) % for selected highly weathered soils from tropical Queensland.

potential also vary with the valence of the counterion. Thus when pH is one unit higher than pH o the negative charge is increased by more than threefold, as is indicated by the ratio (sinh 1.15 x 2/sinh 1.15 x 1) = 3.5, when a divalent cation replaces a monovalent cation. Increases in the surface charge on synthetic hematite due to an increase in electrolyte valency have been demonstrated by Breeuwsma and Lyklema (1971). This explains why many soils will adsorb more cation equivalents from a divalent electrolyte than from an equal concentration of a monovalent electrolyte, all other things being equal.

(e) e-Dielectric Constant of the Medium When considering moist soils in the field, one is dealing with dilute aqueous soil solutions, so that the dielectric constant of the medium approximates that of water and could be assumed

50

Chemistry

to be a constant. It should be remembered, however, that the surface charge is directly proportional to V€ and that laboratory estimates of surface charge will be in serious error when nonaqueous solvents are used. Nonaqueous extractants are often used to wash excess salts in a number of laboratory methods, and this practice alters the dielectric constant and therefore the surface charge.

(j) T - The Absolute Temperature Equation 3.3 predicts that the surface charge varies as...;'T. However, since any variation in the absolute temperature throughout the year is relatively small for soils of the tropics, changes in surface charge due to this variable will be slight. Having discussed the factors that influence the sign and magnitude of the charges on clay surfaces, it should be pointed out that any laboratory method aimed at estimating this charge must take such factors into account. This necessity is elaborated upon further in Chapter 6, which deals with analytical methods. Application of Theories The curves in Figure 3.7 show that the simple Gouy-Chapman theory can be used qualitatively to explain the variation of surface charge with pH, electrolyte concentration, and valency. However, Figure 3.10 shows that when experimental data obtained by van Raij and Peech (1972) were compared with curves predicted by the Gouy-Chapman and Stern theories, the latter was more accurate for high potentials and high ionic strength. We shall now consider those aspects of the Stern theory that would be important for our discussion of the applicability of double layer theory to variable charge soils. The Stern model {Figure 3.4) allows for a compact layer adjacent to the solid surface, up to which counterions could approach. If we now postulate that certain counterions may actually enter the compact layer, we may predict what effect this would have on surface charge characteristics. This type of adsorption would be more than simple electrostatic attraction and could be termed specific adsorption. As we have already used this term in connection with the chemical bonding of certain anions to the oxide surface, we now distinguish between

51

Chemistry ••

•

,,

,••

." •.. . .'

. ,'.

'

,'."

..,," -..

.., ,.... ,

~.

~

1N

"
'.

l1li":

I •

:.. .,r

•

..

O·lN •

•

G)

. ,.:" ,• ..'." ••• . ,, . ,.,e. ,

C

....

~

O·OlN

• ••

.. • , •

.... ..' ...,-'" ,.0·001

".. •...

surface potential Figure 3.10. Variation in net surface charge with surface potential for some highly weathered soils from Brazil at four electrolyte concentrations. Solid line is GouyChapman calculation and dashed line is Stern calculation (van Raij and Peech, 1972).

high-affinity and low-affinity specific adsorption with the former referring to chemical coordination to the surface metal ion (chemisorption) and the latter to adsorption in the Stern layer. Low-affinity specific adsorption of a cation would induce additional negative charge in the oxide surface by proton desorption. If the surface had been at net zero charge before low-affinity specific adsorption, it would now be net negatively charged, and the pH would have to be lowered to cause proton adsorption to establish a new pH value of zero net surface charge, pH o . Conversely, the low-affinity specific adsorption of an anion would result in the pH o value being raised (Figure 3.11). Such effects have been observed with CaH and S04 - for hematite (Breeuwsma and Lyklema, 1971) and soil materials (Keng and Uehara, 1973). This model would further predict that at the plane between the compact layer and the diffuse layer, lowaffinity specific adsorption of a cation would cause this plane to be more positive, and the adsorption of an anion would

52

Chemistry

+6

+

+ + + + + +

era

+13

+6

+

-1

+ 504 504

+

"C

+

;:)

.r:: ..lO:

-;

+

~

+ c:{i

c:r,= 0 a; = -6

504 )

+ + += - +-: + + _.:. 10H-

+ + + 504 + + 504 + + + 504 + 504 + - +~ + +50 4 + tIC c'd C1j=-14 cf2= +1

+

Figure 3.11. Charge reversal in the diffuse layer brought about by adsorption of S04 in the Stern layer. The induction of additional positive charge onto the surface (protonation) results in a higher value of pH o• but the pH where ad =0 (IEP) is lowered.

cause the plane to be more negative. The pH value where this plane has zero net charge (ad = 0) is the isoelectric point (IEP). Thus low-affinity specific adsorption of a cation raises the IEP because the pH would have to be raised to make ad = 0, whereas low-affinity adsorption of an anion lowers the IEP. The GouyChapman equation relating ad to pH would then be written as (3.15) It should be noted that in the absence of specific adsorption,

IEP = pH o , so that ad = a 0 and Equation 3.15 is then synonymous with Equation 3.3. One could, therefore, have the situation where even though pH o has been raised by the presence of an anion, more cations are retained nonspecifically in the diffuse layer than if the anion were absent. Conversely the presence of a specifically adsorbed cation would reduce the capacity of the diffuse layer to hold cations. Ayers and Hagihara (1953) demonstrated that S04' retarded K+ leaching, and Wann and Uehara (1978) showed that adsorption of CaH into the Stern layer caused charge reversal in an oxisol. With high-affinity specific adsorption, the adsorbed ion

Chemistry

53

becomes part of the surface and transfers its charge to the solid. A cation adsorbed in this way on an oxide surface at zero net charge would cause a net positive charge, and the pH would have to be raised to reach a new pHo , and a new IEP. The highaffinity specific adsorption of an anion would lower both pHo and the IEP. The lowering of pHo of goethite by silicate has been reported by Hingston et al. (1967), and the lowering of pHo of an oxisol by phosphate has been reported by Wann and Uehara (1978). Such a lowering of pHo increases the surface negative charge, and increases in the cation exchange capacities of soil following the addition of superphosphate to laboratory soil samples has been widely reported. It should be noted here that the addition of superphosphate involves the application of phosphate, sulphate, and calcium. As discussed, the phosphate and sulphate would increase cation retention, and the lowaffinity specific adsorption of calcium could actually cause a decrease in the adsorption of nonspecifically adsorbed ions such as potassium. Ayers and Hagihara (1953) showed that in the case of an inceptisol developed from volcanic ash, soluble calcium phosphate was less effective than ammonium phosphate in reducing potassium losses by leaching. We have now considered those variables that will affect the behavior of a soil containing predominant amounts of variable charge colloids. Such soils, though of utmost agricultural importance in many areas, are as rare as soils that have mineralogies that are entirely permanent charge. Soils Containing Mixtures of Permanent and Variable Charge Colloids The discussion so far has dealt with soils that behave as pure, variable charge systems. Although it is true that such extreme examples can be found, soils that contain both permanent and variable charge colloids are more common, and both types of charge may occur on a single particle. In this section we present a model for describing the surface charge characteristics of mixed systems and discuss some of the consequences arising from the model.

54

Chemistry

The total surface charge an indifferent electrolyte is

0T

in a mixed system suspended in

(3.16) where subscripts T, p, and J) denote total, permanent, and variable charge, respectively. Since 0p is constant, it requires no further elaboration, but recalling Equation 3.3, in which 00 is replaced by oJ)' oJ) =

Y2 smh . 1.15 z(pH - pH) 2nekTJ IT o [

and substituting it into Equation 3.16, we have aT

= ap

+

[2n;kT]Ii sinh 1.15 z(pHo - pH)

(3.17)

remembering that pH o is the suspension pH where the variable surface charge oJ) is zero. If we now define the suspension pH where the total surface charge 0T is zero as the point of zero net charge (PZNC), then we can substitute PZNC for pH in Equation 3.17 for the condition oT = O. We therefore have

o = "» +

[2n:kT] Ii sinh 1.15 .(pH, - PZNC)

(3.18)

For a 1: 1 indifferent electrolyte, if PZNC is within one unit of pH o , sinh 1.15 z(pH o - PZNC) = 1.15 (pH o - PZNC) so that Equation 3.18 may be written explicitly for PZNC as

or (3.19) When 0p is negative, Equation 3.17 is illustrated graphically as in Figure 3.12. For the total charge (depicted by the curves)

Chemistry

55

pH

+ Figure 3.12. Dependence of total net surface charge in mixed systems on the point of zero net charge (PZNCI. pH, magnitude of the permanent charge (ap ), pH o, and electrolyte concentration.

to be zero, an amount of positive charge equal in magnitude to

the permanent negative charge has to be generated on the variable charge surfaces. This is accomplished by lowering the suspension pH. Thus the greater the permanent negative charge (a~ > a~), the more acid the system must be for aT = O. Equation 3.19 and Figure 3.12 show that the PZNC is dependent upon the electrolyte concentration. As the electrolyte concentration is increased, the difference between pH o and PZNC becomes smaller. Since an increase in electrolyte concentration increases net charge on the variable surface, less acid is required to produce the condition a v = -ap ' This model allows us to directly measure the amount of permanent charge in a mixed system and, hence, the variable charge. To do this, the pH at which the variable charge components arc at net zero charge (pH o ) is obtained from poten-

Chemistry

56

tiometric titration data. At this pH, there are equal amounts of cations and anions adsorbed on the variable charge surfaces. The total amount of cations and anions adsorbed at pH o is then measured, and it follows that an excess of cations over anions adsorbed at this pH indicates permanent negative charge, whereas an excess adsorption of anions over cations indicates permanent positive charge. Full experimental details of this approach are given in Chapter 6, but the results for a number of highly weathered soils from northern Australia are summarized in Table 3.1. It can be seen that the PZNC is less than pH o for all but one sample, showing that negative permanent charge is the usual case, but the exception raises the interesting possibility of positive permanent charge. A similar example of the existence of positive permanent charge can be obtained from the data of van Raij and Peech (1972). The model can also be used for estimating the specific surface of a mixed system on the basis of the theory of negative

Table 3.1

pH o obtained by potentiometric titration, and PZNC obtained by adsorption of K+ and Cl- at 0.002M for a range of north Australian soils. For calculation of permanent charge, q>, see text

Sample

PZNC

Cation adsorption at pH o. meq/100 9

Anion adsorption at pH o ap meq/100 9 meq/100 9

Acrohumox (210 - 240 cm)

6.4

7.0

0.9

2.2

+1.3

Acrohumox (90 - 120 cm)

4.1

<2

2.0

0.3

-1. 7

Aerohumox (90 - 120 cm)

4.0

<2

1.7

0.7

-1.0

Eutrustox (150 - 165 em)

6.0

<2

3.0

0.1

-2.9

Haplustox (0 - 10 em)

4.2

3.0

0.2

-0.2

-0.4

Haplustox ( 120 - 135 em)

4.4

3.8

0.3

~.

---

._--- ---- ---- _._-- •.. _- .•.. -

--."~

. - .... -- ... ~

-

-

.-. -.... '.-.--.-.- _ ..

0.15 ~"'--

-0.2 ..

_.,

~,.-.-

Chemistry

57

adsorption of ions developed by Schofield (1949). A charged particle will repel ions with charge of the same sign as that of the particle surface. If this negative adsorption can be measured accurately, the specific surface may be calculated by assuming that the forces causing the repulsion are spread evenly over the particle. Thus in a mixed system, the sign of the variable charge can be made the same as the permanent charge by adjusting suspension pH. This subject is discussed in Chapter 6. Application of Theory to Soil Systems The ultimate test of a theory hinges on its ability to explain the characteristics of soils. It must not only do this, it must do so more effectively than other models currently in use. To test the theory presented, we will apply it to explain soil characterization data. Soil characterization data are normally collected on important "benchmark" soils for the purpose of soil classification. Since different countries use different methods to characterize soils, it will not be possible to make direct comparisons of data, and wherever possible, trends rather than absolute values of data will be discussed.

(a) Soil Systems Rich in Crystalline Oxides of Iron and Aluminum For purposes of illustration, data from two soils, one with permanent charge, high-activity clay and the other with variable charge, low-activity clay are presented in Table 3.2. Data for the high-activity clay soil were obtained from Soil Survey Investigations Report Number 24 (1973), which contains soil survey laboratory data and descriptions for some soils of California. Data for the low-activity clay soils were taken from a similar report (SSIR no. 12, 1967) for the soils of Puerto Rico and the Virgin Islands. The most obvious difference between these soils is their base content indicated by the sum of the bases. This value is the sum of calcium, magnesium, potassium, and sodium ions extracted by one normal ammonium acetate and expressed as milliequivalents of these cations per 100 g soil. If the sum of cations were expressed as milliequivalents per 100 g clay, the differences would even be greater. The clay fraction of the acrorthox from

Chemistry

58 Table 3.2

Depth (cm)

Soil Characterization data from Puerto Rico and California

Clay

Carbon

(%)

(%)

pH H 2O

KC1

KC1 Extr. Bases Sum Cations (meq/lOO 9)

Typic Acrorthox - Puerto Rico

o-

28

55

6.0

5.1

4.3

1.4

2.9

28 - 46 46 - 71

58 60

2.0 1.3

5.0 5.0

4.4 4.7

0.9

0.1

71 - 97

56

0.9

5.2

5.7

97 - 122

60

0.7

5.5

6.1

tr

0.1

122 - 157

54

0.6

5.7

6.4

tr

157 - 178

38

0.2

5.8

6.7

tr

Typic Hap l oxarol l - California 0-8

35

.87

6.7

6.2

22.1

8 - 17

42

.50

7.1

6.6

27.2

17 - 34

34

.35

7.8

7.4

24.3

34 - 45

36

.30

8.3

7.9

25.8

45 - 54

33

.19

B.7

8.1

26.9

54 - 74

28

.17

8.8

8.2

25.9

74 - 84

31

.17

8.9

8.2

33.7

Puerto Rico represents the insoluble residue of tropical weathering. This residue has the mark of low activity imprinted in the soil pH data. In both soils pH increases with depth. The pH for the California soil was measured in a dilute (I. 10) and a less-dilute (l: 1) soil paste. The pH in the 1: 10 system was higher which indicates a soil material with net negative charge. The dilute suspension had a soil solution of lower salt concentration and therefore was more alkaline. This same effect is observed in the surface sample of the Puerto Rican acrorthox. In the Puerto Rican samples, pH was measured in water and again in a one normal potassium chloride (lNKCn solution. The potassium chloride depressed soil pH in the surface horizon, indicating that the material was net negatively charged. But in

Chemistry

59

the lower horizon, potassium chloride raised soil pH, indicating net positive charge. These effects are illustrated in Figure 3.3. A soil material with net positive charge is an anion exchanger and will not retain cations as counterions. This is reflected in the trace amounts of cations measured in the subsoil. Although the subsoil was leached and stripped of bases, the pH increased with depth. This is an excellent example of a highly weathered soil in which soil pH increases with leaching and loss of bases. The unique features of the acrorthox from Puerto Rico can be explained by Equation 3.3: 00

= (

. 1.15 z(pH - pH). 2nek T ) Yz sinh 7r o

In this soil pH o increases with depth. The soil is rich in iron oxide, and the material has a high pH o . In the surface horizon, organic matter with a low pH o lowers the pH o of the oxideorganic matter mixture. One can at least say with certainty that pH o is less than pH 4.3 in the surface horizon and greater than pH 6.7 in the deepest horizon. Another important feature of the acrorthox is that soil pH tends to follow pH o . It is helpful to show again how stability and equilibrium are achieved when pH attains the value of pH o . This is shown by the Nernst equation (3.2): 00 = 2.3(kT/e)(pH\:) - pH).

Equilibrium is reached when the surface potential 0 0 is zero, and this is achieved when pH = pH o . In this state, net surface charge is also zero, and the soil is in a state of low chemical activity. The first three letters of the name acrorthox refer to the extreme state of weathering. Soils rarely attain this state of weathering since the earth's land surface is constantly rejuvenated by natural erosion and redeposition. In the tropics, soils with aerie properties are most frequently associated with basic rock, which weathers easily, or with ancient land surfaces. The largest known area with aerie soils is located in central Brazil surrounding the capital city of Brasilia. Although these soils are very infertile, they are rapidly being brought under cultivation

60

Chemistry

and are proving to be very productive when properly managed. The rejuvenation of aerie soils involves a manipulation of the parameters in Equation 3.3. It involves increasing the salt concentration with fertilizers and raising the quantity of divalent calcium and magnesium with lime to alter the valency factor z and to raise pH. Intensive farming of such soils for crop production requires a heavy application of phosphorus, which lowers pH o to increase the net negative charge of the clay surface and to increase retention of nutrient ions such as potassium. Since phosphorus is expensive, some vegetable farmers on the outskirts of Brasilia use a phosphorus fertilizer rich in soluble silica. The farmer unknowingly increases the silicasesquioxide ratio of the soil in a rejuvenation process. The silica-sesquioxide ratio of the entire soil cannot be measurably increased, but the farmer is successful because silica and phosphorus are adsorbed and concentrated on the oxide surface, so the ratio is significantly increased on the oxide surface where the chemical and physical characteristics of the soil material originate.

(b) Soil Systems with Non-crystalline Materials The variable charge model also applies to non-crystalline soil materials. Soil characterization data from two andisols (Table 3.3 and Table 3.4) are used to test the model. The noncrystalline nature of the soil colloid is indicated by its high, IS-bar water content. The high, IS-bar water content signals another important feature of andisols, namely, their l.!?"yv bElk.· density. This is indicated by the relationship of the gravimetric water cont~ntflg and volumetric water content ()v to the bulk density Ph through the expression (3.20) If one assumes that the volumetric water content at 15-bar water pressure is 30% and inserts this value into Equation 3.20, it is immediately obvious that low bulk densities will result. If we take the extreme example of the 140-145 em depth sample for the hydrandept, a bulk density of Ph = ()v I() g = 30/196 = 0.15 g/crrr' is obtained. Even if we assume that the sample

I

i

i

j

il,

:

Table 3.3 Depth

Total Chemical Analysis

(em)

(%)

Organic Carbon

Nitrogen SiDz

Chemical and physical data for a Typic Hydrandept Exchange bases

A1203

Fe203

Si02 A1203 + Fe203

pH

15 Bar Water Content

(meq/lOO g) Ca

Mg

Na

K

H2 O

KCl

(%)

0-40

5.30

0.41

12.9

24.3

27.6

0.52

2.0

loB 0.1

0.1

5.8

5.6

70

58-65

3.23

0.22

9.84

34.2

26.3

0.33

2.4

0.3 Tr

Tr

6.3

6.4

123

80-B3

2.28

0.16

9.42

35.4

26.0

0.31

1.4

0.3

Tr

Tr

6.4

6.4

99

123-128 3.12

0.18

11.9

32.6

24.5

0.42

1.7

1.0

Tr

Tr

6.4

6.4

132

140-145 2.28

0.14

10.0

30.0

29.3

0.35

2.6

0.3

Tr

Tr

6.3

6.5

196

Source:

Soil Survey Investigations Report No. 29, 1976

Table 3.4

Depth

Chemical and Physical Oata for a Typic Eutrandept

Total Chemical Analysis

Si02

Exchangeable bases

(%)

(em)

(meq/100 g)

A1203 + Fe203

Organic Carbon

Nitrogen

Si02 A1203

Fe203

0-50

3.29

0.25

51. 1 13.9

12.7

50-78

1. 97

0.17

44.1

15.9

7B-90

0.90

0.08

90-133

0.60

133-163

0.47

Ca

11g

3.95

17.2

9.1

16.1

37.9 20.2

0.06

-

15 Bar Water Content

pH

(%)

H2 O

KCl

0.700.70

5.4

4.5

34.9

2.87

22.8 11.7 1.000.70

5.8

4.8

39.5

18.4

2.03

33.7 19.2 1.80 0.50

6.6

5.6

73.2

35.8 22.2

20.2

1. 74

35.8

6.7

5.6

98.9

36.8 21.4

20.2

1.83

34.4 26.5

6.6

5.7

78.7

Na

K

23.8 2.10 0.30 1.90 0.20

....

0\ Source:

Soil Survey Investigations Report No. 29, 1976

62

Chemistry

was 60% water by volume at IS-bar pressure, the bulk density would still be 0.3 g/cm 3 . In the soil taxonomy a bulk density of 0.85 g/cm' or less is required for andepts (andisols). A combination of low bulk density and high, IS-bar water content alerts us to still another accessory characteristic, namely, a high specific surface. Since cation exchange capacity is the product of specific surface and surface charge density, one would expect a very high CEC for the hydrandepts. But if we look for the effective CEC defined as the sum of calcium, magnesium, sodium, potassium, and exchangeable aluminum, we discover values less than 3 meq/IOO g of soil. This highly leached soil is not very acid and has little or no KCl extractable aluminum. The hydrandept is obviously a more highly weathered soil than the eutrandept. The main difference lies in the fact that the pH in INKCl remains unchanged or rises over that of the pH measured in water in the hydrandept and decreases in the case of the eutrandept. This means that the hydrandept is net zero or net positively charged and the eutrandept is net negatively charged. As a consequence the hydrandept is rich in adsorbed anions, and the eutrandept is rich in adsorbed cations. The dominant anion in the hydrandept is the sulfate ion; 4-6 meqllOO g of this anion can be measured in the subsoil of hydrandepts. Hydrandepts are rare even in the tropics, but they tell us a great deal about the consequences of soil weathering under warm and humid conditions. If one compares the silica-sesquioxide ratio of the hydrandept and the eutrandept, the cause for their characteristics can be seen: The hydrandept has lost most of its silica. As was shown in Chapter 2, weathering is largely a desilication process. So long as a soil is rich in silica in the clay fraction, the soil will have a high CEC. This high CEC is related to a pH o which is located in the very acid range. The eutrandept has a pH o of about 3.5, so in Equation 3.3, (pH o - pH) is about -3.0, and the surface charge density ao would necessarily be high. In the hydrandept (pH o - pH) is near zero or positive and ao is zero or positive. If the acrorthox contains a small quantity of non-crystalline material as it most likely does, the non-crystalline fraction would be low in silica and would be very much like the material

Chemistry

63

that makes up the hydrandept. On the other hand, the noncrystalline material in the haploxeroll would be like that of the eutrandept. For purposes of illustration we have considered extreme endmembers of soils, but it turns out that many of the problem soils of the tropics are in the intermediate stage of weathering. Three examples are shown in Table 3.5.

(c) Soil Systems with Mixtures of Variable and Permanent Charge Minerals As has been indicated, soils that consist entirely of variable charge minerals are rare even in the tropics; soils usually contain a mixture of variable and permanent charge minerals. In the tropics the variable charge minerals dominate, and in the temperate regions the permanent charge minerals dominate. There are always exceptions to this generalization, but if one wanted to study soils with aerie properties, one would not search for them in Europe or North America. Data from three soils are presented in Table 3.5 to illustrate problem tropical soils. Profile A is dominated by variable charge minerals, profile C is dominated by permanent charge minerals, and profile B contains a more even mixture of variable and permanent charge minerals. If we study the data of all three soils simultaneously, we see a number of interesting trends. The pH measured in water increases from soil A to soil C, whereas the pH in INKCI shows a slight decrease. In soil A, the pH in water and INKCI are almost identical, indicating that the soil pH is very near pH o . The soil pH is extremely acid (pH 3.8-4.4) but the KCI extractable aluminum is low. In soil C, the pH is higher (pH 4.9-5.2), but the KCI extractable aluminum is many times higher, even though the clay content is about the same as soil A. Soil C is rich in montmorillonite, and that mineral is unstable at the pH of the soil and releases large quantities of aluminum ions. Soil B contains a small quantity of montmorillonite and has acid properties intermediate between those of soil A and soil C. If all three soils are examined together, one can see that contrary to what one would expect, extractable aluminum increases as the pH of the soil solution increases. If one examines

64

Table 3.5

Depth (cm)

Selected soil characterization data from Thailand (Soil A and B) and Brazil (Soil C) Clay

(%)

H2 O

pH

KC1

KC1 extr.A1 meq/lOOg

Sum

",;

of Bases meq/lOOg

!.'

.'i:, I),

Soil A.

o-

,A

Typic Haplustox (Kaolinite with some amorphous material)

10/14

51

4.4

4.3

0.12

4.13

"

,. I

10/14 - 36 62

4.2

3.9

0.15

1.84

36 - 60

60

3.9

3.8

1. 70

0.98

60 - 86

63

3.8

3.7

1. 67

1.07

86 - 120

62

3.8

3.7

1.71

0.76

120 - 156

63

3.9

3.8

1.88

0.68

156 - 185

59

3.9

3.9

1. 95

0.51

185 - 220

60

4.1

3.9

1.68

0.48

Soil B.

o-

18

Typic Pa1eustu1t (Kaolinite with some montmorillonite) 5

5.5

5.2

tr

3.43

18 - 29

8

5.4

4.9

tr

2.57

29 - 41

12

4.2

3.9

0.67

1.68

41 - 63

17

3.8

3.6

1.88

1.41

63 - 85

24

4.0

3.7

2.57

1. 29

85 - 110

21

4.0

3.7

2.37

1. 34

110 - 135

20

4.0

3.6

2.02

1.71

135 - 162

21

4.0

3.6

2.08

2.08

162 - 195

21

4.0

3.6

1. 91

2.46

195 - 210

22

4.0

3.7

0.79

4.24

24 37 - 56 71 - 95 107 - 135 -135+

59 66 66 70 80

4.9 5.2 5.1 5.0 5.1

3.8 3.8 3.6 3.6 3.6

8.6 10.9 14.8 12.9 8.9

::!

;.

.

.i

Soi1 C. Typic Pa1ehumu1t (montmorillonite)

o-

.e

4.3 1.9 5.6 11.2 15.3

/.

Chemistry

65

the pH measured in 1NKCI, the relationship between pH and aluminum is better, but still unclear. What is very clear is the difference in the pH measured in 1NKCI and in water or the l>pH defined as l>pH = pH K C1 - pHH

0

(3.21)

2

The sign and magnitude of l>pH correspond to the sign and magnitude of the surface charge. In soil A, l>pH is zero or a small negative value, but it is a large negative value in soil C. A positive, zero, or small negative l>pH value (less than -0.5) generally indicates a soil dominated by variable charge minerals. Acid soils with l>pH values near zero can be expected to have low extractable-aluminum content. If the l>pH value is a large negative value, nothing can be said about whether the charge is variable or permanent, but a high, negative surface-charge density is indicated. An acid, sandy soil rich in quartz may show a large, negative l>pH value. This simply indicates that the surface charge density on quartz is high and is negative in sign. On the other hand, a large, negative l>pH value associated with high levels of extractable aluminum is a sure sign of a sample rich in permanent charge minerals. This is the case in soil C. Soil acidity and aluminum toxicity is frequently cited as a serious food-production constraint in the tropics. Few would disagree with the fact that soils A, B, and C in Table 3.5 each have a soil acidity problem. The important difference is that the soil acidity problem can be more easily corrected in soil A than in soil C, even though soil A is more acid than soil C and both soils have about the same clay content. A recognition of this difference is especially important because the inputs required to correct soil acidity are about 5 to 10 times greater for soil C than for soil A. It is clear that soil characterization data collected to support soil surveys and soil classification can be used to reduce cost and minimize error in agricultural development projects. This can be achieved by applying the appropriate model to each soil. In the examples provided in Table 3.5, the variable charge model applies to soil A, the permanent charge model to soil C, and the hybrid model to soil B. Most

66

Chemistry

soil characterization data contain sufficient information to indicate which model to choose. Implications for Soil Management (a) Controlling the pH

The surface charge characteristics of soils are of central importance in soil management because the majority of the reactions that control nutrient availability, and many of a soil's physical properties, are dependent upon the physicochemical processes that occur at the soil particle surface. We have demonstrated the importance of pH in determining surface charge in variable charge soils, so it is obvious that the correct manipulation of soil pH is crucial to the proper management of such rolli. Since the days of Edmund Ruffin, who increased the productivity of his land by applying oyster shells to it, liming has been an important agricultural practice, and the beneficial results of raising soil pH are well known-at least for soils whose clay mineralogy is predominantly of the permanent charge type. In those soils, a reduction in base saturation leads to acidity and a subsequent breakdown of 2: 1 clays. The aluminum released occupies the charge sites, and the effect of the lime is to raise the pH, precipitate aluminum as the hydroxide, and replace the precipitated aluminum with calcium. However, the situation in the case of variable charge soils is different. Aluminum (and iron) in those soils exist as minerals that are stable at a pH as low as 5.0, with the metal ions "buried" beneath the particle surface and not easily available for exchange with other cations. As such they pose no threat to plant growth until soil pH drops to a level where the oxides are dissolved, thus bringing the aluminum into solution, sometimes in toxic quantities. Enormous quantities of lime are not required to overcome aluminum toxicity in variable charge soils, only enough to raise the pH to about 5.5, which allows for some residual effect. In fact the theory described earlier shows that attempting to increase the pH of variable-charge mineral soils beyond a certain level, say pH 6, could be very costly. An examination of Figure 3.3 or Figure 3.7 shows that as suspension pH moves away from pH o , there is a rapid increase in surface charge. This

1

67

Chemistry

reflects the sinh function of Equation 3.3. If the derivative with respect to pH of Equation 3.3 is plotted against pH (do., /dpH vs pH), we see that a material with variable charge clay is weakly buffered at pH o but is highly buffered on either side of pH o . At any pH the buffering capacity increases with specific surface, electrolyte concentration, and valence of the counterion as shown in Figure 3.13. Hydroxyl ions formed by the hydrolysis of the carbonate ion create charge by deprotonation of surface hydroxyls, and consequently do not raise soil solution pH. In short, there is a resistance to an increase in soil pH. The reaction of lime with a variable surface-charge material is as follows: CaC0 3

~

Catt + C0 3 =

C0 3 = + H20

~

HC0 3 - + OH-

M-OH]O + OH-

~

M-O]- + H20

, ,

-

d.CY

1

dpH

I I

I I I

I

p

I 0

pH

»

Figure 3.13. Relationship between buffering capacity and pH in a variable charge system at two electrolyte concentrations. The buffering capacity is the product of the specific surface and the derivative of surface charge density with respect to pH.

68

Chemistry

This is the reason for the high buffering capacity of soils containing appreciable amounts of variable charge clays. The experimental curves shown in Figure 3.7 were determined in monovalent electrolytes, but referring back to Figure 3.3 we see that the slopes of the curves are much steeper if a divalent electrolyte is used, and this explains why measuring the lime requirement with NaOH often grossly underestimates the amount of CaC0 3 required to produce a given pH change. Theory would also predict that the application of ammonium sulphate to a variable charge soil would cause the pH to rise as a result of proton adsorption as the sulphate ion entering the Stern layer causes pH o to increase. An increase in pH would initially occur, based on laboratory studies with K2 S04 (e.g., Mekaru and Uehara, 1972), but this would soon be masked by the acidity produced when the NH4 + ion was oxidized to N0 3 -. Thus some liming would always be required to counteract the acidifying effect of ammoniacal fertilizers. The effect of adding a fertilizer that consists of indifferent electrolyte ions, e.g., potassium, is to cause a change in pH as illustrated in Figure 3.14. As discussed earlier, an increase in electrolyte concentration causes an increase in surface charge. If the clay surface is already net negatively charged, then more negative charge is created by deprotonation of the surface hydroxyl groups. The hydrogen thus released to the soil solution causes soil pH to decrease from pHI to pH 2 • Note that if the addition of potash is accompanied by sufficient lime to keep the pH constant, the increase in net negative charge is even greater (C I to C3 instead of C I to C2 ) , and if the pH declines naturally to pH 2 without the addition of fertilizer, the net negative charge would decline from C I to C4 along curve nt. It should be obvious from these considerations that maintaining the pH of variable charge soils at levels sufficiently high to precipitate aluminum, and preferably a little higher to take advantage of the increased cation-exchange capacity, is of practical importance. (b) The Retention of Cations and Anions As we have just seen, an increase in pH will increase the net

Chemistry

69

.. --_ ... -._._._ .... _._--------_.

Q)

~ C

..c

------.-----_._---------_.--_._--------.-----.--

u

Q)

c

+ pH Figure 3.14. Changes in net surface charge that might be observed when a variable charge soil is fertilized with an indifferent electrolyte such as KCI and where nz is greater than nl'

negative charge and, therefore, the capacity of the soil to retain cations, and recalling our discussions centering around Equation 3.3, we know that it is actually the difference between soil pH and pH o that governs the magnitude of surface charge. There are a number of practical considerations as to why soil pH cannot be increased too greatly in variable charge soils. As already mentioned, the costs involved in liming such a highly buffered system would be too great, and there is the danger of a micronutrient deficiency at high pH. Nevertheless, we need to increase the cation exchange capacity of these low-activity clays, and one alternative is to lower the value of pH o . One way of lowering pH o would be to increase the organic matter content of the soil. Organic matter has a low pH o , so an overall lowering of the soil pH o can be achieved by sorption of large organic anions onto particle surfaces, thereby masking

70

Chemistry

some positive charge. Increasing the organic matter content is difficult to achieve under warm and humid conditions when the soil is regularly cultivated. Where a continuous cover is maintained, such as in a pasture, it should be possible to maintain higher levels of organic matter and, consequently, the capacity of a variable charge soil to retain cations. In situations where it is not possible to build up organic matter levels, inorganic soil amendments such as phosphate or silicate could be used to lower the pH o value. In a laboratory study, Wann and Uehara (1978) showed that the application of 1500 ppm of phosphorus to an oxisol lowered the pH o from near 5.0 to below 3.5 in the presence of calcium. Their data also indicated that the presence of too much calcium would cause charge reversal when calcium was adsorbed into the Stern layer, and this would mean that even though surface charge density was greatly increased, the capacity to retain indifferent nutrient cations such as NH 4 and potassium would be reduced. Therefore, it would appear that concentrated superphosphate would have a more beneficial amendment effect than single superphosphate, owing to the lower calcium/phosphorus ratio of the former. Field experiments have shown that the cation exchange capacities of a hydrandept and of a gibbsihumox were significantly increased and losses of calcium, magnesium, and potassium were reduced by the application of triple superphosphate (Gillman and Fox, 1980). Figure 3.15 shows the increase in CEC of the surface horizons of three soils as a function of the phosphorus recovered in a boiling HCI extract. The CEC generated by an application of phosphorus is masked by the fact that the soils were limed to pH 6 with coral limestone or calcium silicate with a resultant increase in CEC for all treatments. The unlimed CECs of the hydrandept and the gibbsihumox were about 5 and 3 meq/lOO g, respectively, so that the effective CEC was doubled by the combined effect of liming and fertilizing. In many cases, it would not be economical to apply phosphate merely to increase the CEC of a soil, but as variable charge soils generally require applications of phosphorus for plant growth, the increase in CEC comes as an added benefit. Gillman and Fox (1980) showed that for the gibbsihumox that had been limed with calcium silicate, the increase in CEC

71

•

16

• Hydrandept

Figure 3.15. The effect of phosphorus application on the CEC of three soils with variable charge minerals.

72

Chemistry

was evident to a greater depth. Such an effect was not obvious when limestone was used to raise the soil pH. Apparently, the application of both phosphate and silicate to the soil caused either or both to move deeper into the subsoil as a result of competition for sorption sites. This would suggest a way of increasing the subsoil CEC so that the need for deep incorporation of lime might be avoided. It is important that subsoil CEC be increased to allow a greater retention of calcium, for this in turn will enable the plant roots to extract water and nutrients from a larger soil volume. The beneficial effects of adding silicate to highly weathered soils whose silicon levels have been reduced by leaching are well established. A number of reasons have been proposed to explain these effects, such as the reduction of phosphate sorption, the increase in soil pH and calcium supply, and the increase in cation exchange capacity. But the relatively high cost and unavailability of silicate place this material at a disadvantage, and cheaper sources of soluble silica are required. As it happens, tropical soils that contain variable charge clays often occur near outcrops of basic rock, and often the very nature of the easily weatherable basic rock results in the formation of highly leached soils. The basic rock can be a source of silica and basic cations such as calcium, magnesium, and potassium. In a laboratory experiment Gillman (1980) showed that the application of crushed basaltic scoria to an oxisol from northern Australia caused an increase in CEC. As shown in Figure 3.16, this increase was related to the contact time between scoria and soil, the rate of application, and the fineness of the scoria. Application rates were high (up to 300 t/ha) because of the low cost involved in obtaining the material. The real benefit apparently derived from the amendment was the large increases in exchangeable calcium, magnesium, and potassium. The magnitude of these exchangeable ions was small when compared to what was applied, so the residual effect of increased exchangeable-cation content should be evident for many years. Results for the most finely ground material are presented in Table 3.6. These results could in part explain the favorable response of sugarcane to large applications of crushed basaltic rock obtained

_e

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o 12 o.<,

CT 11 Q)

E '-"

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, 3

•

•

•

.> e

.

Lm /.

./

..

~

<,

.

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.- /.-

.-

./

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_./·coarse

.._--.control. 200 t/ha

100t/ha

/.. ._./

,

,

,

,

,

,

I

,

6

9

12

3

6

9

12

3

•

•

,

,

,

6

9

12

300 t /ha

INCUBATION TIME (mths) Figure 3.16. Effect of incubation time, application rate, and particle size of crushed basaltic scoria on the CEC of an Oxisol.

i::.l

74

Chemistry

Table 3.6

Increase over control in exchangeable calcium. magnesium, and potassium. when finely ground basalt scoria was applied at 3 rates and incubated at 2SoC in a moist condition for 12 months

(t/ha)

Potassium (kg/ha)

Magnesium

Calcium (kg/ha)

Application Rate of crushed scor t a

(kg/ha)

Applied

Exchangeable

Applied

Exchangeable

Appl ied

Exchangeable

7,000

400

6,500

180

1000

50

200

14,000

500

13,000

240

2000

70

300

21,000

700

19,500

290

3000

85

100

in Mauritius before World War II and summarized by D'Hotman de Villiers (1961). The basalt was called "mother rock" and the amending process "rejuvenation," signifying the replenishment of the leached soils with materials formerly present. Phosphorus Next to soil acidity, phosphorus deficiency is the most frequently encountered agronomic problem in the humid tropics, but that does not mean that nitrogen, potassium, trace elements, or other nutrient deficiencies or excesses are not important. The cure for all is well known j the rule of thumb is to lime acid soils and apply fertilizers to nutrient deficient soils. But if the cure is known, there is less agreement on how best to diagnose the problem and administer the cure. The phosphorus problem is unique because its solution entails the application of chemical and physical principles. We have already cited the effects of phosphorus on pH o and therefore the surface charge characteristics of variable charge materials. Phosphorus is not only an essential plant nutrient, it also serves as a soil amendment, Physical principles come into play when we begin to search for efficient ways to apply phosphorus fertilizers. A considerable part of this section deals with the physical theory of fertilizer placement developed by de Wit (1953), and thus this section serves to link the chemistry and physics of soils with variable charge clays.

Identifying the Phosphorus Problem Although phosphorus deficiency is a worldwide problem, it

Chemistry

75

appears to be more acute in the humid tropics where large tracts of land remain unused because of the inordinate amount of phosphorus required to correct the deficiency. This is particularly true in regions where there are acid, heavy-textured soils that are high in oxide and hydrous-oxide mineral content. The extreme examples would be the highly weathered soils that have been derived from volcanic ash. The evidence suggests that the most serious phosphorus deficiency problems occur in soils containing variable charge minerals. Phosphorus is a problem in those soils not simply because it is deficient, but because of the huge amounts of phosphorus that need to be added to obtain good growth. In many cases, the total amounts of native phosphorus are quite high, but the element is present in forms that render it unavailable to plants. Phosphorus added as fertilizer, especially in soluble form, is immediately sorbed onto metal oxide surfaces, and the strength of the chemical bonds formed makes it difficult for plants to compete with the surface for this nutrient. Figure 3.17 illustrates graphically what is meant by the phos-

A

-.-

""'C

CD

>-

X Phosphorus App lied Figure 3.17. Relationship between crop yield and phosphorus applied for a low P-fixing soil (A) and a high P-fixing soil (B).

76

Chemistry

phorus problem. Crop yield has been plotted according to the result of phosphorus applied to two different soils for a crop showing the same phosphorus deficiency symptoms. A farmer whose soil shows a response similar to curve A does not have .a serious phosphorus problem, since a large yield increase is obtained for a relatively small phosphorus application X. But that farmer's counterpart, growing the same crop on soil B, does have a problem because very little is gained from a phosphorus application X and the cost of adding a sufficient amount of fertilizer to obtain yields approximating those obtained on soil A would be prohibitive. Obviously more phosphorus has been sorbed onto soil particle surfaces in soil B, which means the phosphorus is less available to plants. Different soils can be tested for their phosphorus sorption curve by a relatively simple laboratory analysis that equilibrates the soil with increasing concentrations of phosphate under a standard set of conditions; by measuring the phosphorus concentrations at equilibrium, the amount adsorbed can be calculated. Phosphorus sorption curves for four soils of different mineralogies are presented in Figure 3.18. That figure shows that as the mineralogy changes from a permanent charge system (haplustoll) to the more highly weathered, oxidic soils, culminating in the hydrandept, which is dominated by amorphous material, the amount of phosphorus sorbed at any equilibrium concentration (e.g., 0.2 ppm) increases. Curves A and B of Figure 3.17 might correspond to the haplustoll and hydrandept soils respectively. It is now generally believed that the effectiveness of the applied fertilizer phosphorus largely depends on the phosphorus remaining in the soil solution, as it is from this source that the plant adsorbs this element. Therefore, the value of the sorption curve lies in its ability to predict the amount of phosphorus that has to be added to a soil to achieve a given solution concentration. The curve, however, tells us even more. As plants deplete a soil solution's phosphorus, the solution must be continuously recharged if good growth is to be maintained. Recharge occurs when phosphorus is desorbed from the soil surface, and this will happen in sufficient quantity only if the soil has a large capacity to sorb and therefore desorb phosphorus. The clue to

Chemistry

700

77 EQUILIBRIUM CONDITIONS 0.01 COCl2 6 DAYS

~

.....

25'C

5000

~

::L.

Q ILl

CD

a::

0

(J)

~

1000 - - 0.01

0.1

P IN SOLUTION

rlAPLUSTOLL

02

1.0

(ppm)

Figure 3.18. Phosphorus sorption curves for four clayey soils with different clay mineralogies (Fox, 1978).

. this capacity is the steepness of the phosphorus sorption curve, and it can be seen that the steepness increases from haplustoll to hydrandept. Thus, even though the more oxidic soils require more phosphorus to achieve a given level of phosphorus in solution, they have the compensating value of being able to supply phosphorus to the soil solution as it is taken up by plants. Different crops require a different amount of soil solution phosphorus for optimum growth, and Figure 3.19 illustrates the phosphorus requirements of five root crops. It is immediately obvious from this figure why cassava is widely grown by subsistence farmers in many tropical areas, since optimum yields of cassava are obtained at very low levels of phosphorus in solution. At the other extreme is the Irish potato, which has a large phosphorus requirement for a good yield. The subsistence farmer normally chooses crops that will give adequate yields even under the most adverse conditions of phosphorus stress. Some of the high-yielding, early maturing, nutrient-rich maize and wheat varieties that brought about the green revo-

Chemistry

78

100

..-'.", _ - - - ~ ----- .,.c:::::::::--_ --..c:.....---.. ---..,."

/ " /

.>

<,

"X .... .....

,/

•• 0

", /

~

80

: -'-...

~

/

90

....... .' .' .

~

,

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~

~

.........

--

-

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.. 0

~

. .- .'

-~-

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/

....

><

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E 70 '+-

0

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>-

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Q)

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40

o

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o f-"-_..L.-

.------ Cassava .................... Potato -'-'-'-'- Sweet-potato ----Taro . ----- Yams -----L_-'------'--~_....a....-.___'__ ____

0030060/2 025 05

.1

.2

4

.8

1.6

P in solution (ppm) Figure 3.19. Effect of phosphorus in the soil solution on the relative yields of five root crops (Vander Zaag, 1979).

lution have a high phosphorus requirement. The small farmer who farms a high phosphorus-fixing soil cannot use these new varieties unless there is some practical means to overcome the phosphorus problem. It is the responsibility of the researcher to identify, understand, and ultimately develop the means to counter this problem. The Cause ofPhosph 0 rus Fixation

The phosphorus sorption curves shown in Figure 3.18 clearly illustrate the wide differences in phosphorus adsorption among

Chemistry

79

soils. To maintain a phosphorus concentration of 0.2 ppm in the soil solution, the haplustoll would require little or no addition of phosphorus over what is already present, but the hydrandept would require about 2800 J1.g of phosphorus per gram of soil to attain the same level of phosphorus in solution. This is equivalent to about 3.0 t/ha of phosphorus or 30 tonnes of single superphosphate per hectare! The other soils display intermediate phosphorus sorption curves. There are a number of reasons for the observed differences in phosphorus adsorption, among which are 1. 2. 3. 4.

differences in surface reactivity differences in specific surface differences in capacity to occlude phosphorus the presence of species that compete with phosphorus for adsorption sites

1. Differences in surface reactnnty. The layered silicates, such as the smectites, do not adsorb much phosphorus. This is illustrated in Figure 3.18 by the haplustoll because it has montmorillonite as the dominant mineral species, with some kaolinite. The oxides and hydrous oxides of iron and aluminum, on the other hand, adsorb large quantities of phosphorus. Let us consider the eutrustox and the haplustoll examples in Figure 3.18. The eutrustox has a specific surface of about 100 m 2/g, and the haplustoll can be assumed to have at least that specific surface and most probably double that value in view of its high montmorillonite content. Both soils are heavy clay soils. If one takes a conservative estimate for the haplustoll and assumes equal specific surface for the haplustoll and the eutrustox, one can see that when the soil phosphorus concentration is 0.2 ppm, the amount of phosphorus sorbed on the eutrustox per unit surface area exceeds that on the haplustoll by two orders of magnitude. Surface reactivity thus refers to the quantity of phosphorus adsorbed per unit surface area at any soil solution phosphorus concentration. The high phosphorus-fixing capacity of some tropical soils is related to the high affinity of the mineral surface for phosphorus. 2. Differences in specific surface. Soils that contain minerals

80

Chemistry

with high surface reactivity can still show a wide variation in their capacity to sorb phosphorus. The differences can be attributed to the degree of subdivision of the reactive particles and/or to the number of reactive particles; in other words, the total surface area of the reactive particles per gram of whole soil. Thus a spectrum of soils that contain only quartz in the sand fraction and hydrated iron oxides in the clay fraction will show increasing phosphorus adsorption as the quartz content decreases or the iron oxide content increases. Thus there are many red, acid soils in the tropics that adsorb relatively small amounts of phosphorus because they are diluted with inert quartz particles. The presence of other mineral species in the clay fraction can also influence the specific surface of iron oxides. Thus, Gallez et al. (1976) showed that the specific surface of iron oxides in some Nigerian soils that were derived from basalt was about 300 m 2/g, but the specific surface was only about 50 m 2/g for iron oxides that were derived from acidic parent rocks. The low values were attributed to a strong association between kaolinite and the iron oxides, which leads to a net loss of reactive surface. As a result, the phosphorus sorption of Nigerian soils that were derived from acidic rocks was much lower than those that were derived from basic rocks (juo and Fox, 1977). 3. Differences in capacity to occlude phosphorus. In soils such as the hydrandept (Figure 3.18), the concept of specific surface loses its conventional meaning. The hydrandept consists not of minerals with well-defined surfaces but of mineraloids, which are amorphous, and in some respects amorphous minerals behave like liquids. In their fully hydrated state, they may flow and coalesce, which can cause old surfaces to be buried and new ones to be formed. A surface with adsorbed phosphorus may coalesce with another surface, and the phosphorus ion that was once adsorbed on that surface is then occluded. New surfaces of high reactivity can be exposed through flow and deformation to adsorb more phosphorus from solution. Soils with amorphous oxides and hydrous oxides of iron and aluminum adsorb the larg.est quantities of phosphorus. The electron micrograph of Figure 2.9 illustrates the nature of amorphous inorganic oxides and hydrous oxides. The crystal-

Chemistry

81

line oxides and hydrous oxides-mainly hematite, goethite, and gibbsite-are embedded in an amorphous matrix. The crystalline oxides and hydrous oxides are themselves less than 50 A in diameter along their longest axis, but even these highly subdivided particles probably do not adsorb as much phosphorus as the amorphous matrix that binds them into aggregates. In Figure 2.11, an electron micrograph of a hydrandept shows that a greater proportion of the mass is amorphous. The gelatinous film that stretches across the holes in the substrate of the grid appears in some cases to have shrunken and ruptured upon drying. Those greatly magnified pictures of the soil constituents help to explain phosphorus occlusion and its role in phosphorus fixation. 4. Presence of competing species. Some organic and inorganic ions other than phosphates are adsorbed onto oxide and hydrous oxide surfaces. Phosphorus appears to be the most competitive adsorbate at least among the inorganic species, but the presence of other anionic species can reduce the phosphate sorption capacity. Thus Roy et al. (1971) found that less phosphorus was adsorbed by a range of Hawaiian soils when silicate was added as basic slag. Unfortunately, though, the effect was greatest on the soils that already had the lowest phosphorus sorption capacity. Apparently, calcium silicate, which hydrolyzes to monosilicic acid in solution, partially adsorbs onto the surface and thus reduces the number of sites available for phosphate sorption. Similarly, the presence of organic matter reduces the phosphorus sorption capacity. In Figure 3.20 the phosphorus sorbed at 0.2 ppm solution concentration was appreciably lower for samples collected from the soil surface than for subsoil samples containing the same oxide content. It should be remembered, however, that surface soils could contain greater amounts of adsorbed silica as a result of plant recycling, and this situation could also result in a reduced phosphorus-sorption capacity. Correcting the Soil Phosphorus Problem

The problem of phosphorus fixation in acid, high-clay, highsesquioxide soils can be lessened by a number of management practices. These include liming with calcium carbonate to cor-

Chemistry

82

•

1.2

z

•

o

.-..

Q,

a:

o

0·8

en w--;p.&

...

•

•

-..;;,;

• •• •

CO)

~E Q,-0·4

en

o

~ Q,

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0·2

• 14

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30

Figure 3.20. Effect of organic matter content in reducing phosphorus sorption on high sesquioxide (R 2 0 3 ) soils. The lower and upper curves represent surface and subsurface horizons (Gillman, 1970) .

rect soil acidity, liming with calcium silicate, applying fertilizer in bands, and applying less-soluble phosphate fertilizers. Each of these practices will be discussed with respect to their limitations and special advantages. a. Liming with calcium carbonate. Liming with calcium carbonate is generally the first management practice that comes to mind when the question of high phosphorus fixation in acid soils is raised. But unless an experiment is designed to separate out the pure effects of such liming on phosphorus nutrition, the results lead to confusion because lime corrects other effects such as calcium deficiency, aluminum toxicity, and trace-element imbalances. The correction of these soil factors invariably results in increased phosphorus uptake even though the lime had

Chemistry

-

~ CD '0

e

-

7r------r-....------,----,--.--,.--,.--.-----,--.----,----,--,

6-

:L

5-

o

4

w

CD

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83

pH 5.2 pH 6.0

------------

3

20

40

EQUILIBRIUM

I

I

60

80

TOTAL

PHOSPHATE

100

120

(fLM)

Figure 3.21. Phosphate sorption on kaolinite as a function of pH (Chen et al. 1973).

no effect on reducing phosphorus sorption. When the effect on these soil factors is removed from consideration, the pure effect of lime application on reducing phosphorus fixation is generally small. This observed effect appears at first to be in contradiction to what is known about the dependence of phosphorus adsorption on solution pH. A typical set of data illustrating the pH dependence of phosphorus adsorption is provided in Figure 3.21. An immediately favorable impression of liming is given by a cursory examination of Figure 3.21, but a more careful study of the same data reveals why liming and the correction of soil acidity do not lead to better phosphorus nutrition in plants. If one follows the family of curves from right to left, one discovers that the wide differences in the phosphorus sorbed at high concentrations virtually disappear at low concentrations. In fact the curves coalesce into a single curve, and it is this coalesced region of the phosphorus sorption curve that is relevant to the agronomist. It is now generally recognized from the work of Fox and his coworkers that even plants that require high phosphorus

84

Chemistry

concentrations need less than 0.1 ppm phosphorus in the soil solution. In this low concentration range the reduced effect of high pH on phosphorus fixation is masked by other uncontrolled variables, and it is generally difficult to demonstrate the direct benefits of liming in field experiments. Liming to reduce phosphorus sorption is important in systems that contain 2: 1 clay minerals, in which acidity releases free aluminum. Applied phosphorus is then precipitated as aluminum phosphate, so that the application of lime to eliminate soluble aluminum has a direct and important effect on phosphate fixation. b. Liming with calcium silicate. The effect of silicate on phosphorus sorption has already been mentioned. When silicates are applied to soils, however, it is not at all clear whether the increased phosphorus uptake is due to reduced phosphorus fixation or to improved plant health caused by an increased uptake of silica. Although silica is not considered to be an essential nutrient element, it does contribute to greater crop resistance to insects, diseases, and wind damage. The greatest response to calcium silicate application is generally obtained with plants that normally have a high tissue-silicon content, such as rice and sugarcane. On the other hand, marked improvement in growth has been observed with crops such as lettuce (Silva, 1973). The lettuce response to calcium silicate is most likely due to a greater availability of soil phosphorus when calcium silicate is applied. In most liming experiments that compare calcium silicate with calcium carbonate on highly weathered soils, calcium silicate is at least equal to or superior to calcium carbonate. Compared on a cost-benefit basis, the higher cost and unavailability of calcium silicate puts it at a disadvantage. Calcium silicate should be used only when the returns justify its use. c. Fertilizer placement. The problems of fertilizer placement and nutrient uptake by crops have been elegantly summarized by de Wit (1953) and van Wijk (1966, p. 3). Their analyses explain the contradictory results that are frequently obtained when phosphate fertilizer is broadcast or applied in bands of varying widths. Unfortunately there do not appear to be many tropical field experiments based on their analyses. Their

Chemistry

85

analyses offer great promise for the development of extensive areas of the tropics in which the single most important limiting soil factor for crop production is high phosphorus fixation. The essence of de Wit's (1953) analysis of fertilizer placement is summarized in Figure 3.22. Curve 1 of that figure illustrates the classic S-shaped nutrient uptake, or yield versus broadcast fertilizer-application rate, curve of a high phosphorus-fixing soil. Curves 2, 3, and 4 represent the relationship for the same soil when the nutrient is applied in bands of decreasing width. The band width increases to the right until curve 1 represents coalescing bands equivalent to broadcast application. The vertical line A, equal to a phosphorus application rate of 100 kg/ha, intersects all curves and illustrates the effect of band width on 1

10,000

2 8000

3

A .-..

-

6000

-.-

4000

e .s::.

4

01

~ "-'

"'CI

•••••• __ B

G)

>-

2000

100

200

300

400

500

Phosphorus appln. rate(kg/ha) Figure 3.22. The relationship between crop yield and phosphorus application when phosphorus is concentrated in bands of varying widths. Curve 1 is phosphorus broadcast and mixed in the entire area. Curves 2, 3, and 4 represent phosphorus applied in bands of decreasing width.

86

Chemistry

yield. A soil that is virtually incapable of supporting a crop with 100 kg/ha of broadcast phosphorus will produce nearly 50% of maximum yield with the same amount of fertilizer applied to a narrow band. The advantage of banding phosphate fertilizers is well known. What is generally not understood is the sensitivity of nutrient uptake to band width. An individual not familiar with the relationships of Figure 3.22 could easily conclude that if a small amount of fertilizer placed in a band is beneficial, a larger amount placed in the same band would be even better. If one follows the path of the narrow band (curve 4) one soon discovers that the early advantage gained by applying low rates of phosphorus in a narrow band is quickly lost when higher rates are applied. On the other hand one can increase the application rate and band width simultaneously and not measurably increase the phosphorus uptake. This is illustrated by the horizontal line B, which shows an increase in the phosphorus application rate from 60 to 140 kg/ha with no increase in uptake. Although de Wit's analysis of the fertilizer placement problem is subject to criticism, it at least provides a basis for designing rational field experiments. He presents a solution in the form of a hypothesis that lends itself to easy verification, and if found to hold true, it can have an important impact in the development of the tropics where phosphorus is a major food production constraint. A more detailed description of de Wit's analysis and some of the assumptions contained in the analysis follow. It begins with an examination of nutrient uptake from solution cultures. When the entire root mass Xb of a plant is immersed in a nutrient culture, the plant will remove Ub grams of the nutrient in question. If the plant is also allowed to remove the nutrient from an identical solution but with only a part Xr of the root mass immersed in the solution, Ur grams of the nutrient will be taken up by the plant. If one plots Ur/Ub as a function of XrlXb, a relation such as that shown in Figure 3.23 is obtained. The results show that one-half of the root mass or XrlXb = 0.5 can remove three-quarters as much nutrient as the entire root mass. The experimental relationship between Ur/Ub and XrlXb is

87

Chemistry

(3.22) and this relationship appears to be independent of crop type and nutrient solution concentration. The last two properties of Equation 3.22 enable one to utilize it for diverse crops under field conditions. In the field Ub, Ur, Xr, and Xb take on new meanings. They are according to de Wit (1953) Ub = uptake rate from broadcast fertilizer Ur = uptake rate from banded fertilizer Xr = width of the fertilizer band Xb = distance between the crop rows 100r-

~

80

60

20

, ,, "

~

~

~

,,

,

0·2

, ,,

,~'

,,

,

, ,,

0·4

,

, ,,

,

, ,,

0·6

,

, ~

, ,,

,

, ,,

,

, ,,

~

~

0·8

1·0

Figure 3.23. The relationship between nutrient uptake ratio Ur/Ub and the fraction of roots Xr/Xb immersed in a nutrient solution (de Wit, 1953).

88

Chemistry

For a given depth of incorporation, XrlXb represents the fraction of soil volume that has been fertilized. The physical analysis requires that the concentration of fertilizer per unit mass (or volume) of soil be identical in the banded and broadcast treatments. This means that the ratio XrlXb represents the fraction of broadcast fertilizer applied to the whole field in bands. Thus when Xr/Xb = 0.5, the amount of banded fertilizer applied per hectare of whole field is one-half the amount that would be applied to the whole field as a broadcast treatment. Most agronomists are accustomed to thinking in terms of equal rates on a hectare basis and so generally think of squeezing more fertilizer in a narrow band. But the analysis is based not on that premise but on equal concentration per unit of fertilized volume, and therefore, the soil solution concentration in the banded and broadcast areas is the same. Thus the phosphate sorption isotherms and the phosphorus remaining in the soil solution become powerful parameters in the analysis. A key element in the transfer of laboratory nutrient culture results to field situations lies in the fact that Equation 3.22 does not have a nutrient concentration term. This being the case, Equation 3.22 holds so long as the ratio of the solution concentration in the banded and broadcast soil treatments remains at unity. This implies that the soil solution concentration may change with time, but the change must be synchronous. The probability of this ratio remaining constant over a cropping cycle is greatest in soils with high phosphorus-fixing capacity. Soils with steep sorption curves (high phosphorus fixers) would exhibit less change in solution phosphorus due to plant uptake or leaching than low phosphorus-fixing soils for the same quantity of phosphorus removed. This is fortunate, since the potential benefit from fertilizer placement is highest in high phosphorus-fixing soils. The use of data from phosphorus sorption curves concerning soil solution phosphorus concentration to predict phosphorus uptake and crop yield in combination with phosphorus placement analysis offers a way to increase fertilizer use efficiency. The question that is answered by this integrated approach is, How can the yield response curve, such as the one shown in Figure 3.17, be altered to benefit the farmer? The answer, based

Chemistry

89

on Equation 3.22, can be computed and displayed in a form similar to Figure 3.22. The result will vary with crop and soil, but the result can be made independent of season if yield or uptake is expressed as a relative yield, i.e., as a fraction or percentage of the maximum. To generate curves 2, 3, and 4 in Figure 3.22 it is only necessary to have on hand uptake or yield data as a function of increasing broadcast application of phosphate fertilizer. Such data result in curve 1 (XrlXb = 1) in Figure 3.22, and Equation 3.22 is employed to generate curves 2, 3, and 4 or any number of desired curves. Examples of computed data points for Figure 3.22 are given in Table 3.7.

Table 3.7

Calculation of yield from banded fertilizer (Ur) and corresponding banded application rate (Rr), for various band widths, using yield (Ub) from broadcast application rat" (R)

Broadcast Data

Xr Xb

Ur ~ Ub(

Xr )0.44 Xb

Rr = R ( Xr ) Xb

R = 100 kg/ha Ub= 2500 kg/ha

1.0 0.75 0.50 0.25

2500 2200 1840 1360

100 75 50 25

R = 200 kg/ha Ub = 6000 kg/ha

1.0 0.75 0.50 0.25

6000 5290 4420 3260

200 150 100 50

R = 300 kg/ha Ub = 8750 kg/ha

1.0 0.75 0.50 0.25

8750 7710 6450 4750

300 225 150 75

R = 400 kg/ha Vb = 9750 kg/ha

1.0 0.75 0.50 0.25

9750 8590 7190 5300

400 300 200 100

R = 500 kg/ha Ub • 10,000 kg/ha

1.0 0.75 0.50 0.25

10,000 8810 7370 5430

500 375 250 125

90

Chemistry

Curve 1 of Figure 3.22 shows us that 90% of maximum yield is obtained when phosphorus is broadcast at a rate of 320 kg/ha. lf a phosphorus sorption curve for this soil were available, we could ascertain the phosphorus soil solution concentration that corresponds to this application rate. For curves 2, 3, and 4, the phosphorus application rates that give 90% of maximum yield are 240, 160, and 80 kg/ha respectively. These rates are expressed on a "whole field" basis, but if we were to calculate the application rate within each band, they would all be 320 kg/ha. In other words, the soil solution phosphorus concentration corresponding to 90% of maximum yield for a particular crop is a single value for any band width. This is a significant result. It tells us that if a farmer has a specified amount of phosphorus to be applied on a given area for a particular crop and knowledge of the concentration of phosphorus in the soil solution necessary to obtain maximum yield for that crop, the farmer can consult a phosphorus sorption curve constructed for that soil to read off the phosphorus application rate and then band the fertilizer to achieve that banded application rate. As an example, suppose a farmer has 100 kg of phosphorus to apply to 1 ha of land and the requirement for the crop is 0.01 ppm phosphorus in solution. A phosphorus sorption curve might show that 400 kg/ha of phosphorus are required to achieve such a level in the soil solution. The farmer would therefore band the fertilizer so that only a quarter of the field was fertilized (i.e., XrlXb = 0.25) in order to achieve the highest yield possible for that amount of fertilizer. It should be remembered that factors such as the source of phosphate must be accounted for in the analysis, and this is done in the construction of the phosphate sorption curve. Thus if rock phosphate is the source, a phosphorus sorption curve based on that material must be used. It may also be necessary to obtain a new sorption curve every few years as the isotherm shifts with long years of phosphorus fertilization. From the foregoing discussion we have reached a trivial but immensely practical conclusion. Faced with a high phosphorusfixing soil and a small quantity of fertilizer, the fertilizer can be used to best advantage by concentrating it in a band so that the phosphorus concentration in the soil solution is identical

J

Chemistry

91

to the concentration that gives maximum yield in broadcast application. Any deviation from this optimum vlaue, either to higher or to lower concentrations, leads to less than an optimum return per unit of fertilizer input. Although experimental verifications of this analysis may falter at high concentrations owing to horizontal and downward leakage, the analysis deserves full testing in the high phosphorus-fixing soils of the tropics. d. Low solubility of phosphorus fertilizer. There is a natural inclination in any scheme to reduce the inordinate amount of phosphorus fixed by some tropical soils through the use of lowsolubility fertilizers. This strategy has special appeal in regions where rock phosphate is readily available. Rock phosphates vary greatly in solubility, and this quality is related to the crystal chemistry of the phosphate mineral. The relationships between citric-acid soluble phosphorus and crystal chemistry has been worked out by Lehr and McClellan (1972) at the Tennessee Valley Authority (TVA) fertilizer research center. The TVA fertilizer research provides agronomists with a reliable and sound basis for evaluating rock phosphate quality. Rock phosphate has an important place in the development of tropical agriculture, but its place in the scheme needs to be specified. The advantages of using rock phosphate are its lower cost and its sometimes ready availability. One disadvantage is its low solubility, although some would consider that quality to be an advantage. Another disadvantage of rock phosphate is that its solubility decreases rapidly with increasing levels of applied or naturally occurring lime. When the disadvantages are considered it becomes evident that rock phosphate can be used to best advantage in situations where a combination of acid soil and an acid tolerant crop exists. An excellent example of such a combination is the pineapple crop, which can withstand high soluble aluminum (low pH) and can extract phosphorus from solutions of low concentration. In fact, those plants that are acid tolerant appear to be those that can also withstand low phosphorus concentration in the soil solution. Rock phosphate may become an even greater factor in the efficient utilization of acid oxidic soils of the tropics if plant breeders succeed in selecting crops that do well without benefit of liming and that, at the same time, require low phosphorus concentrations in the soil solution.

92

Chemistry

On the other end of the spectrum, some crops do best when the soil acidity is low and the soil solution phosphorus concentration is high. Many of the important food crops, bred and selected for early maturity and high yields in the high base soils of the temperate regions, fall in this category. It may very well be that for many crops, high nutrient concentration is a necessary condition for early maturity and high yields. The wide range in the external phosphorus requirement (phosphorus concentration in the soil solution) of plants suggests that crops that need high phosphorus concentrations are better served with high-solubility fertilizers. The optimal economic use of highsolubility fertilizers is gained through maintaining optimum soil solution concentrations in bands of decreasing width as described in the previous section. The rule of thumb is to compare the different costs of raising the soil solution phosphorus to an optimum level for a particular crop and soil. If this phosphorus level can be reached at less cost with rock phosphate than with super or treble superphosphate, then rock phosphate would be the better phosphorus source. It appears that it would be more economical to use high-solubility phosphorus fertilizers when the crop has a high external phosphorus requirement. This would be especially true if, in addition, the crop required a high base saturated soil.

Residual Effects and Hysteresis Were it not for the long residual effects of phosphorus, there would be little hope of ever developing the high phosphorusfixing soils of the tropics for intensive agricultural use. Figure 3.24 illustrates the effect of phosphorus fertilization on the phosphorus adsorption curves of a soil. The resistance of the fertilized curve to return to the left with time demonstrates the residual effect. In soils that require a heavy initial application of phosphorus, the quantity of phosphorus removed by the crop and lost through leaching is negligibly small compared to the amount that is added. The remaining adsorbed phosphorus continues to supply the soil solution with phosphorus, but at a lower concentration. However, the phosphorus concentration in the soil solution is

Chemistry

3000

93

HYDRANDEPT

~2000 -o CD

::L.

w or::

co

oV> 0..

1000

NO PHOSPHATE 6 TONS SUPERPHOSPHATE 5 CROPS TOMATOES

QOl

OJ

P REMAINING IN SOLUTION (ppm) Figure 3.24. Effect of phosphate application on the phosphorus sorption curve of a Hydrandept (Fox and Searle, 1978).

not a single valued function of the sorbed phosphorus. The slopes of the adsorption and desorption isotherms differ as shown in Figure 3.25, and the slope is steeper for adsorption than for desorption. This difference in slope is commonly termed hysteresis and is an important factor in determining the magnitude of the residual effect. The greater the hysteresis, the smaller the residual effect. Fox (1978) shows that the phosphorus adsorption isotherms shift to the right as the phosphorus application rate increases but the slopes of the adsorption curves remain relatively constant. This shift in the isotherms to the right can be measured many years after the fertilizer has been applied. However, with time after the application of phosphorus, the isotherms progres-

Chemistry

94 500

400

..-..

CJ)

desorption

CJ)

~ 300

....... "a

..

Ql

.t.l

~

adsorption

200

A.

100

0·01

0·1

1·0

P in supernatant (ppm)

Figure 3.25. Phosphorus sorption hysteresis as indicated by the difference in the adsorption and desorption curves.

sively return to the left, and the rate of return depends on the quantity of phosphorus desorbed through plant uptake and leaching losses as well as on hysteresis. High hysteresis is associated with low residual efficiency of the remaining adsorbed phosphorus. The hysteresis factor can be defined as Hysteresis Factor =

~

where A is the amount of fertilizer lost by crop removal and leaching, and B is the quantity of phosphorus that must be added above and beyond A to regain the initial soil solution phosphorus concentration. The case B = 0 represents a special case in which it is only necessary to reapply the amount of phosphorus removed to regain the initial phosphorus concentration, which in most cases would constitute less than 25 kg/ha

Chemistry

95

per harvested crop. This analysis is valid only if the assumption of parallel adsorption isotherms holds up. The two factors that must be considered in the utilization of high phosphorus-fixing soils are the magnitude of the initial application rate and the subsequent hysteresis effect. If the initial or quenching rate is high but the hysteresis factor is low, the initial cost can be amortized over many years and treated as a capital investment cost. Although it is true that an inordinate initial phosphorus input will discourage development, it will be the magnitude of the hysteresis factor that will eventually determine the economic success of agricultural schemes on high phosphorus-fixing soils.

4 Physics Rheology and transport phenomena are the two main subdisciplines of soil physics. Rheology is the science that deals with flow, deformation, and stress-strain relations of matter. It relates to such practical problems as compaction, tillage, trafficability, and bearing capacity of soils. Transport phenomena, on the other hand, belong to that part of soil physics that deals with the flow and retention of water, heat, gases, and solutes. They relate to such practical problems as water infiltration and drainage, soil temperature, soil aeration, and fertilizer and salt leaching. RHEOLOGY

Three equations are required to describe the full range of soil rheologic behavior, and the equations describe matter in the viscous, plastic, and elastic states. Reiner (1960) gives the ideal rheologic equations for viscous, plastic, and elastic materials in shear as

dr T = 'Yl dt

(4.1)

T =

~

(4.2)

T =

p'a

(4.3)

where T is the shear stress, 'Yl is the viscosity coefficient, dr/dt is the change in strain with time, ~ is the tangential yield stress, p. is the shear modulus, and a is the shearing strain. 97

98

Physics

Equation 4.1 applies to an ideal viscous or Newtonian body for which the time rate of strain is directly proportional to stress. A freshly prepared rice paddy approaches characteristics that might be described by Equation 4.1. If we continue to use the rice paddy as an example, the St. Venant model for a perfectly plastic body (Equation 4.2) becomes the more appropriate equation for a field at an intermediate stage of drying. When the paddy is permitted to dry and bake in the sun, a hard crust forms, described by Equation 4.3, that behaves like an elastic, Hookean body. This body responds to shear, compressive or tensile, and hydrostatic stresses. The elastic constants that correspond to these stresses are, respectively, the shear modulus, Young's modulus, and the bulk modulus. The reciprocal of the Young's bulk modulus is the compressibility. The rheology of a soil material is sensitive to water content and the degree to which the material has been worked or puddled. Thus a single rheologic constant is inadequate to describe the rheology of a soil material. Some other means is required to characterize the rheology of soil materials. The Atterberg Constants The most commonly measured rheologic parameters of soil are the Atterberg constants or the liquid limit and the plastic limit. A third parameter known as the plasticity index is obtained by subtracting the plastic limit from the liquid limit. The liquid limit is the water content at which a soil material turns from a viscous to a plastic state, and the plastic limit is the water content at which a soil material turns from a plastic to a near-elastic state. The plasticity index is the water content range over which a soil material is in a plastic state. Rheologically a soil material may be represented as a point on a Casagrande plasticity chart. The point is located vertically by the soil material's plasticity index and horizontally by its liquid limit. The location of a soil material on the plasticity chart determines its suitability for specified uses. A plasticity chart for clay from representative soils is shown in Figure 4.1. The location of a point on the plasticity chart depends,

99

Physics

>< w

C 40

o

Vertisol clays

o

Oxisol

A

Histosol material

C

And isol material

clays

o

o o CH 0

o

Z

>t-

o

V

- 20 t-

o

o

0

0 MH

V)

« .... a. 20

40

LIQUID

60

80

100

LIM IT

Figure 4.1. Plasticity chart showing location of four different types of soil materials.

among other things, on the type and amount of clay in the sample. If only one clay type is considered, a good, direct, linear relationship exists between the plasticity index and the clay content. If soil samples from a wide range of sources are analyzed, the relationship between the plasticity index and clay content becomes less certain. For a given clay content, the plasticity index varies with the clay type, the valence of the exchangeable cations, the salt content, and the organic matter content. Clay Activity To account for the effect of clay type on the plasticity index, Skempton (1953) defined the term clay activity as the ratio of the plasticity index and percent clay. Activity values are normally less than 0.5 for kaolinitic clays and greater than 1.0 for smectite. The clay fraction of the highly weathered soils of the tropics is low in activity. Soils with low-activity clays do not shrink or swell greatly, are generally well aggregated and therefore have higher water intake rates, which in turn reduces hazards from erosion. They can accommodate traffic more readily after heavy

Physics

100

rains and offer less resistance to tillage implements than do soils with high-activity clays of comparable textures. Properties accessory to the above are low cation-exchange capacities of the clay and low retentions of available water. The practical soil scientist must constantly seek to understand the relationships among soil properties. The property that co-varies with the largest number of other properties and therefore has the largest number of accessory features is the most useful property. Although clay content and clay type are powerful indicators of other soil characteristics, those accessory characteristics can be obtained equally well from clay activity. Clay activity has the additional advantage of being easier to measure than clay mineralogy. Furthermore clay activity integrates clay content and clay mineralogy into a single, quantitatively expressible number. Clay Activity and Shear Strength The shear strength of a soil material is the sum of the cohesion and the internal friction. Skempton (1953) showed that the contribution of cohesion to shear strength increases with clay activity. In kaolinitic samples with activity of about 0.4, 80% or more of the shear strength is attributable to internal friction. The activity of some common minerals are listed in Table 4.1. Quartz and muscovite exhibit low activity even when they are Table 4.1

Activity (plasticity index / % clay-sized particles) of common soil minerals Mineral

Activity

Quartz

0.0

Mica (muscovite)

0.18

Kao 1i nite

0.33 - 0.46

Illite

0.90

Ca-montmorillonite

1.5

Na-montmorillonite

7.2

Source:

Skernp ton , 1953

101

Physics

ground to clay size. Although there are other contributing factors to the low activity of clay-sized quartz, mica, and even kaolinite, the primary cause is low specific surface. Thus for the weathered soils of the tropics, their non-cohesive characteristics can be related to low specific surface or low clay activity.

Clay Activity, Swelling, and Aggregate Stability Swelling clays are high-activity clays, and non-swelling clays are low-activity clays. Figure 4.2 shows the relationship between clay activity and the expansiveness of soil materials. Soils with clay activities less than 0.5 are treated as low-expansion materials. Because low-activity clays do not expand upon wetting, aggregates of low-activity clays tend to be more stable and do not slake when wet. Large, water-stable aggregates create large, water-stable pores that in turn lead to high water-intake rates and a low inherent-erodibility of a soil. The fact that extensive areas of the humid tropics with low-activity clays are eroded should not detract from the causal relationship between low-

V

60 ~

!:/ ~

~Q.

J

V

o

a.:

,

J

..: 20

/

~

/

J u.

_\

/

HIGH

1u.6 IUM / ' 1/ ./

%:../

,,'" bf' ...(;)

/

/

VERY HIGH

., ~

Do.9 \)'~

--

--

..,

LOW

20

40

60

CLAY FRACTION OF WHOLE SAMPLE(%) Figure 4.2. Categorization of soil clay according to activity (Morin and Todor, n.d.).

102

Physics

activity clay soil and inherent soil erodibility. The K factor in the universal soil-loss equation is the index of a soil's inherent erodibility (Wischmeier, 1976). Erosion and soil loss in the tropics are largely due to careless management and inattention to conservation measures. Rheology of Soils with Non-crystalline Clay Non-crystalline solids can be looked upon as supercooled liquids. They show no tendency to assume the geometrical forms of crystals and exhibit little or no organization of structure. When highly hydrated, they coalesce with other materials like them and form contact angles with other solids. These non-crystalline materials are most common in soils that form in volcanic ash deposits. Volcanic ash contains non-crystalline glass, which weathers to allophane, imogolite, or hydrated gels of iron and aluminum oxides. Allophane and imogolite are hydrated aluminosilicates with short-range order. Allophane is a hollow sphere, and imogolite is fibrous. They have specific surface of about 1000 m 2/g and a surface charge that is variable. A peculiar characteristic of this type of material is a high liquid limit. On the plasticity chart, they would plot out as OH (high organic) materials. Soils with these characteristics are classified as andepts in Soil Taxonomy (U.S. Soil Survey Staff, 1975) and andosols in the FAa Legend (1974). These characteristics are most clearly expressed in the hydrandepts (Soil Taxonomy, 1975) or humic andosols (FAa Legend, 1974). Another unusual feature of these soils is their bulk density. Most soils have bulk densities greater than 1.0 g/cm", but bulk densities of 0.5 g/cm" are not uncommon in andepts. Field moisture contents of several hundred percent, even under well-drained conditions, are common in hydrandepts. These materials dry irreversibly, and the degree of irreversible drying increases with the magnitude of the liquid limit of the undried sample. Virtually every rheologic property changes markedly with drying. These are the same soil materials that lose much of their cation exchange capacity upon drying (see Chapter 2).

Physics

103

The change in a material's chemical and physical character stems from a reduction in its specific surface. A smeary material that feels like soft butter when fully hydrated turns, upon drying, into hard aggregates that look like freshly ground coffee in color and texture. This effect is commonly observed along road cuts, but it rarely occurs in the field since the topsoil protects the subsoils from drying. The effects of drying on the rheologic character of highly hydrated, non-crystalline materials from andosols are shown in Table 4.2. Since the Atterberg constants are used as indexes of the physical and engineering behaviors of soil materials, it is necessary to take proper steps to determine the constants at the appropriate condition. Andosols, however, can be easily identified on the landscape, and their unusual features rarely go unnoticed or are misinterpreted. The irreversible effect of drying on soil rheology is not confined to andosols, and it is in this situation that misinterpretation of data can occur. The effect of drying on the Atterberg constants for more conventional soils is shown in Table 4.3. In the tropics the rule of thumb is not to air dry or oven dry samples prior to analysis. The importance of this rule increases with the increasing wetness of the environment. TRANSPORT PHENOMENA

The flow of water, solute, heat, and air into and out of soil can be adequately predicted in the laboratory with existing transport models, but it is difficult to predict in the field. The difficulty arises from spatial and temporal variabilities encountered in virtually all field situations. Random or stochastic variability may be accommodated by statistical methods, but systematic variability is better handled through taxonomic stratification of soils into groups that behave nearly alike. Soils with variable charge, low-activity clay, fall into one such group. Soils with these characteristics are not identical, but they have common accessory features that can be used to assess the biological, chemical, and physical qualities of land. In the following section, variable surface charge, low-activity clays and their

Table 4.2

ATTERBERG Location

'-

Effect of drying on Atterberg Constants in Humic Andosols (Dystrandepts, Hydrandepts)

CONSTANT

Drying

Before

~

Drying

After

Plastic Limit

Liquid Limit

Plasticity Index

Hawaii*

183

290

107

nonplastic

Hawaii*

105

152

47

nonplastic

Java t

117

164

47

53

60

7

Javat

145

183

38

73

79

6

80

135

55

nonplastic

New Gui nea I

78

132

54

nonplastic

New Guinea r

76

146

70

nonplastic

New Guinea t

Plastic Limit

Liquid Limit

J.

Source:

* Sato (1971) t Morin and Todor (no date)

Plasticity Index

Physics

105 Change in Atterberg Limits After Drying at Various Temperatures and Time Periods. (LL and PL are the liquid limit and the plastic 1imit )

Table 4.3

Air As Recei ved Dried

6 hours at 50°C

24 hours at 50°C

6 hours at 105°C

24 hours at 105°C

IV-5

LL PL

63.4 39.1

62.2 31.2

60.1 27.2

60.7 28.7

57.3 27.4

55.1 28.4

G12- 1

LL PL

54.3 22.4

47.6 22.9

47.1 22.2

44.8 24.1

41.8 23.1

41.9 22.4

DAH-3

LL PL

51.8 29.8

44.6 26.8

44.7 23.9

45.5 22.9

42.8 19.8

42.9 20.3

Gll-4A

LL PL

45.2 21.7

40.0 21.0

40.5 21.6

41.9 21.8

41.0 24.8

37.6 20.7

Gll-4B

LL PL

36.5 38.2

34.5 30.2

36.1 31.4

36.8 29.1

36.0 26.3

35.2 28.2

Gll-6B

LL PL

29.0 21.4

26.5 16.6

26.2 15.6

26.5 14.8

25.8 14.1

24.7 13.8

OKODEE

LL PL

65.0 27.2

62.9 27.8

58.3 29.4

58.5 28.5

49.4 24.4

46.0 25.9

BEKWAI

LL PL

61.6 28.2

53.4 25.5

54.4 26.2

53.9 24.8

44.4 23.6

42.7 23.8

LL PL

45.7 23.2

44.2 23.5

44.4 25.9

44.4 25.9

43.4 26.2

44.4 26.1

TAt~ALE

5

* Source: Morin and Todor (no date)

accessory properties will be examined as they relate to the physics of transport processes. Soil Water To characterize the flow of water in soils, three conditions must be known: the initial condition or antecedent water content, the boundary condition, and the transport coefficients. This simply means that how water applied to a soil moves depends on the wetness of a soil at the time the water was applied (initial conditon), on whether the water was applied by furrow, flood, drip, or sprinkler irrigation (boundary condition), and on the texture, structure, and pore size distribution of the soil (transport properties).

Physics

106

The accessory properties of variable surface charge, low-activity clays that affect water retention and flow are related to soil structure and the resultant pore size distribution. These soil properties are integrated into two precise terms, namely, the water capacity C and the hydraulic conductivity K. A third term, the water diffusivity, is the ratio of the hydraulic conductivity to the water capacity. The water capacity is the slope of the water content-pressure curve. All three terms vary with water content. Tropical soils with low surface-charge density, low-activity clays, are generally better aggregated than high-activity clays. Since pore size between aggregates increases as the aggregate size increases, many highly weathered, clayey soils of the tropics have moisture release curves similar to sandy soils as shown in Figure 4.3. Like sandy soils, strongly aggregated soils attain field capacities at comparatively low tensions. Soils with 60% to 80% clay may thus reach field capacity at a soil water pressure

70

....... ~50

~

-.

-- .. -.-.

~

I-

Z IU

I-

Z

o

vso

B

IU

Ill: ;:)

lll)

o :::e 10

10

TENSION (em)

Figure 4.3. Water desorption curve for a Eutrustox (A) and a Torrox (B). Source: Sharma and Uehara (1968).

Physics

107

of 0.1 bar. In addition most of the plant-available water is removed by the time the pressure is one bar. Between 1- and IS-bar pressure, the water content-pressure relationship is nearly flat, even though the volumetric water content may still be 20% to 30%. When information on field capacity is not available it is advisable to select the water content at 0.1 bar from a moisture release curve to estimate available water, but frequently the 0.3-bar water content provides a better estimate of available water. This discrepancy arises because the water content at 0.1 bar obtained from laboratory-determined moisture release curves overestimates the water content at 0.1 bar in the field. The data supplied in Table 4.4, showing the incomplete saturation at zero pressure in the field, offer an explanation for this apparent discrepancy. In the field, entrapped air within and between aggregates prevents the soil from becoming fully saturated. Table 4.4

Comparison of 85% Total Porosity with Measured "Field Saturated" Soi 1 Water Content.

(1)

(2)

Depth

Total Porosity

85% Total

(% by Vol)

(% by Vol)

(em)

(3) Poros i ty

(4) Measured Field Saturated D

(5) Saturation %

(4)/(2)

(% by Vo 1)

(%)

PLOT NO.2

20

61. 2

52.0

45.7

74.7

40

54.2

46.1

46.0

84.9

60

51. 7

43.9

44.4

85.9

80

54.1

46.0

45.7

84.5

PLOT NO.4

20

54.9

46.7

45.2

82.3

40

57.1

48.5

49.2

86.2

Source:

Unpub11shed data from Green. Rao. and Balasubram1an

Physics

108

For well-aggregated clay soils, 85% saturation at zero pressure appears to be common. Moisture extraction from a soil that is 85% saturated at zero pressure results in a moisture release curve that falls below that of a soil core that has been fully saturated in the laboratory prior to determining the water release curve. Thus the selection of water content at 0.3 bar from laboratory data compensates for the lower saturation in the field. Laboratory water-release curves are useful for assessing the pore size distribution of aggregated soils as shown in Figure 4.4, but field data should be used for estimating water storage for plant use.

16

A

12 --. l')

E u

-..

~

., t-

-a 8

-.. --.

I

I

•

I',

,

• ••, , •,

:

. I

I

,

•,,

4

,, ,

I I'

I

•• •,

I

•

.... ....'::'.

, ,,

,,I

I

I

,

: •• B • ,

,, •, , • •

I

~.~-~

Figure 4.4. Biomodal pore size distribution of aggregated soils from a Eutrustox "A" and Torrox "B." Source: Tsuji et al. (1975).

Physics

109

Well-aggregated tropical soils with low-activity clays are also very difficult to disperse. Since clay dispersion is essential for a particle-size distribution analysis, Soil Taxonomy (1975) permits the use of the relationship % clay = 2.5 X IS-bar gravimetric water content

(4.4)

to estimate clay content. This relationship is not valid for soils that are high in organic matter, non-crystalline materials, or high-activity clays. One can readily see that an andosol with IS-bar gravimetric water content (see Table 3.3) in excess of 40% would not fit this relationship. The factor of 2.5 varies with the specific surface of the clay; the higher the factor, the coarser the clay. An analysis of data for Brazilian soils (Lepsch and Buol, 1974) shows values nearer to 3.0 for oxisols. The low specific surface of low-activity clays and their tendency to form water-stable aggregates contribute to their low water-holding capacity, their high saturated hydraulicconductivity, and their rapid drop in conductivity with increasing negative pressure. The water held within the aggregates, however, resists drainage even at negative pressures in excess of 100 bars (see Figure 4.3). In tillage operations, the shearing stress of the implement is adequate to squeeze water from the nearly saturated aggregate. Free water accumulates on the implement surface, and large masses of soil particles eventually adhere to the implement. In Hawaii this problem is avoided by bolting teflon sheets to the plow face. Solute Transport Prolonged leaching under warm and humid conditions results in soils with low capacities to supply and retain one or more important plant nutrients. This results from a tendency of highly weathered materials to achieve a state of minimum surface potential. The potential on the surface of variable charge, low-activity clay is kT

00 = -

e

H+ In H+ 0

Physics

110

where 0 0 is surface potential, k is the Boltzmann constant, T is the absolute temperature, e is the electron charge, H+ is the hydrogen ion concentration, and H~ is the hydrogen ion concentration when 0 0 is zero. The tendency of H+ to approach H~ was termed isoelectric weathering by Mattson (1932). Under certain conditions, H~ corresponds to the hydrogen ion concentration at the isoelectric point. As H+ approaches H~, the charge on the clay surface diminishes, aggregation of clay particles is favored, an open structure develops, and soluble fertilizers leach through the soil with relative ease. The relationship between H+, H~, 0 0 , and solute transport is summarized in Equations 4.5 to 4.12.

(4.5)

(4.6) (4.7) (4.8)

(4.9)

Nze 1 + (A/Mn) exp -(t/Jd Z€+
(4.10)

kT

(4.11)

(4.12)

111

Physics

D Co X

= = = z = v = t = P{3 = ex = CEC= Y = S = = = = N =

°0 °1 °2 z e A P M

1/I d ¢ k T n €

5

00 H+

= = = = = = = = = = = = = =

H;EP = H+0 =

fluid dispersion coefficient (cmv/sec) influent cation concentration (rneq/cm") relative solution cation concentration depth of the column (em) average interstial flow velocity (em/sec) time (sec) bulk density (g/cm") pore fraction cation exchange capacity (meq/g) relative cation concentration in the exchanger specific surface (em 2 /g) surface charge density (esu/cm") charge in the Stern or compact layer (esu/cm") charge in the diffuse layer (esu/crrr' ) number of adsorption sites on cm 2 of surface (l015/ cm2) counterion valence electron charge Avogadro's number density of water molecular weight of water potential on the plane between the Stern and diffuse layer (statvolts) specific adsorption energy Boltzmann constant absolute temperature electrolyte concentration (ions/ern") dielectric constant of water thickness of the Stern layer surface potential (statvolts) hydrogen activity in soil solution hydrogen ion activity at isoelectric point hydrogen ion activity when 1/10 = 0

Equation 4.5 is a solute transport model that has been successfully used (Lai and Jurinak, 1971, 1972). It can be solved numerically to indicate the whereabouts of solute moving through a soil column provided the initial and boundary conditions are known. The whereabouts of a nutrient cation will

Physics

112

depend on, among other things, the cation exchange capacity of the soil, but the CEC in turn depends on the surface charge 00, which depends on the charge in the Stern layer and diffuse layer and on the surface potential. The relationships among the various parameters in each equation are indicated by connecting lines. The most important conclusion that can be drawn from Equations 4.5 to 4.12 is that the low nutrient-retention capacity of highly weathered soils is not a permanent and irreversible feature of those soils and that leaching losses of costly fertilizer can be measurably reduced. Experimental evidence supporting this thesis has been reported by Wann and Uehara (l978a, 1978b). Figures 4.5, 4.6, and 4.7 illustrate the extent to which leaching losses from soils with low-activity clays can be reduced. The data show that the application of silicate amendment or phosphate fertilizer is effective in reducing leaching losses of calcium, magnesium, and potassium. Silicate and phosphate ions, when adsorbed on the clay surface, alter the surface chemistry by

1200

o :z:: v 0( .. III

800

..

I!;. ..

1110 "'Q,

....... :::eg) :::>E -

...

V

.

• .... . •

A

J::.

400

~

I!;.

v

. '

•

••••• 4 LP OSi

C

C-

.'

~ \;

0(

•

11·--

... ....11A---

C HPOSi

•••• ••••••••••• LP HSi

•••

0

0

0

HP HSi

~.---

o

o

600

1200

1800

2400

LEACHATE VOLUME Cml) Figure 4.5. Cumulative Ca leached from a Gibbsihumox as a function of Leachate volume and P and Si application rate. (LP = 100 ppm P, HP = 750 ppm P,OSi = zero Si, HSi = 968 ppm Si]. (Syed-Fadzil, 1972)

113 100

.... . ' Q I.lol

J:

75

U

< I.lol

=: Cii

....

0

0------=

A-

en

A~.......••

I.lol

e < =:

A:"'" 0 HP 051 ••••••• LPHSI

60.

50

2 E Z

... ••• -A LP 051

. . . ., " . 0 'O;;s-°,'-

"'00.

.."

• A- .-

25

0a.=;

o

~

600

." ."

••• '

~~

.O-----O

HP HSI

......

.....

..J

1200

1800

2400

LEACHATE

VOLUME (ml)

Figure 4.6. Cumulative Mg leached from a Gibbsihumox as a function of leachate volume and P and Si application rate. (LP = 100 ppm P, HP = 750 ppm p. OSi = zero Si, HSi = 968 ppm Silo (Sved-Fadzll, 1972)

.. ' .' •• -4LP OSi

200

Q I.lol

J:

150

U

< I.lol-

_ - - - 1 0 H P 051

... 0 0. 10 0

=:Cii

~_ _O-_--lO HP HSI

=>E

50

600

1200

1800

2400

LEACHATE VOLUME [rnl )

Figure 4.7. Cumulative K leachedfrom a Gibbsihumox as a function of leachate volume and P and Si application rate. (LP = 100 ppm p. HP = 750 ppm p. OSi = zero Sl, HSi = 968 ppm SL) (Sved-Fadzil, 1972)

Physics

114

lowering Ho in Equation 4.7. This results in a surface with a higher negative charge. Figure 4.8 illustrates another aspect of solute transport predicted by Equations 4.5 to 4.12. In the most highly weathered oxisols, calcium occurs in trace amounts so that the applied calcium can be treated as total calcium content. When the sum of the calcium in the plant (roots and tops), leachate, and soil is measured, the value falls short of the calcium applied. The calcium adsorbed in the Stern layer is not removed by conventional extraction procedures and is unaccounted for in the calcium budget. For example, Wann and Uehara (l978a) found that in an oxisol sample treated with increasing amounts of

CALCIUM • 0 0

%

UI

UI

0

0

0

LPOSi

_11111111111111 11111111.

LP MSi

_:1111111111 11111 _11111 II

LP HSi HPOSi

_:11111111 III

HP HSi

~II '6' ! -':lU ~ CD -

:lU _

%

-Q Qn

()-~::r

>()-2,

-0

.... %

Z

CD

:lU :lU

o

> ....

-

r-

0

~

0

Z

Figure 4.8. Distribution and recovery of Ca applied to soil in leachate, plant tissue, and soil. (Syed-Fadzil, 1972)

0 0

Physics

115

ammonium phosphate, the quantity of calcium extracted with one normal ammonium acetate solution decreased with an increasing rate of phosphorus application. This effect is illustrated by the data in Table 4.5. For soils with a pH within one unit of pH o , when z = 1, T = 25°C, and soil solution electrolyte concentration n < 0.01 normal, Equations 4.7 to 4.12 can be simplified to

ao = 1.67 X 10-6 n'lz (pH o - pH)

(4.13)

Equation 4.13 applies to the most weathered and impoverished soils of the tropics in their natural state. For these soils, nand (pH, - pH) are almost always very small numbers, so that ao and therefore the CEC are small. When these soils are cleared for intensive farming, they are limed and fertilized. Liming raises pH, and fertilizer application increases n and frequently lowers pH o. The end result is a gradual increase in ao, a partitioning of ao into al and az, and the eventual need to use the Stern model to describe nutrient retention and solute transport. Unlike soils with permanent charge mineralogies, the surface charge characteristics of soils with variable charge and their effects on solute transport can and should be treated as a management variable.

Table 4.5

Effect of P level on CEC and extractable bases in an Oxi so1.

P applied (ppm)

P source

Cation exchange capacity NH40Ac, pH 7

Extractable bases (meg/100 g) Mg Ca Na K

(NH 4)2 HP04

0

10.80

3.53

1.48

0.10

0.11

(NH4)2 HP04

100

10.96

3.49

1.42

0.10

0.10

(NH 4)2 HP04

500

12.05

3.26

1.39

0.08

0.11

(NH 4)2 HP04

1,500

14.63

2.80

1.35

0.10

0.10

------Source:

Wann and Uehara, 197Ba

_ •• ,,_·..__ ,,_._,,0_'--- __•• ____

116

Physics

Soil Temperature It is often said that one of the truly unique features of tropical soils is their soil temperature regimes. This statement refers to the fact that tropical soils have mean summer and mean winter temperatures that differ by less than 5°C. As summarized in Table 4.6, such soil temperatures are termed iso-temperature regimes in Soil Taxonomy (1975). It turns out, however, that iso-temperature regimes extend beyond the tropics in the Southern Hemisphere and frequently turn to non-iso-temperature regimes within the tropics in the Northern Hemisphere. In Soil Taxonomy (1975) soil temperature is generally measured. at a depth of 50 em, where the diurnal temperature fluctuation is small. In the absence of soil temperature data, the rule of thumb has been to add 1°e to the mean annual air temperature to obtain the mean annual soil temperature. New evidence suggests that 2°e or more are required to relate air temperature to soil temperature. Air and soil temperature data from three elevations in Peru are provided in Tables 4.7, 4.8, and 4.9 and the relationship between soil and air temperature that was proposed by Toy et al. (1978) appears to fit the Peruvian data. The equation that relates the mean monthly soil temperature to the mean monthly air temperature is T, = 2.337 + .986 Ta

(4.14)

where Ts is the soil temperature and Ta is the air temperature in "c. Toy et al. (1978) measured soil temperature at 5 em to obtain Equation 4.14, the Peruvian data were measured at 20 em, and the soil taxonomy requires the measurement of soil temperature at the 50-em depth. However, the mean annual soil temperature varies little with depth and is virtually independent of depth below 10 em. In the tropics elevation strongly affects soil temperature as can be seen from the data in Tables 4.7,4.8, and 4.9. As a rule, air temperature decreases 0.6°e for each 1000-m increase in elevation. However, at a given elevation, air temperature and therefore soil temperature increases with increasing proximity

.'

117

Table 4.6 Soil temperature regimes Difference between mean summer and mean winter soil temperatures < 50C > 50C

Mean annual soil temperature

°c

> 22

i sohyperthermi c

hyperthermi c

isothermic

thermic

8-15

isomesic

mesic

8

isofrigid

frigid

15-22

<

Table 4.7

Measured and estimated soil temgerature (oC) at 20 cm degth. San Ramon (Peru). Latitude 11 07' South, Longitude 75 20' West, elevation: 800 m.

Air Temp * Month

Year

°c

January February March June July August September December Mean Correction **

1977 1977 1977 1976 1976 1976 1976 1976

22.7 22.5 23.2 21.6 21.4 21.5 22.1 23.1 22.3

Estimated Soil Temp. Soil :remp* Method A(l) Method B(2) 25.7 25.2 26.0 23.5 23.6 23.9 23.8 26.4 24.8

23.7 23.5 24.2 22.6 22.4 22.5 23.2 24.1 23.3 1.0

24.7 24.5 25.2 23.6 23.4 23.5 24.1 25.1 24.3 2.0

* Source: L. Manrique (International Potato Center, unpublished data) ** Amount added to each air temperature value to estimate soil temperature (1) Soil Taxonomy (1975)

= 2.337 + 0.986 Ta. where Ts and Ta are soil and aIr temperatures in oC (Toy et al.. 1978).

(2) Ts

118

Measured and estimated soil te~perature (oC) at 20 cm d5pth. La Molina (Peru). Latitude 12 05' South, Longitude 76 57' West, elevation: 238 m.

Table 4.8

Month

Year

Air Temp* °c

January February March June July August September October November December Mean Correction**

1975 1975 1975 1975 1975 1975 1975 1975 1975 1975

21.1 22.8 22.2 14.1 14.4 14.8 15.0 16.8 18.0 21.6 18.1

Soil Temp* °c 25.1 24.1 25.4 18.4 16.0 15.5 15.1 18.1 22.0 25.0 20.5

Estimated Soil Temp. Method A(l) Method 6(2) 22.1 23.8 23.2 15.1 15.4 15.8 16.0 17.8 19.0 22.6 19.1 1.0

23.1 24.8 24.2 16.2 16.5 16.9 17.1 18.9 20.1 23.6 20.1 2.0

*

Source:

**,

Amount added to each air temperature value to estimate soil temperature

L. Manrique (I nternationa1 Potato Center, unpublished data)

(l)

Soil Taxonomy (1975)

(2)

Ts = 2.337 + 0.986 Ta, where Ts and Ta are soil and air temperatures in Uc (Toy et al., 1978).

Table 4.9

Measured and estimated soil tomperature (oC) at 20 cm $epth. Huancayo (Peru). Latitude 12 00' South. Longitude 75 05' West, elevation: 3200 m.

Month

Year

Air Temp* °C

November December January February Average Correction**

1976 1976 1977 1977

13.9 13.4 14.1 13.4 13.7

Estimated Soil Temp. Soil Temp* Method A(l) Method 6(2) °c 18.6 17.7 16.9 14.6 17.0

14.9 14.4 15.1 14.4 14.7 1.0

16.0 15.5 16.2 15.5 15.8 2.0

L. Manrique (International Potato Center, unpublished data)

*

Source:

**

Amount added to each air temperature value to estimate soil temperature

(1)

Soil Taxonomy (1975)

(2) Ts = 2.337 + 0.986 Ta, where Ts andTa are soil and air temperatures in °c (Toy et a1., 1978).

Physics

119

to the equator. Thus isothermic temperature regimes begin at higher elevations in Kenya near the equator than in Hawaii on the northern edge of the tropics. The isothermic temperature regime is considered to be one of the most comfortable climates for human habitation. Managing Soil Temperature

Although plant breeders have managed to extend the range of crop adaptation, soil temperature still restricts the performance of many important food and industrial crops. Cassava and Irish potato, for example, are two root crops that have very different soil temperature requirements. Cassava performs best in isohyperthermic temperature regimes, and Irish potatoes do better in isothermic or cooler soil temperature regimes. Duke (1978) has made an inventory of plants on the basis of their environmental requirements. He believes that when two plant species grow together and one grows best in locations with temperatures between T 1 and T 2 and the other in temperatures between T 2 and T 3, one can assume that the effective temperature of that location is T 2 • Thus he claims that the temperature indicated by the occurrence of two species that have a common upper and lower temperature limit is a better measure of the effective temperature of that location than measured temperatures, particularly if the plants are perennials. Certain management practices may be instituted to lower or raise soil temperatures to extend the growing area and period of a crop. Soil temperatures may be lowered by irrigation and/or with a straw mulch-Table 4.10 shows the effect of irrigation on soil temperature. Whereas Equation 4.14 predicts that at about ZSoC air temperature, the mean monthly soil temperature is approximately ZoC higher than the air temperature, irrigated soils have a mean monthly temperature that is about ZoC lower than the air temperature. The high heat capacity of water combined with the effects of evaporative cooling has a measurable effect on soil climate. Soil temperature can also be altered by surface mulches. Plastic mulches, which act as a vapor barrier between soil and atmosphere, raise soil temperature. Straw mulches, which permit vapor loss from soil, tend to lower soil temperature.

Physics

120

Table 4.10

Air and irrigated soil temperature * at Waipio site (Island of Oahu, Hawaii). Tropeptic Eutrustox, clayey, kaolinitic. isohyperthermic. Latitude 21 0 North, Longitude 157 0 West, elevation: 150 m.

Month

Year

~:~~ §~r

January February March Apri1 May June July August September October November December Mean

1979 1979 1979 1978 1978 1978 1978 1978 1978 1978 1978 1978

21.5 21.4 23.0 24.9 24.7 26.5 27.1 26.6 26.8 26.0 25.0 22.8 24.7

* Benchmark Soi1 Project.

Mean aoil Temp C 20.2 20.5

26.8 24.0 23.0 22.5, 22.8

University of Hawaii.

Figure 4.9 shows air and surface temperatures for bare and straw-mulched soils. A combination of straw mulching and irrigation effectively reduces soil temperatures. Ekern (1967) examined the effect of mulches on the growth of pineapple and concluded that an increase in soil temperature was beneficial to growth. The mulches tested by Ekern (1967) were paper and plastic mulches, and clear plastic was most effective in increasing soil temperature. Table 4.11 shows soil temperature data measured at 7.5 em depth for different types of mulches. When a clear plastic cover is used, radiant energy evaporates the water beneath the plastic cover, but the cover acts as a vapor barrier so the water vapor moves downward and condenses in the cool soil. Since condensation is an exothermic reaction, the heat generated causes the soil temperature to rise. When low soil temperature limits crop performance, plastic mulches can have an important beneficial effect on yield. Heat Flow in Soils

Heat moves by conduction, convection, and radiation. The

121 35

....... }J w

---ar:

30

::::l

l-

e(

ar: w

lL

25

:IE

u.I

I...I

20

0

en

12

8

4

16

20

24

TIME (hrs) Figure 4.9. Surface soil temperatures as a function of time of day for bare soil (A), soil mulched with 20 T/ha straw (B), irrigated soil with full potato canopy (C], and air temperature (D I,

Table 4.11

Soil temperatures at a nominal 7.5 cm depth in the plant line for sunny as opposed to cloudy periods at Wahiawa, Oahu, beneath pineapple planted in November 1953. Ekern, 1967.

Mul ch treatment

Sunny, rainy period 1954 Jan 2 Jan 22 Feb 26

Sunny, warming period 1954 Jan 17 Jan 18 Jan 16

Average daily soil temperature, (oC) Unmulched Oiled kraft paper Asphalt saturated felt paper Black 1.5 mil polyethylene Clear 1.5 mil polyethylene

20.6 21. 1

20.6 21.4

23.0 23.6

19.4 19.7

1g. 7 20.0

20.0 20.0

21.1

21.4

23.9

19.4

19.7

20.3

22.2

22.5

25.0

19.7

19.7

19.7

22.8

23.0

26.4

20.3

19.4

20.3

18.9

20.0

19.4

68

99

31. 8

18.8

Ai r temperature (oC) 20.3

19.4

22.2

Sunlight, ly/day 498

368

501

100 (est.)

Rai nfa 11, mm 0.0

0.0

0.8

31j , 1

, 'L

122

Physics

soil parameters needed to describe heat flow by conduction are the heat capacity, the thermal conductivity, and the thermal diffusivity. All three parameters vary with water content as shown in Figure 4.10. A dry soil has a low thermal conductivity because air in the interaggregate pores is a poor conductor of heat.

....-u>a 0au

...a -.

(a)

Ql

J:.

0

>

...>... .-

.~ u

:::)

"'tI C

0 u

(b)

a

.

E Ql

...

J:.

....;

>-

.-III

-:::)

(c)

"'tI

a

.

E Ql

...

J:.

volumetric water content

Figure 4.10. Relationships between heat capacity, thermal conductivity, thermal diffusivity, and water content.

Physics

123

However, the increase in thermal conductivity with increasing water content is more than compensated for by the increase in the heat capacity of wet soils so that the thermal diffusivity, which is the ratio of the thermal conductivity to the heat capacity, decreases as water content increases. This is another reason why wet soils are cold soils. The thermal diffusivity can be calculated from the attenuation of the peak of the temperature wave measured at two depths or from the time shift in the temperature peak at two depths. These relationships are D =

1I'(Z2 - Zl)2

P(ln T1 and

D

=

-

In T2 ) 2

P(Z2 - Zl )2 411' <1>2

(4.15)

(4.16)

where D is the thermal diffusivity, Z is the depth at which temperatures are recorded, T is the amplitude of the temperature wave at the depths in question, P is the 24-hour period, and is the phase shift. These parameters are identified in Figure 4.1l. One can see that penetration of the temperature wave into the soil is the measure of thermal diffusivity. The thermal diffusivity is related to the heat capacity C and thermal conductivity K through the relation D = K C

(4.17)

The thermal diffusivity decreases as water content increases because the heat capacity rises faster than the thermal conductivity. If D is estimated from Equations 4.15 or 4.16, it is clear that this value will vary-for the same depth increment in the same soil with the same water content-with the nature of the surface mulch. If the mulch is a vapor barrier the temperature wave penetrates deeper into the subsoil, and D will be higher. Since D is not entirely attributable to heat flow by conduction, [)

Physics

124

Z1 -.--- •••

I I I I I,

.-IL:::E: LLI

C

••I

1 Z2 _ _ __ _ _ _

• __ •

_

11 ME .... Figure 4.11. Temperature (n attenuation and phase shift (cI» in the diurnal temperature wave as a function of soil depth (Z).

estimated from Equation 4.15 or Equation 4.16 is referred to as an "apparent" thermal diffusivity. The fact that surface and subsoil temperatures can be purposely altered by the proper choice of mulches is important to soil management. Highly aggregated, low-activity clays are poor conductors of heat because unlike quartz, clay has a low thermal conductivity. But more important, the aggregates are saturated with water even when the interaggregate pores are drained. The low thermal conductivity of the interaggregate air gaps and the high heat capacity of the moist aggregates keep subsoil

Physics

125

temperatures low. In regions of the tropics where the isohyperthermic temperature regime meets the isothermic regime, soil temperature frequently becomes a major production constraint for crops that prefer high soil temperatures. When a practical substitute crop that is suited to lower soil temperatures is not available, mulching can overcome some of the problem. Soil Air Soil air forms a reciprocal relationship with soil water. Drain- . age may be viewed as water displacement by air, and wetting as air displacement by water. The displacement of water by air becomes increasingly difficult as the pore size decreases. The relationship between pore water pressure and pore size is p = 2acosw

r

(4.18)

where P is the pore water pressure, a is the surface tension of water, w is the solid-liquid contact angle, and r is the pore radius. If r is large, the gravitational forces are adequate for drainage. As Figure 4.4 shows, an adequate volume of large pores keeps well-aggregated, highly weathered soils well aerated. In general, the size of the interaggregate pores increases with increasing aggregate size but the intra-aggregate pore size remains small and constant. Since the intra-aggregate pores frequently remain saturated with water for extended periods, reducing conditions can occur in the aggregate interior. These conditions increase the solubility of iron phosphates and contribute to the mineral nutrition of plants. However, when naturally well-drained, high oxide soils are artificially submerged for prolonged periods, iron toxicity frequently becomes a serious problem. Like most soil constraints, soil aeration is crop specific. What is a constraint for one crop may be a desirable condition for another. Some of the most densely populated regions of the tropics occur on the deltas of large rivers, where oxygen stress in the root zone is high. The major sources of food and income in such areas come from crops adapted to the low root-zone

Physics

126

oxygen supply. Examples of such crops are cereal crops such as rice, root crops such as taro, and fiber crops such as jute. Soil aeration becomes a problem when land farmed to crops that require good soil aeration deteriorates physically. A loss of organic matter with the subsequent breakdown of soil structure is one cause of the physical deterioration of soil. Poor soil structure in turn renders the soil more susceptible to compaction. Increasing farm mechanization adds to the overall problem by creating tillage and traffic pans. . What can one say about physical deterioration in soils with variable charge clays? Figure 4.12 illustrates what has happened to the productivity of soils with variable and permanent charge clays after about 100 years of intensive cultivation in Hawaii. The record for the last 40 years tells the story. Forty years ago, the highest sugar yields were obtained on naturally fertile vertisols and mollisols. Those soils contain high-activity, permanent charge clay and are rich in bases. Sugar yields were considerably lower in the oxisols, since the low-activity, variable charge clays of the oxisols were low in bases and deficient in phosphorus. Rising labor costs forced the sugar growers to mechanize their farm operations, and improved varieties, better fertilizers applied in larger doses, and better 0.7

s

~

"'

~

o

..J LaJ

>=

0.6

r-"

0.5 0.4

............ r - " ••••••• 0 ••• •••• ••

-

,...-

..- ...... ..... ..........

0.3 0

~

01

o

......................

0

III 01

0

..,•

0

!!!

!!!

I

It)

....0

!!! I

...... ...... ......

..... .......

VERTISOLS MOLLISOLS

~

0

~

0

01

III 01

0 .... 01

I

I

I

..,0

0

III 01

01

0

It)

!!!

OXISOLS

0

III

e

10- YEAR PERIOD

Figure 4.12. Sugar yields over 40·year period on some soils in Hawaii.

Physics

127

management resulted in higher yields. But by 1950-1960, sugar yields from oxisols had equaled those from the vertisols and mollisols, and now the highest yields are consistently harvested from oxisols. Oxisols possess excellent physical properties so that when their fertility problems were corrected, they surpassed the vertisols and mollisols in performance. Many of the tropical soils perform today as those in Hawaii did in 1930. The superior physical properties of soils with variable charge clays will not be fully utilized until their nutrient deficiencies are corrected. In the end it will be the chemical versatility and the physical superiority of soils with variable charge clays that will raise them to a place of prominence along with other productive soils.

5 Extent and Distribution In order to use the information in this book, it is necessary to know to which soils the information applies. Since virtually every soil contains some variable charge material, some means to match the book's contents to named kinds of soil is necessary. To do this we need to define and set limits to the surface charge continuum as shown in Figure 5.1. According to Figure 5.1, if 60% or more of a soil material's specific surface is of the variable charge type, it is categorized as variable charge. The same percentage applies to the permanent charge category. Soil materials that do not meet either of these requirements are placed in the mixed category. For example, a sample that is by weight 95% variable charge quartz sand and 5% permanent charge smectite will fall in the

<

0/0

60

100

o Variable Charge

60

40 0/ 0

~

40

Mixed Charge

Permanent Charge

o

permanent charge

variable charge

100 ----->

Figure 5.1. Scheme for classifying soil materials into permanent, mixed and variable charge categories. 129

130

Extent and Distribution

permanent charge category because the combined variablecharge surface from quartz and smectite would constitute less than 10% of the sample's specific surface. Similarly, a clay sample containing 80% kaolinite and 20% smectite would still fall in the permanent charge category. Given these definitions, it is now possible to estimate the extent and distribution of variable surface-charge soils in the tropics and the world with the help of Table 5.1. Soil names in Table 5.1 are those of the FAO Legend (1974). There is a high Table 5.1

Distribution of the Major Soils of the World

Soil Associations dominated by

Area in 1000ha

Percent

Fluvisols

3]6,450

2.40

Gleysols

622,670

4.73

1,330,400

10.10

Andosols

100,640

0.76

Vertisols

311,460

2.36

Solonchaks and Solonetz

268,010

2.03

1,175,980

8.93

Xerosols and Kastanozems

895,550

6.79

Chernozems, Greyzems

407,760

3.08

Cambisols

924,870

7.02

Luvisols

922,360

7.00

Podzoluvisols

264,120

2.00

Podzols

477,700

3.63

Planosols

119,890

0.91

Acrisols and Nitosols

1,049,890

7.97

Ferralsols

1,068,450

8.11

Lithosols, Rendzinas and Rankers

2,263,760

17.17

Histosols

240,200

1. 82

Miscellaneous land units (icefields, salt flats, rock debris, shifting sands, etc.)

420,230

3.19

13,180,390

100.00

Reyosols and Arenosols

Yermosols

and Phaeozems

Total world land area Source:

Dudal, 1976

Extent and Distribution

131

degree of correspondence between the soil units of the FAO Legend and those of the Soil Taxonomy (U.S. Soil Survey Staff, 1975) because the taxonomic keys of both systems are based on somewhat similar diagnostic horizons. The FAO system was designed for a different purpose, and therefore it does not have the taxonomic depth of the soil taxonomy. It is, however, ideally suited for examining soils on a global scale. Only four soil units listed in Table 5.1 fall clearly into the variable charge or permanent charge category. These are the andosols, ferralsols, and podzols, which are variable charge, and the vertisols, which are permanent charge. Histosols or the organic soils can also be placed in the variable charge category. Variable charge materials in andosols and podzols differ so much from those in ferralsols that they deserve to be called by different names. We shall use the terms andic, spodic, and oxic, respectively, for variable charge materials in andosols, podzols, and ferralsols. Andie and spodic materials are non-crystalline and are characterized by high specific surface, high exchange capacity, and high water retention. Oxic materials are generally crystalline and possess low specific surface, low exchange capacity, and low water retention. On the other end of the surface charge spectrum, materials with permanent surface charge are said to have vertic properties. Vertic materials are crystalline and are characterized by high specific surface, high exchange capacity, and high water retention. The reader should note the similarity in the characteristics of andic, spodic, and vertic materials. We will remark on this point shortly. One can readily see from Table 5.1 that andosols, podzols, and vertisols are not extensive in the world. Ferralsols, on the other hand, constitute 8.11% of the world's land area. This percentage is even more remarkable when one considers the facts that ferralsols are, for all practical purposes, tropical soils and that they normally occur on flat to gently sloping land. If the tropics constitute 38% of the world's land area or 5 billion ha, then ferralsols occupy 21 % of the land area of the tropics. There are some who feel that the 8.11% value for ferralsols is excessive. This would be true if ferralsols were equated to the oxisols of the soil taxonomy, but it turns out that the ferralsols of the FAO Legend arc similar but not identical to the oxisols.

132

Extent and Distribution

Both soils have oxic properties, but that property alone is not sufficient to classify soils as oxisols. For this reason ferralsols are more extensive than oxisols. In the strictest sense, oxic materials must have a cation exchange capacity determined with neutral, one normal, ammonium acetate of 16 meq orless per 100 g clay. In order to qualify as a ferralsol or oxisol a soil must also have a 30 em section of this material in the soil solum. Anyone who has looked for these qualities knows they are not very common, even in the tropics. It is mostly for this reason that the extent of ferralsols reported in Table 5.1 is suspect. In order to meet the 16 meq/100 g clay requirement, an oxic material must be relatively free of andiclike or spodiclike, noncrystalline material. As the CECil 00 g clay increases, we say that the clay activity increases. This is an unfortunate use of the term activity since the increased CEC may be caused by vertic or spodic and andiclike materials. Although vertic, spodic, and andic materials have in common a high exchange capacity, high specific surface, and high water retention, vertic materials impart high rheologic activity to a soil, but spodic and andic materials do not. Properties that co-vary with, or are accessory to, vertic materials differ greatly from properties accessory to spodic and andic materials. If one wishes to use CEC as a mark of the cause of soil behavior, one must know the origin of the surface charge. High-CEC spodic or andic materials generally do not shrink or swell as do vertic materials. This statement, along with all that it connotes, is illustrated by the position of vertic and andic materials on the plasticity chart shown in Figure 4.1. Depending on the nature and origin of surface charge, the rheology and therefore the physics of soils diverge with increasing CEC and specific surface. Non-crystalline spodic and andiclike materials lose their extensive surface area and therefore their exchange and water retention capacities when they crystallize or turn into sheets, nodules, or ironstones as shown by the relationship between weathering and specific surface in Figure 2.2. Owing to the continuing rejuvenation of the earth's land surface, a parcel of land rarely attains the age necessary to form an oxic horizon. In the tropics the chance that an oxic horizon will form is greater because warm and humid conditions prevail year-round

Extent and Distribution

133

and the rejuvenating action of glaciation is confined to the summit of the high mountains. But even so, the more usual situation is a soil that has not yet become oxic. Many of the acrisols and nitosols fall in this category, i.e., they have not yet become oxic. Unlike the ferralsols, acrisols and nitosols are not entirely tropical soils, but a large proportion of the world's acrisols and nitosols do occur there. Those that occur in the tropics contain variable charge materials, and acrisols are also the main variable-charge soils of the temperate regions. Another important soil of the tropics with variable charge material is the luvisols. Luvisols occur extensively in both the tropics and the temperate zone, but they are more likely to contain permanent charge material when they occur in the temperate regions. The acrisols and nitosols correspond for the most part to ultisols, and the luvisols to the alfisols of the U.S. soil taxonomy. It is now becoming increasingly apparent that ultisols and alfisols with variable charge clays behave differently from those with permanent charge clays. In 1975 an International Committee on Low Activity Clays was formed and given the responsibility to study and reclassify ultisols and alfisols with oxiclike properties. So far, clay activity has been entirely based on exchange capacity, but in the future it may become necessary to add a rheologic parameter to distinguish between materials that gain exchange capacity from variable charge materials from those that gain it from permanent charge minerals. If we now add to the low-CEC, variable-charge clays of the ferralsols the higher-CEC but still variable-charge materials of the acrisols, nitosols, luvisols, cambisols, and arenosols, we estimate conservatively that 60% of the land area of the tropics is mantled with variable charge materials. An estimated distribution of the world's land area with variable, mixed, and permanent charge materials is given in Table 5.2. Accessory Characteristics The number of properties that co-vary with and are accessory to oxic, spodic, andic, and vertic materials is immense. If we superimpose organic matter or histic materials on the vertic-

134 Table 5.2

Extent and Distribution Estimated world distribution of soils with variable, mixed and permanent charge minerals Tropical Zone area in billions

Surface Charge

%

of hectares

Temperate Zone area in billions of hectares %

%

World area in billions of hectares

variable

60

3.0

10

0.82

29

3.82

mixed

30

1.5

45

3.68

39

5.18

permanent

10

0.5

45

~

~

4.18

100

5.0

100

8.18

100

13.18

andie-oxic triangle, a picture very similar to Figure 2.5 emerges. To know the properties of the four end-members is to know the properties of the intermediate members. But soil science is not simply a science of materials. Identical materials behave differently if they are organized and arranged differently. Superimpose on material diversity an even greater diversity in pedologic organization, and we have the natural body we call soil. Because soil diversity arises more from differences in arrangement and organization of materials than from material differences, there are more soils than there are soil materials. To use soil information efficiently and intelligently, it is necessary to organize what we know about soils in a classification scheme. Most soil classification systems separate soils on the bases of material properties such as CEC, the state of the material such as particle size and base saturation, and the arrangement and distribution of materials in the profile and soil climate. In the soil taxonomy, mineralogy appears in the family or fifth category of classification. However, a closer examination of the system shows that in many instances the higher categories are separated on the basis of accessory mineral properties. Thus the material we have presented in this book is more meaningful if it is linked to the soil names of a soil classification system. The soil name is a condensed statement of what we know about the soil. Names are based on defined characteristics, but it is the properties that are accessory to these characteristics that are important for soil interpretation.

Extent and Distribution

135

Soil Interpretation In order to predict the behavior of named kinds of soils, one must be able to relate the soil properties employed to classify and name soils to the soil performances. A soil performs well for a particular purpose if its characteristics match the requirements for that use. Soil interpretation is the process by which one matches soil characteristics to land-use requirements, and for this reason soil interpretation can only be as good as the knowledge used to classify the soils. The extensive distribution of variable charge materials in the soils of the tropics suggests that these materials deserve considerably more attention than they now receive. However, new knowledge gained about soil materials will not aid soil interpretation unless the knowledge is incorporated into a soil classification system. It is significant that there are three international committees currently examining the classification of soils with variable charge materials. These committees are responsible for reclassifying the ultisols and alfisols with lowactivity clays, the oxisols, and the andepts. For the first time there is hope that the soils of the tropics will be studied in a systematic manner and that the results will be organized and condensed in a soil classification system.

6 Analytical Methods In this chapter some of the methods currently in use for determining the surface charge characteristics of soils are reviewed, and their applicability to soils containing variable charge colloids is discussed. A number of new methods for characterizing variable charge soils are also described. Cation Exchange Capacity Cation exchange capacity is commonly determined at a standard pH of 7.0 or 8.2 in an electrolyte solution whose ionic strength is brought to an undefined ionic strength by washing with alcohol. One such method is the ammonium acetate (pH 7.0) method, in which the soil sample is stripped of cations with pH 7, 1M NH40Ac to saturate the exchange complex with ammonium ions. Excess ammonium acetate is removed with ethanol, and the adsorbed ammonium is either determined by direct distillation or is measured after extraction with a strong NaCI solution (Soil Survey Investigations Report no. 1). In closely similar methods, sodium acetate at pH 8.2, potassium acetate at pH 7.0, and barium chloride at pH 8.2 are used as the saturating solutions. Although such methods should be quite suitable for soils in which the clay minerals are of the constant charge type, it should be obvious from our earlier discussion on the factors that influence surface charge that these methods should not be used for soils that contain significant amounts of variable charge colloids. If the soil pH is lower than pH 7, raising the pH to

Analytical Methods

138

7 or 8.2 artificially creates additional charge on the surface. The use of a monovalent ion such as NH 4 , potassium, or sodium lowers the surface charge density since divalent cations usually dominate the exchange complex in soils. Using non-aqueous solvents such as ethanol or methanol lowers the electrolyte concentration and brings into play the dependence of electrolyte concentration on variable surface charge. In addition the use of a non-aqueous solvent reduces the dielectric constant relative to that of an aqueous medium, so that surface charge density is artificially reduced. In short, the methods described above alter most of the parameters controlling charge density as described by Equation 3.3,

°0 = lr. 2n€kT]Y2 1r

. 1.15 z(pHo - pH) sinh

Thus, though such methods might be useful for certain purposessuch as for classifying soils-they should be used with great caution because soil interpretation based on them could be very misleading. An alternative method for estimating CEC that appears to avoid the above-mentioned pitfalls has been designated the effective cation exchange capacity (ECEC). In this method, the cations extracted with 1M NH 40Ac are summed with the aluminum extracted with 1M KCl (Soil Survey Investigations Report no. 1), and this method undoubtedly gives values close to the actual capacity of many soils to retain cations under field conditions. Problems arise however when a soil contains free salt, as it is difficult to remove the free or non-exchangeable cations without affecting the exchangeable cations. Attempts to remove free salt with water could result in the hydrolysis of the exchangeable cations, and of course, the reduction in electrolyte concentration would lower the surface charge density and cause a loss of exchangeable cations. The use of non-aqueous solvents such as ethanol for salt removal would also cause a loss of exchangeable cations because both the electrolyte concentration and the dielectric constant would be reduced. In some variable charge soils, conventional extraction with 1M NH 40Ac does not completely remove the divalent cations that are presumably held in the Stern layer. In addition, the

Analytical Methods

139

aluminum removed by 1M KCI would not always be exchangeable. According to Amedee and Peech (1976) at least some of the aluminum extracted by 1M KCI comes from the solubilization of amorphous aluminum. What is required is a method that will measure the soil's capacity to adsorb divalent cations from an aqueous solution at a pH and ionic strength similar to those encountered in the field. One such approach would be to saturate the soil with an unbuffered divalent-cation electrolyte solution-at an appropriate ionic strength-centrifuge and decant most of the supernatant, estimate the small volume of the remaining solution by weight determinations, and remove the divalent cation with another electrolyte containing a different divalent cation. The CEC is the amount of the first divalent cation removed, allowing for entrained cations. This approach attempts to use values for pH, n, e, and z that are appropriate to field situations. As outlined, this method is somewhat tedious because several washings are normally required to remove all of the index cation. The procedure can be simplified by using a method described by Bascomb (1964) in which the soil is saturated with barium. Magnesium is then permitted to compulsively exchange with barium when the former is added as MgS0 4. Magnesium becomes the index cation, and a sufficient amount of MgS0 4 is added to achieve the appropriate ionic strength. The use of unbuffered solutions throughout ensures that soil pH is maintained. It is not necessary to remove the magnesium once the correct equilibrium has been established, because if the amount of MgS0 4 added is known, then it is necessary only to measure the magnesium in the equilibrium solution to determine the adsorbed amount. Thus for 100 g of soil,

x Soil-Ba + y BaCl2 (entrained) + z MgS0 4 ~x

Soil-Mg + Y MgCl2 + (z-x-y) MgS0 4 + (x+y) BaS04.J,

where x, y, and z are expressed in milliequivalents, then CEC = Mg removed from solution =

z - [y + (z-x-y)]

- x mcq/LOt) g

140

Analytical Methods

This simple and inexpensive method for determining CEC can be extended to allow for the estimation of the anion exchange capacity and also of the exchangeable cations present (Gillman, 1979). A full description is given in the section on the measurement of charge characteristics. Anion Exchange Capacity The anion exchange capacity (AEC) is not normally determined, because most studies have been conducted on soils in which the AEC was small or insignificant. The recent interest in tropical soils has created an awareness of the importance of AEC, for in some soils AEC can be significant. Meaningful AEC values can be obtained by saturating the anion exchange sites with an anion under the appropriate conditions of pH and ionic strength and then replacing the index anion with some other anion. As polyvalent anions appear to have specificity for oxide surfaces, a monovalent anion such as chloride or nitrate might be preferable. The compulsive exchange method described later shows how the AEC can be estimated by chloride adsorption. Exchangeable Cations The basic exchangeable cations present in soils are usually removed with a concentrated salt solution using a cation not usually present on the exchange complex. The NH~ ion is often used, but problems can arise in the case of some oxidic soils in which there appears to be a specificity for divalent cations. In such cases, the use of a divalent cation such as barium will remove more calcium and magnesium than will NH~. The difficulty, then, is in deciding whether a divalent cation adsorbed in the Stern layer is exchangeable and whether it is available to plants. It should also be mentioned that if the salt used to remove exchangeable cations is adjusted to pH 7.0 or 8.2, an increase in surface charge density on variable charge colloids will result in an incomplete removal of cations. The problems arising from separating free salts from exchangeable cations have been

Analytical Methods

141

discussed in the section on cation exchange capacity, and the removal of basic exchangeable cations by the compulsive exchange method is described in the following section. Charge Characteristics Measurement by Compulsive Exchange Throughout this book we have emphasized the need for methods suitable for assessing the charge characteristics of soil materials with variable surface charge. Since the term variable charge implies that the surface charge is dependent on solution pH, electrolyte composition and concentration, and on the dielectric constant of the medium, the conditions under which the measurements are made should be similar to conditions in the field. The procedure outlined below attempts to meet these conditions. • Place 2 g of soil plus 20 ml of 0.1M BaCl2 solution in a preweighed 30 ml centrifuge tube and shake for 2 hours. Centrifuge and retain the supernatant solution for analysis of calcium, magnesium, potassium, and sodium. . • Wash the soil with 3 X 20 ml of 0.002M BaCI2 , shaking for 1 hour between centrifugations. Discard the supernatant liquid. • After 3 washings, weigh the tubes plus the moist soil to estimate the amount of entrained BaCl2 solution. • Add 10 mlofO.005M magnesium sulphate and shake gently for 1 hour. • Establish the electrical conductivity of a 0.0015M MgS0 4 solution at the existing ambient laboratory conditions and check the conductivity directly in suspension, adjusting it to that of the reference solution by adding 0.005M MgS0 4 solution or water. Shake gently overnight and adjust the conductivity if necessary. • Weigh the tubes to estimate the volume of MgS0 4 or water added, and after centrifugation, determine magnesium and chlorine in the supernatant.

Analytical Methods

142

If we define the following terms, volume of entrained BaCl2 = Vl ml = V2 ml volume of added MgS0 4 final supernatant volume = V3 ml magnesium cone. of supernatant = c, meq/ml then, if MgS0 4 was added, CEC (meq/l00 g) = 50 (0.01 V2 - C l V3 ) and if water was added, CEC (meq/lOO g) = 50 (0.1 - Cl V3 ) and furthermore, if CI concentration of nominal 0.002M BaCl2 = C2 meq/ml = C3 meq/ml CI concentration of final supernatant then

Notes 1. If tubes of 30 ml capacity are used, then the use of O.OOSM MgS0 4 is limited to soils with a CEC of less than 10 meq/100 g. For higher CEC values, a more concentrated MgS04 solution is required. For example, 0.01M MgS0 4 is appropriate for CEC values between 10 and 20 meq/100 g, and so on. 2. For routine analyses, the final magnesium concentration need not be measured, and it can be assumed to be 0.003 meq/ml. 3. If CEC and AEC at some other pH or ionic strength are required, the desired pH can be attained during washings with 0.002M BaCl2 using Ba(OHh or HCI, and the reference MgS0 4 solution can be set at any desired ionic strength. 4. The ionic strength of 0.006 used is considered to be representative for surface horizons of highly weathered soils. Actual soil solution ionic strength I may be obtained by measuring the electrical conductivity EC of a 1: 5 soil:water suspension and applying the formula (Gillman and Bell, 1978) 1= 0.0446 EC - 0.000173

Analytical Methods

143

in which the soil solution has been equilibrated at a soil moisture tension of 0.1 bar.

Surface Net Charge and pHo In this procedure the sample is titrated with H+ and OHat three or four electrolyte concentrations, and by running blank titrations the amount of H+ or OH- adsorbed over a range of pH values and salt concentration can be calculated. A net adsorption of H+ indicates net negative charge. At some pH value, equal amounts of H+ and OH- are adsorbed so that the surface has zero net charge. This is the point at which net surface charge is independent of salt concentration, so the titration curves should intersect at this point, which we refer to as pH o (See Figure 6.1). As explained in Chapter 3, pH o depends on the choice of electrolyte, so NaCl, CaClz, or Na z S0 4 might be chosen depending upon the particular needs of the analyst. Thus if the soil being studied has mainly calcium as the exchangeable cation, then CaClz might be the appropriate electrolyte to use. The following procedure generally follows the steps outlined by van Raij and Peech (1972). 1. Place 4 g of soil (oven dry equivalent) into each of 60 50-ml beakers and arrange them into 4 rows of 15 beakers. 2. Add 10 ml of electrolyte, the concentration of which is 2M, 0.2M, 0.02M, and 0.004M for rows 1, 2, 3, and 4 respectively. 3. Designate the middle beaker in each row as "zero." 4. Add increasing amounts of O.IM acid to the beakers on the left of zero, e.g., 0.25 ml, 0.5 ml, 1.0 ml, 1.5 ml, 2.0 ml, 2.5 ml, and 3 ml, and similarly increasing amounts of O.IM base to the beakers on the right of zero. 5. Add distilled water to bring the total volume in each beaker to 20 ml. 6. Allow to equilibrate for 4 days, stirring occasionally. 7. Construct titration curves from the titration with acid and base of each of the 4 electrolyte solutions. These are the blanks.

144

Analytical Methods

-

..i

Q)

0)

-'" Q)

pH 2 pH1

C

pH Figure 6.1. Variation of net surface charge with pH and electrolyte concentration (n). The sign of surface charge is obtained from sign of .6pH = pH 2 - pH!.

8. After 4 days record the pH of the suspension and calculate H+ or OH- adsorbed as the amount added less the blank reading of that pH. 9. Plot H+ or OW adsorbed (actually I'H" - rOH-) against pH for each electrolyte concentration on the one graph. 10. If there is sufficient variable-charge colloid present, the curves will intersect at a common point (usually over a range of 0.2 pH units). This is the pHo. Frequently the 1M curve does not pass through the common point. As salt concentrations of this magnitude are not normally encountered in the field, this 1M curve can be disregarded. 11. By adjusting the pH axis to pass through pHo, the vertical scale now indicates surface net charge for a range of pH and ionic strength values.

Analytical Methods

145

This method can be considerably shortened if only the pH o value is desired. 1. Place 4 g of soil (oven dry equivalent) in each of 10 to 15 50-ml beakers and adjust the pH with acid or alkali to span the expected pH o value. Add 0.5 ml of O.IM electrolyte solution and bring to 20 ml with distilled water. 2. Allow the soil to equilibrate for 4 days, stirring occasionally, and then record the equilibrium pH. Designate these values pH.oo2M . 3. Add 0.5 ml of 2M electrolyte solution, shake gently for about 3 hours, and record the pH. Designate these values pH.o5M · 4. For each beaker calculate .0,pH = [pH.o5M - pH.oo2M ] and plot .0,pH versus pH.oo2M to reveal the point where .0,pH = o. This is pH o - the pH value which is independent of salt concentration.

Electrolytes that could be used include NaCl, CaCI2, and Na2S04, and it is suggested that CaCl2 be used because calcium (or magnesium) is normally the dominant cation in most soils. Sign of Net Charge To determine whether a sample has net negative or net positive charge, it is only necessary to measure the pH in a very dilute electrolyte solution and again in a more concentrated solution. Referring to Figure 6.1

so that if the soil has a net negative charge, the sign of .0, pH is negative. Conversely, the soil is net positively charged if .0, pH is positive. As explained in Chapter 3, the type of electrolyte will influence the position of pH o. Thus, referring to Figure 6.2, if the soil pH falls between pHo KC1 and pHONaZS04' .0,pH between two KCl concentrations would be negative, indicating a net negative

Analytical Methods

146 KCI

CD

...

0)

o .c u

CD

C

ApH

~H

neg.

pos,

t-

...

pHONa SO 2 4

soil pH

pH Figure 6.2. Variation of net surface charge with pH, nature of electrolyte, and electrolyte concentration (n). n2 >nl' For a given electrolyte, l',pH = pH n - pH n . 2

1

charge, but zspl-l would be positive between two Na2S04 concentrations, indicating a net positive charge. Note that if l',pH is negative for the KCI system, it will be more negative for the CaCl2 system. Measurement of Permanent Charge in a Mixed System The measurement of the amount of permanent charge (op) in a soil system composed of both permanent charge and variable charge components is based on the assumption that at pH o• de-

Analytical Methods

147

termined in the absence of specific adsorption, there is equal adsorption of cations and anions on the variable charge surfaces. Any excess cation or anion adsorption at pH o is a measure of the permanent negative and permanent positive charge respectively. To measure pH o in the absence of specifically adsorbed cations or anions the sample should first be washed with a strong, indifferent electrolyte solution (e.g., 1M) at a slightly elevated pH to ensure removal of anions such as 50 4. To measure permanent charge proceed as follows. 1. Wash about 100 g of soil (oven dry equivalent) with 2 or 3 500-ml portions of 1M KCI with pH adjusted between pH 7 and pH 8 with KOH. 2. Wash with distilled water until suspension electrolytic conductivity is approximately equal to that of a 0.002M KCI reference solution. 3. Place 4 g of soil (oven dry equivalent) in each of 10 50ml beakers and adjust the pH with KOH or HCI to span the expected pH value, adding a 0.002M KCI solution to bring the final volume to 20 ml. 4. Allow the soil to equilibrate for 3-4 days, stirring occasionally, and then record the equilibrium pH. Designate these values pH.ooZM ' 5. Add 0.5 ml of 2M KCI, shake gently for about 3 hours, and record the pH. Designate these values pH.0 5M ' 6. For each beaker calculate 6.pH = [pH.05M - pH.ooZM ] and plot 6.pH versus pH.ooZM to reveal the point where 6.pH = O. This is pH o . 7. Choose one or more of the beakers, transfer the contents to 30 ml preweighed centrifuge tubes, and adjust the pH to pH o. 8. Centrifuge and remove but retain the supernatant solution for K+ and cr analysis. Weigh the tubes to estimate the volume of entrained solution. 9. Remove the K+ and CI- with 5 20-ml washes with 0.5M NH4N0 3 and determine K+ and Cl" in the combined washings. 10. Allowing for entrained KCI, calculate the adsorption of K+ and cr as meq/lOO g.

Analytical Methods

148

11. Permanent charge ap (meq/I 00 g) ads.].

= -

[K+ ads. - Cl-

Measurement of Point of Zero Net Charge The point of zero net charge (PZNC) is the pH at which there is equal adsorption of cations and anions from an indifferent electrolyte of a sample containing a mixture of variable and permanent charge minerals. The PZNC is determined as follows. 1. Place 4 g of soil (oven dry equivalent) into each of 8 30-ml preweighed centrifuge tubes with 20 ml of 1M KCI and shake for 1 hour. 2. Centrifuge and discard the supernatant solution. Wash twice with 20 ml of 0.2M KCI and then 3 to 5 times with O.OIM KCl. 3. Adjust the pH to span the expected PZNC and allow to equilibrate over 3 to 4 days before recording the pH. 4. Centrifuge and remove but retain the supernatant solution for K+ and Cl" analysis. Weigh the tubes to estimate the entrained KCl. 5. Remove the K+ and cr with 5 20-ml washes with 0.5M NH 4N0 3 and determine K+ and Cl" in the combined washings. 6. Allowing for entrained KCI, calculate the adsorption of K+ and CI- as meq/lOO g. 7. A plot of K+ ads. and Cl" ads. versus pH will reveal the point where K+ ads. = cr ads. This point of intersection of the 2 curves corresponds to PZNC. Specific Surface Ion adsorption measurements may be used to estimate the surface area of colloidal materials. Schofield (1949) has derived an equation from the Gouy-Chapman theory relating negative co-ion adsorption to the specific surface and total surface charge. For a 1: 1 electrolyte, the equation is -y- =

2NS ylz{3N

(6.1 )

Analytical Methods

149

where 'Y, the total surface charge, and 'Y-, the negative co-ion adsorption, are expressed in meg/100 g. The electrolyte concentration N is expressed in meq/rnl, and the specific surface in cm 21100 g. If a O.OIM, 1:1 indifferent electrolyte is chosen for ion adsorption measurement, Equation 6.1 becomes 'Y- = 6.1

X

10-9 S - 3.774 'Y

X

10- 17 S2

(6.2)

For a system composed of a mixture of minerals with permanent negative and variable charge, the positive variable charge can be made very small by choosing a pH well on the alkaline side of pH o, so that negative anion adsorption can be attributed to surfaces that are nearly all negatively charged. A similar procedure was employed by Barber and Rowell (1972). If pH o is relatively high, e.g., pH 6, negative adsorption of cations may be measured by choosing a pH on the acid side of pH o . In a mixed system, negative adsorption of cations is determined from the difference between total cation adsorption and permanent negative charge. The surface area measured in this way is attributable to the variable charge component SV' By combining the Gouy-Chapman theory and the Nernst equation, a simple relationship between Sv and the negative adsorption of cations 'Y; can be obtained as follows. The charge deficit equation is

=[nekTJ'h [exp ze0 2kT 1] 0

0_

where 1963)

0_

_

(3.9)

is the surface charge deficit in esu/crn! (van Olphen,

00

and

so that

11'

0_

kT

r:: -

=[n~:TJ'h

e

2.3 (pH o - pH)

[ex P[1.15 z(pH o - PH)}-I]

(3.2)

(6.3)

At (pH o - pH) = 2 and in a O.OIM 1: 1 electrolyte at 25°C, Equation 6.3 leads to

150

Analytical Methods

o

= -1777 X 0.8997 (esu/cm") = -5.46 X 10-9 (meq/crrr")

Now

'Y~ (meq/100 g) =

or

Sv (m 2/g) = 1.83 X 10 2 'Y;

-u_ (rneq/cm")

X

Sv (cm 21l00 g) (6.4)

Thus Sv is simply obtained by measuring the negative adsorption of cations in a O.OlM 1: 1 electrolyte two pH units below pH o . The procedure is similar to that outlined for the measurement of PZNC but measured at a single pH value, i.e., 4 pH units above pH o or 2 units below pH o. Sand Sv are calculated according to Equations 6.2 and 6.4, respectively. The following example for the calculation of specific surface illustrates the computations involved when using the GouyChapman theory. 1. The pH o of an oxisol was found to be pH 4.5. Adsorption of K+ and Cl" at pH 8.5 from a O.OlM KCI solution was 10.7 meq/100 g and -0.3 meqllOO g, respectively. The specific surface is calculated in the following manner. At pH 8.5 assume that surface charge is entirely negative and that negative adsorption occurs over the whole surface. Then negative anion adsorption 'Y; = 0.3 meqllOO g. The total negative charge 'Y

=

-(cation ads. + negative ads.)

= -(10.7 + 0.3) = -(11.0 meq/100 g)

From Equation 6.2 = 6.1 X 10-9 S -

0.3

= 6.1

X

10-9 S +

3.774 X 10-17 S2 'Y

\~~6

X

10-17 S2

0.343 S2 + 6.1 X 108 S - 0.3 X 10 17 = 0 or

s2 + 17.78 X 108 S - 0.87 X 10 17 = 0

Analytical Methods

151

Thus S(cm 21l00 g) =

=

±) 316.13 X 10 16

-17.78 X 10 8

+ 34.8 X 10 16

2

0.48 X 10 8

S = 48 m 2/g

2. The pH o of a subsurface sample of a eutrustox occurred at pH 6.0. At this pH the adsorption of K+ and Cl" from a O.OlM KCI solution was 4.0 and 1.1 meq/100 g, respectively. At pH 4.0 the respective values were 2.0 and zero meq/100 g. The specific surface of the variable charge component is calculated in the following manner. Permanent charge

0p

= -(cation ads. - anion ads.) at pH o

=

-(4.0 - 1.1)

= -2.9 meq/100 g

Cation adsorption by variable charge surface at pH 4.0 = cation ads., at pH 4.0 - lap]

= (2.0 -

2.9) = -0.9 meq/100 g i.e., negative cation adsorption

. Equation . 6 .3,0_ = Using where n (ions crn")

'Y; = 0.9 meqllOO g

[n€kTJYz [exp {1.15 z(pHo-pH)1}-1] 21T

= 0.01 X 10- 3 X 6.02 X 10 23

€(esu 2/dyne-1cm 2 ) = 80 (ergs) = 4.12 X 10- 14 at 25°C z = -1 (pH o - pH) = 2

kT

Thus,

o

= -(1777 X 0.8997) esu/cm/ =

-

1777 X 0.8997 rneq/cm/ 29.2 X 10 10

= -5.48

X 10-9 meq/cmf

Analytical Methods

152

Now

thus

Sv

= _ 'Y; a

= =

(1.83 X 10 8 X 0.9) cm 2 / 100 g

165 (m 2 /g)

References Amedee, G., and Peech, M. 1976. The significance of KCl-extractable Al (III) as an index to lime requirements of soils of the humid tropics. Soil Sci. 121 :227-233. Atkinson, R. J., Posner, A. M., and Quirk, J. P. 1967. Adsorption of potential-determining ions at the ferric oxide-aqueous electrolyte interface. J. Physical Chemistry 71:550-558. Ayers, A. S., and Hagihara, H. H. 1953. Effect of the anion on the sorption of potassium by some humic and hydrol humic latosols. Soil Sci. 75:1-17. Balasubramanian, V., and Kanehiro, Y. 1978. Surface chemistry of the hydrandepts and its relation to nitrate adsorption as affected by profile depth and dehydration. J. Soil Sci. 29:47-57. Barber, R. G., and Rowell, D. L. 1972. Charge distribution and the cation exchange capacity of an iron-rich kaolinitic soil. J. Soil Sci. 23:135-146. Bascomb, C. L. 1964. Rapid method for the determination of cationexchange capacity of calcareous and non-calcareous soils. J. Sci. Food Agric. 15 :821-823. Bolt, G. H. 1955. Analysis of the validity of the Gouy-Chapman theory of the electric double layer. J. Colloid Sci. 10:206-218. Breeuwsma, A., and Lyklema, J. 1971. Interfacial electrochemistry of haematite (a- Fe203)' Dis. Faraday Soc. 52:324-332. Chapman, D. L. 1913. A contribution to the theory of electrocapillarity. Phil. Mag. 25:475-481. Chen, Y. R.; Buder, J. N.; and Sturn, W. 1973. Adsorption of phosphate on alumina and kaolinite from dilute aqueous solutions. J. Colloidal and Interface Sci. 43 :421-436. de Wit, C. D. 1953. The physical theory on placement of fertilizers. Vers!. vor Landbouwk. Ondcrz (Gravenhage) Vo!. 59, no. 4, 71 pp. D'Hotman de Villicrs, 0. 1961. Soil rejuvenation with crushed basalt in 153

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Food and Agricultural Organization of the United Nations-UNESCO. 1974. Soil map of the World. Vol. 1. Legend. UNESCO, Paris. Fox, R. L. 1978. Studies on phosphorus nutrition in the tropics. In Mineral nutrition of legumes in tropical and subtropical soils. Ed. C. S. Andrew and E. J. Kamprath. CSIRO, Brisbane, Australia. Fox, R. L., and Searle, P.G.E. 1978. Phosphate adsorption by soils of the tropics. Chap. 7 in Diversity of soils in the tropics. Ed. M. Drosdoff. ASA, SSSA, Madison, Wis. Gallez, A; juo, AS.R.; Herbillon, A J. 1976. Surface and charge characteristics of selected soils in the tropics. Soil Sci. Soc. Amer. J. 40:601608.

Garrels, R. M., and Christ, C. L. 1965. Solutions, minerals, and equilibria. Harper and Row, New York. Gillman, G. P. 1970. Some aspects of the chemistry of red basaltic soils in north Queensland. M.Sc. Diss. James Cook Univ. of Nth Qld. Aust. __. 1974. The influence of net charge on water dispersible clay and sorbed sulphate. Aust. J. Soil Res. 12:173-176. __. 1979. A proposed method for the measurement of exchange properties of highly weathered soils. Aust, J. Soil Res. 17: 129-139. __. 1980. The effect of crushed basalt scoria on the cation exchange properties of a highly weathered soil. Soil Sci. Soc. Amer, J. In prep. Gillman, G. P., and Bell, L. C. 1976. Surface charge characteristics of six weathered soils from tropical north Queensland. Aust, J. Soil Res. 14:351-360. __. 1978. Soil solution studies on weathered soils from tropical north Queensland. Aust. J. Soil Res. 16:67-77. Gillman, G. P., and Fox, R. L. 1980. Increasing the cation exchange

capacity of variable charge soils in the field with superphosphate. In prep.

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Gouy, G. 1910. Sur la constitution de la charge electrique a la surface d'un electrolyte. Ann. Phys. (Paris) 9:457-468. Grahame, D. C. 1947. The electrical double layer and the theory of electrocapillarity. Chern. Rev. 41:441-501. Hashimoto, I., and Jackson, M. L. 1958. Rapid dissolution of allophane and kaolinite-halloysite after dehydration. 7th National Conference on Clays and Clay Minerals. Pergamon Press, New York. Hingston, F. J.; Atkinson, R. J.; and Quirk, J. P. 1967. Specific adsorption of anions. Nature 215: 1459-1461. Hingston, F. J.; Posner, A. M.; and Quirk, J. P. 1972. Anion adsorption by goethite and gibbsite: I. The role of the proton in determining adsorption envelopes. J. Soil Sci. 23 :177-192. Jones, R. C., and Uehara, G. 1973. Amorphous coatings on mineral surfaces. Soil Sci. Soc. Amer. Proc. 37:792-798. juo, A.S.R., and Fox, R. L. 1977. Phosphate sorption capacity of some benchmark soils in West Africa. Soil Sci. 124:370-376. Kanehiro, Y., and Sherman, G. D. 1956. Effect of dehydration-rehydration on cation exchange capacity of Hawaiian soils. Soil Sci. Soc. Amer. Proc.20:341-344. Kelly, W. P. 1944. Mattson's papers on "The laws of soil colloid behavior" ... review and comments. Soil Sci. 56 :443-456. Keng, J.C.W., and Uehara, G. 1973. Chemistry, mineralogy, and taxonomy of oxisols and ultisols. Soil and Crop Sci. Soc. of Florida 33:120-126. Lai, S.. H., and J urinak, J. J. 1971. Numerical approximation of cation exchange in miscible displacement through soil columns. Soil Sci. Soc. Amer. Proc. 35:894-899. __. 1972. Cation adsorption in one-dimensional flow through soils: A numerical solution. Water Resour. Res. 8 :99-107. Lehr, J. R., and McClellan, G. H. 1972. A revised laboratory reactivity scale for evaluating phosphate for direct application. Bulletin Y-43. Tennessee Valley Authority, Muscle Shoals, Ala. Lepsch, I. F., and Buol, S. W. 1974. Investigations in an oxisol-ultisol toposequence in S. Paulo State, Brazil. Soil Sci. Soc. Amer. Proc. 38:491-496. Mattson, S. 1927. Anionic and cationic adsorption by soil colloidal materials of varying Si0 2/Al20 3 + Fe203' Proc. First International Congo Soil Sci. (Washington) 2 (Comm. 11):185-198. __. 1928. The electrokinetic and chemical behavior of the aluminosilicates. Soil Sci. 25: 289-311. __. 1931. The laws of soil colloidal behavior: V. Ion adsorption and exchange. Soil Sci. 31: 311-331. __. 1932. The laws of soil colloidal behavior: IX. Amphoteric reactions

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Conf. Soil Mech. and Found. Eng. 1:57-61. Stern, O. 1924. Zur Theorie de elektrolytischen doppe1schieht. Z. Electrochem.30:508-516. Syed-Fadzil, S. F. 1972. Ion retention and movement in soils with variable charge colloids. M.S. Thesis. Univ. of Hawaii. Toy, J. T.; Kuhaida, A. J., Jr.; and Munson, B. E. 1978. The prediction of mean monthly soil temperature from mean monthly air temperatures. Soil Sci. 126:181-189. Tsuji, G. Y.; Watanabe, R. T.; and Sakai, W. S. 1975. Influence of soil microstructure on water characteristics of selected Hawaiian soils. Soil Sci. Soc. Amer. Proc. 39:28-33. Uehara, G., and Jones, R. C. 1974. Particle surfaces and cementing agents. In Soil Crusts. Ed. J. W. Cary and D. D. Evans. Agr. Expt, Sta., Univ, of Arizona Tech. Bull. 214. U.S., Department of Agriculture, Soil Conservation Service. 1972. Soil survey laboratory methods and procedures for collecting soil samples. Soil Survey Investigations Report no. 1. Govt. Printing Office, Washington, D.C. U.S., Department of Agriculture, Soil Conservation Service, Soil Survey Staff. 1960. Soil classification, a comprehensive system, 7th Approximation. Govt, Printing Office, Washington, D.C. __.1975. Soil taxonomy: A basic system of soil classification for making and interpreting soil surveys. USDA Agric. Handbook No. 436. Govt. Printing Office, Washington, D.C. Vander Zaag, P. 1979. The phosphorus requirements of root crops. Ph.D. Diss. Univ. of Hawaii. van Olphen, H. 1963. Introduction to clay colloid chemistry. Interscience Publishers, New York. van Raij, B., and Peech, M. 1972. Electrochemical properties of some oxisols and alfisols of the tropics. Soil Sci. Soc. Amer. Proc. 36: 587-593. van Wijk, W. R., ed. 1966. Physics of plant environment. 2d ed. NorthHolland Publ. Co., Amsterdam. Wann, S. S., and Uehara, G. 1978a. Surface charge manipulation in constant surface potential soil colloids: I. Relation to sorbed phosphorus. Soil Sci. Soc. Amer. J. 42: 565-570. __. 1978b. Surface charge manipulation in constant surface potential colloids: ll. Effect on solute transport. Soil Sci. Soc. Amer. J. 42: 886-888. Wischmeier, W. H. 1976. Use and misuse of the universal soil loss equation. J. Soil and Water Conservation 31:5-9. Yopps, J. A., and Fuerstenau, D. W. 1964. The zero point of charge of alpha-alumina. J. Colloid Sci. 19:61-71.

Index Acid soil, 7, 20,65,74,75,80,82, 91-92 See also Rocks; Tropical soils, defined Aerie soils, 59-60,63 Acrisols, 13 3 Acrorthox, 57,58,59,62 Adsorption. See Cations, adsorbed; High-affinity specific adsorption; Ions, adsorption; Low-affinity specific adsorption; Phosphorus, adsorption AEC. See Anion exchange capacity Aeration, 97,103,125-127 Africa, 11 Aggregates, 22, 25(illus.), 103, 106, 107, 109 and dehydration, 25 and pore size, 106, 108, 125 stability of, 101 and thermal conductivity, 122, 124-125 Agricultural development projects, 65, 95 Alcohol, 13 7 Alfisols, 18, 19, 133, 135 Alkaline, 58 Allophane, 102 Aluminosilicates, 23,45, 102 Aluminum, 4, 13,65,68,82, /'il)

138,139 amorphous, 139 free, 84 as hydroxide, 66 -interlayered chlorites, 20 ions, 63 oxide, 43, 45(chart), 46, 79, 80, 102 phosphate, 84 soluble, 91 trivalent, 31-32 Amedee, G., 139 Amendment. See Soil amendment Ammonium, 138, 140 acetate, 132, 137 phosphate, 53, 115 sulphate, 68 Amorphous materials, 22-23,45 See also Hydrandepts, amorphous materials in Andepts. See Andisols Andes, 11 Andie materials, 131, 132, 133-134 Andisols, 17(chart), 18, 19(chart), 60, 6l(tables), 62, 13 5 cation exchange capacity of, 30 and gravimetric water content, 109 humic, 102, 103, 104(table) variable charge, 131

160

Andosols. See Andisols Anion exchange capacity (AEC), 140,142 Anions, 33, 39,40,43,46,47, 48,50,51-52,56,62,69, 147, 148, 150 See also Anion exchange capacity Aqueous soil solution, 49 Arenosols, 13 3 Aridisols, 18-19 Aridity. See Aridisols Arid regions, 23 Atterberg constants, 98-99, 103, 104(table), 105(table) Australia, 12,48, 49(chart), 56, 72 Ayers, A. 5., 52, 53 Balasubramanian, V., 28 Barber, R. G., 149 Barium, 139 chloride, 137, 141, 142 Basalt, 80 Basaltic scoria, 22, 23(table), 72, 73(chart), 74(table) Bascomb, C. L., 139 Bauxite, 10 Bearing capacity, 97 See also Rheology Black earths. See Montmorillonite Bolt, G. H., 39,42-43 Boltzmann constant, 110 Borneo, 11 Botswana, 12 Boundary condition, 105, 111 Bragg, Sir Lawrence, 3 Brasilia (Brazil), 59,60 Brazil, 11, 51(chart), 59, 64(table), 109 Breeuwsma, A., 49 Buffering capacity, 67-08, 69 Bulk density, 8, 23, 60, 62, 102 Bulk modulus,98 Calcium, 13, 53, 70, 72, 74(table), 141 deficiency, 82

Index

divalent, 60 carbonate, 48, 68, 81, 82-84 chloride, 143, 145, 146 phosphate, 53 replaces aluminum, 66 retention, 72, 112, 114-115 silicate, 70, 81-82, 84 California, 57, 58 Cambisols, 13 3 Carbonates, 16,67 See also Calcium, carbonate; Free carbonates Casagrande plasticity chart, 98, 99(chart) Cassava, 77, 78(chart), 119 Cation exchange capacity (CEC), 7,13-14,40,42,112,115 of constant charge system, 41-42 determination, 137-140, 142 of minerals, 29-30 and phosphate,S 3 and silica-sesquioxide ratio, 28, 29 subsoil, 72 of synthetic soils, 21 of variable charge soils, 46, 47-48, 69,70, 71(chart), 72, 131, 132, 133 See also Effective cation exchange capacity; Hydrandepts, cation exchange capacity of; Oxisols, cation exchange capacity of Cations, 2-3, 72, 137 adsorbed, 62, 147, 148, 149-150, 151 distribution of, 33 divalent, 49, 138, 139, 140 exchangeable, 40,99, 138, 140-141, 143 hydrated, 39 low-affinity specific adsorption of, 51,52 monovalent, 49 nutrient, 13, 70, 111 positively charged, 33, 53, 56 retention of, 46, 47,53,59,69, 70 CEC. See Cation exchange capacity

Index

Cementation, 28 See also Crust cementation Chad,12 Chapman, D. L., 33 Chemical bond, 3 Chemical change, 11, 14 Chemisorption. See High-affinity specific adsorption Chloride, 43, 140 See also Potassium, chloride Chlorine, 141, 147, 148, 150, 151 Chloritic class, 20 Citric acid. See Phosphorus, citric-acid soluble Clay cation exchange capacity of, 42, 99, 100 content of soil, 7 dispersion, 109 fraction, 2,4, 14,43, 57-58, 99, 101(chart) as heat conductor, 124 mineralogy, 7, 100 minerals, 3-4, 9(table), 10, l1(chart), 12, 31, 84. See also Smectite non-crystalline, 102-103 particles, 9, 109, 110 plasticity of, 99. See also Clay activity soils, 8, 9, 20-21, 57, 64(table), 65,66,81,106 specific surface of, 9, 101, 109 surface charge of, 40, 110 swelling, 101-102 variable charge, 68, 109, 126-127 Clay activity, 4-5,99-102, 103, 106, 109, 13 3 Colloidal behavior, 3 Colloidal soil charge of, 46 crystalline, 21 fraction, 3, 5, 46 heterogeneity, 44-45 organic, 32 permanent charge, S3-S 7

161

silica-sesquioxide and silica alumina ratios of, 21 surfaces, 31, 32, 33, 39,42, 148-152 variable charge of, 53-57, 137 Colombia, 11 Compaction, 97 Compact layer. See Stern layer Condensation, 120 Constant surface charge, 2, 3,25, 31, 34(chart), 35, 38(chart), 40, 41,48,66-67,69,74,137 and counterion valence, 48-49, 67 of crystal, 31-32, 35 densities, 13, 14,25,28,30,33, 62,65 and soil management, 66-74 and subsistence farming, 14 variation, 37(chart) Constant surface potential, 2, 3, 4,31,32,35-36,41,50 and counterion valence, 48-49 and pH, 36-37,48 Coordination number, 13 Coral limestone, 70 Coulombic interaction energy. See Ions, coulombic interaction energy between Counterions, 31, 33, 35, 37,40, 47,48-49,50,59,67 Crop yield and acidity, 91-92 and phosphorus, 75(chart), 76, 77, 78(chart), 85-91 Crust cementation, 23 Crust formation, 98 Crystal. See Constant surface charge, of crystal Crystalline materials, 131 Crystalline particles, 22 Crystallographic analysis, 3 Decomposition, 17 Deformation, 25, 97 Dehydration, 23, 25, 27(table), 28 Density. See Bulk density; Exchange dcnsi ty j Particle densi ty

162

Department of Agriculture. See Soil Conservation Service Deprotonation, 33(chart), 43, 68 Desilication, 62 Desorption. See Phosphorus, desorption; Proton desorption; Water, desorption de Wit, C. D., 74,84,85,86 D'Hotman de Villiers, 0., 74 Dielectric constant, 35-36, 37, 39,49-50,138,141 Diffuse layer, 33, 34(chart), 38(chart), 39,40,52,112 Diffusion forces, 33 Diffusivity. See Thermal diffusivity , Water, diffusivity Double layer. See Electrical double layer Drainage, 11, 97, 125 Drying, 103, l04(table), 105(table) Duke, J. A., 119 ECEC. See Effective cation exchange capacity Ecuador, 11 Effective cation exchange capacity (ECEC), 62, 68, 138 Ekern, P. C., 120 Elasticity, 8, 23, 97, 98 See also Liquid limit Electrical conductivity, 8 Electrical double layer, 33, 34(chart), 35, 37, 38(chart), 39,40,41,42 Electrical neutrality, 33, 39 Electrolyte solutions, 35-36, 37(chart), 40, 41, 42, 43, 45(charts), 47, 48, 50, 5l(chart), 67,115,137, 138,141,143,144,145,150 ambient, 25,32 indifferent, 54, 55, 68, 69(chart), 147, 148, 149 unbuffered, 139 valency, 49, 68 Electron microscopy, 22, 24(illus.),

Index 26(illus.), 29(illus.), 80, 81 Electrons, 31, 110 Electrostatic forces, 33,42,50 Elevation, 116, 119 Entisols, 18, 19 Equator, 11, 119 Equilibrium, 59, 76, 83(chart) solution, 10, 33, 35,40 Erosion, 99, 101-102 Ethanol, 137, 138 Eutrandept, 61(table), 62, 63 Eutrustox, 77(chart), 79, 106(chart), 108(chart), 151 Evaporative cooling, 119 Exchange density, 41, 42 Exothermic reaction, 120 FAO (Food and Agriculture Organization). See Legend Farm mechanization, 126 Feldspar, 9, 13, 22 weathering of, 10, 12, 13 Ferrasols, 131, 132, 13 3 Fertility, 11, 13, 14, 126-127 Fertilizers, 5,48,60,115,126 ammoniacal, 68 banding, 84-86, 87, 88, 89(table), 90 broadcast, 87-90, 91 leaching of, 112 phosphorus, 74, 75. See also Phosphorus placement of, 84-86 soluble, 110 See also Phosphate, fertilizer Fluoride anions, 43 Food production constraint, 65, 86, 125 Fox, R. L., 70,83, 93 Free carbonates, 4 Gallez, A., 80 Gas retention, 97 Gibbsihumox, 70, 112(chart), 113(charts) Gibbsite, 10, 12, 13

Index

Gillman, Gavin P., 22, 70, 72 Glaciation, 133 See also Pleistocene glaciation Goethite, 45(chart), 53 Gouy, G., 33 Gouy-Chapman theory, 33-35, 37,39,40,42,50, 51(chart), 52, 148, 149, 150 Grahame, D. C., 39 Gravel, 25 Gravimetric water content, 60, 109 Green revolution, 77-78 Hagihara, H. H., 52, 53 Halloysite, 16 Haploxeroll,63 Haplustoll, 76, 77, 79 Hardness, 8 Hashimoto, 21, 23 Hawaii, 81, 109, 119, 120(table), 121(table), 126(chart), 127 Heat capacity. See Volumetric heat capacity Heat flow, 120, 122-125 Heat retention, 97, 103 Hematite, 3, 32, 36, 45(chart), 51 synthetic, 49 High-affinity specific adsorption, 51, 52-53 Hingston, F. J., 53 Histic materials, 133 Histosols, 17(chart), 18, 131 Hookean body, 98 Hydrandepts, 23, 60, 61(table), 63,77, 79, 102 amorphous materials in, 25, 76, 80, 81 cation exchange capacity of, 25, 27(table), 28,62, 70, 102 phosphorus application on, 93(chart) silica-sesquioxide ratio of, 62 Hydraulic conductivity, 106, 109 Hydrogen ion, 110

163

Hydrolysis, 48, 67,138 Hydrous oxides, 4, 16, 17(chart), 19(chart), 75, 79 crystalline, 80-81 inorganic, 80 Hydroxyl ions, 48, 67 replacement, 43, 44(chart) Hydroxylated surface, 33(chart), 43,46,67,68 Hysteresis, 93-95

IEP. See Isoelectric point Illite. See Mica Imogolite, 102 Inceptisols, 18, 19(chart), 53 India, 12 Indonesia,S Initial condition, 105, 111 Inorganic soils, 18, 19(chart), 32,81 Inosilicates, 19 International Committee on Low Activity Clays, 133 Ions adsorption, 3, 32, 35, 38, 42, 52-53,62,148-152: specific, 43, 50, 56-57 competitive, 81 coulombic interaction energy between, 39 equilibrium of, 43 exchangeable, 40 hydrated, 43 metal, 43, 44(chart), 51,66 monovalent, 138 nutrient, 60 polarization energies of, 39,43 repulsion of, 43 size, 39 soluble, 13 substitution, 2: isomorphous, 3, 13-14 See also Anions; Cations; Counterions

Index

164

Iron oxides, 4, 43,46, 79, 80 gels, 102 See also Goethite; Hematite Iron phosphates, 125 Ironstones, 132 Irrigation, 23, 120 Isoelectric point (IEP), 52, 53 Isoelectric weathering, 110 Isohyperthermic temperature regimes, 119, 125 Iso-ternperature regimes, 116, 119 Isothermic temperature regime, 119, 125 Isotherms, 93-94, 95 Jackson, M. L., 21, 23 Java, 5 Jones, R. C., 21, 23 Jute, 126 Juvenile soils, 5, 12(chart) See also Inceptisols Kanehiro, Y., 25, 28 Kaolinite, 9, 10, 13, 21, 64(table), 79,83(chart) clay, 99, 100, 101, 130 and feldspar, 10, 12 and iron oxide, 80 as phyllosilicate, 16 Kelly, W. P., 3,4 Kenya, 119 Lateric soil, 1 Laterite, 23 Latosol, 1 Leaching, 3,46,53,59,62,72, 74,88,94,97,109,110,112, 113(charts) Legend (FAD, 1974), 102, 130-131 Lehr, J. R., 91 Lettuce, 84 Lime, 7,60,66,67,68,69,70, 74,81-84,91, 115 Limestone, 72 See also Coral limestone Liming. See Lime

Liquid limit, 98-99, 102, 104(table), 105(table) Low-affinity specific adsorption, 51, 52, 53 Luvisols, 133 Lyklema, J., 49 McClellan, G. H., 91 Magnesium, 13, 70, 72, 74(table), 112,139,141,142 divalent, 32, 60 sulphate, 139, 141, 142 Maize, 77 Manganese oxide, 43 Mattson, Sante, 3,4,21, 110 Mature soils, 5, 12(chart) Mauritius, 74 Mechanization. See Farm mechanization Methanol, 138 Mexico, 12 Mica, 10, 12, 13,43,101 See also Muscovite Microclimate variations, 10-11 Mineralogy and soil taxonomy, 17, 19-20,

134 See also Minerals; Mixed Mineralogies Mineraloids, 80 Minerals distribution of, l1(chart) diversity of, 11 fraction of, 8-9 properties of, 8(table) in soils, 15(chart), 16 See also Cation exchange capacity, of minerals; Clay, minerals; Permanent charge minerals; Silicate minerals; Variable charge minerals; X-amorphous minerals Mixed charge. See Permanent charge minerals, mixed; Variable charge minerals, mixed Mixed mineralogies, 20-23, 25 Mohr, E.C.J., 5

Index

Mollisols, 18, 19(chart), 28, 126-127 Monosilicic acid, 81 Montmorillonite, 2, 9, 10, 11, 12, 13,20,21,23, 26(illus.), 63, 64(table), 79 Morocco, 12 Mulches, 124 paper, 120, 121(table) plastic, 119, 120, 121(table) straw, 119-120, 121(chart) Muscovite, 100 Nernst equation, 36,46, 59, 149 Newtonian body. See Viscosity Nigeria, 80 Nitosols, 13 3 Nitrate anions, 43 Nitrogen, 74 Nodules, 13 2 Non-aqueous solvents, 50 See also Ethanol; Methanol Non-crystalline materials, 16, 23, 25(illus.), 26(illus.), 27(illus.), 102-103, 109, 131, 132 dehydration of, 25, 27(table), 28 fraction, 21, 62-63 and mixed mineralogies, 20-23, 25 rehydration, 27(table), 28, 29 silica and alumina, 21, 22(table) and soil composition, 17-18, 19(chart) specific surface of, 25, 28-29, 132 and variable charge model, 60, 62-63 Non-iso-ternperature regimes, 116 Nutrients plant, 3, 13, 72, 84, 85-91, 92, 109,125 soil,66,69, 74,112,115,127 soluble, 1-2 See also Cations, nutrient Olivine, 22 Organic matter, 4, 16, 23, 81,

165

82(chart), 109, 133 functional groups of, 32 pH value, 46, 59,69-70 and plasticity index, 99, 102 and soil composition, 17 Oxic materials, 131, 132, 133-134 Oxides, 16, 17(chart), 19(chart), 42,43,75 crystalline, 80-81 inorganic, 80 See also Hematite; Hydrous oxides; Manganese oxide; Sesquioxides Oxidic soils, 45, 47, 76, 91,125 Oxisols, 17(chart), 18, 19(chart), 114 and adsorption, 52, 53 cation exchange capacity of, 28, 30,47,72,73(chart), 115(table), 131-132, 135, 150 kaolinitic, 21 montmorillinitic, 21 and phosphorus, 70 specific surface of, 28, 109 sugar yields of, 126, 127 Oxygen stress, 125-126 Oyster shells, 66 Particle density, 8, 13 Particle size distribution, 7, 9, 109 and soil taxonomy, 17 and specific surface, 22 Pauling, Linus, 3 Pedogenic development. See Soil formation Peech, M., 39, 50, 56, 139, 143 Permanent charge minerals , 2 , 3, 48,57,115 concept of, 4, 5 density, 13 measurement of, 146-148,151 mixed, 30, 53, 55, 56, 57, 63-66, 129, 134(table) and phyllosilicates, 16, 19-20

166

and soil classification, 129-130, 131, 133, 134(table) and weathering, 14 See a/so Colloidal soil, permanent charge; Haplustoll Permeability, 7, 8 Peru, 116, 117(table), 118(tables) pH-dependent minerals. See Variable charge minerals Phosphate, 47,53,70,72 anions, 43 equilibrium, 76 fertilizer, 112 rock, 90, 91, 92 See a/so Ammonium, phosphate; Calcium, phosphate; Iron phosphates; Superphosphate Phosphorus, 60, 70, 71(chart), 74-78,81-84,85-91,92-95, 115, 126 adsorption, 78-81, 92-93 citric-acid solu ble, 91 desorption, 76, 94 fixation, 85, 91, 92,95 low solubility of, 91-92 occluded, 80 residual effect, 93. See a/so Hysteresis See a/so Food production constraint; Sorption, phosphorus pH values, 46-47, 49, 50, 51, 52,53,54,55-56,62,63,65, 68-69, 83(chart), 84, 13 7, 139, 140, 143-146, 147, 149, 150,151 See a/so Constant surface potential, and pH; Soil pH; Variable charge minerals, mixed; Variable charge minerals, and pH Phyllosilicates. See Permanent charge minerals, and phyllosilicates Pineapple, 91, 120, 121(table) Plastic state, 97, 102, 104(table), 105(table) See a/so Casagrande plasticity

Index

chart; Liquid limit Pleistocene glaciation, 4 Podzols, 131 Point of zero net charge (PZNC), 54,55,56 measurement of, 148, 150 Polarization energies. See Ions, polarization energies of Poles, 11 Porosity, 8, 9, 28-29, 107(table), 124, 125 Potash,68 Potassium, 10, 13,60,68, 70, 72, 74(table), 112, 141, 147, 148, 150,151 acetate, 13 7 chloride, 58-59, 69(chart), 146 hydroxide, 147 monovalent ion, 138 Potato Irish, 77, 119 sweet-, 78(chart) See a/so Yams Potentiometric titration, 48, 55-56 Proton adsorption, 68 Protonation, 33(chart) Proton desorption, 51 Puerto Rico, 57, 58, 59 Pyroclastic sediments, 1 PZNC. See Point of zero net charge Quartz, 9,45,80,100,101,124 sand, 129-130 Regurs. See Vertisols Rehydration. See Non-crystalline materials, rehydration Rejuvenation, 74, 13 2-13 3 Rendzinas. See Vertisols Rheology, 7, 14,98-103,132 defined,97 equations for, 97-98 See a/so Atterberg constants Rice, 84, 126 Rice paddy, 98

Index

River sediments. See Pyroclastic sediments Rocks, 80 See also Phosphate, rock Rock weathering, 12, 72 Rowell, D. L., 149 Roy, A. C., 81 Ruffin, Edmund, 66 St. Venant model, 98 Salts, 50, 58,60,99, 140 free, 138, 140 Sand,25,80 Sandy soils, 8, 106 Saturation, 107, 108 Schofield, R. K., 57 Scoria. See Basaltic scoria Sediments. See Pyroclastic sediments Senile soils, 5, 12(chart) Serpentinitic class, 19 Sesquioxides, 21, 23, 29, 81, 82(chart) Shear modulus, 98 Shear strength, 100-101 Sheets, 132 Sherman, G. D., 25, 28 Silica, 10,62, 72, 81, 84 amorphous, 21, 23 as cement, 23 non-crystalline, 26(illus.) pH value, 46 and porosity, 29 soluble, 60, 72 Silica-alumina ratio, 21 Silica-sesquioxide ratio, 21, 23, 25, 60,62 See also Cation exchange capacity, and silica-sesquioxide ratio Silicate minerals, 3,15,16,31,70, 72,79,112 anions, 43 classes of, 19-20 and goethite, 53 as slag, 81

167

Siliceou s soils, 46 Silkon, 13,43,72,84 Silt, 25 Skempton, A. W., 99, 100 Slag, 81 Smectite, 17(chart), 18, 19(chart), 22, 79, 99, 129-130 See also Montmorillonite Smith, Guy D., 18 Sodium, 13, 141 acetate, 137 chloride, 137, 143, 145 monovalent ion, 138 sulphate, 143, 145 Soil air. See Aeration Soil amendment, 74, 112 Soil characterization, 57-66 Soil classification. See Soil taxonomy Soil Classification, 7th Approximation (1960), 4 Soil composition. See Minerals, in soils; Soil constituents; Soil taxonomy, and soil composition Soil composition tetrahedron, 17, 18 Soil Conservation Service (U.S. Department of Agriculture), 4 Soil constituents, 15-16,21-22 Soil formation, 12, 19 Soil interpretation, 135 Soil management, 2, 7, 14, 102, 119-120 and mulches, 124 and surface charge characteristics, 66-74 Soil mineralogy, 7-8, 134 See also Clay, mineralogy Soil pH, 3, 7,28,36,47-48, 58-59,60,63,66,67,68, 72,115,137-138 Soil physics. See Rheology; Transport phenomena Soil properties, 7, 8, 106, 134-135

168

Soils, 130(table) See also individual types Soil science, 2, 21,42, 134 Soil scientists, 3,4, 7, 100 Soil Survey Investigations Report Number 24 (1973),57 Soil systems, 2, 57 Soil taxonomy, 4-5,109,129-135 and bulk density, 62 and cation exchange capacity, 137-140 and mineralogical classes, 16 and soil composition, 17-23 and temperature, 116 United States, 133 Soil Taxonomy (1975), 4, 16, 18, 19, 102, 116, 131 Soil water, 105-109 and heat flow, 122-123 and soil air, 125 Solum, 132 Solutes retention, 97, 103 transport, 110-115 Sorption, 69, 72, 76 phosphorus, 76, 77, 78-79, 81, 82(chart), 83, 88, 90, 93, 94(chart) Specific surface, 9, 12, 13, 14,30, 56-57,62,103 and buffering capacity, 67 measurement of, 148-152 and phosphorus sorption, 79-80 and seasonal changes, 28 and soil classification, 129, 131 and weathering, 12, 22 See also Clay, specific surface of; Non-crystalline materials, specific surface of Spodic materials, 131, 132, 133 Spodosols, 18, 19(chart) Stern, 0., 38 Stern layer, 38, 39, 50, 51, 52,68, 112, 114, 138, 140 Stern theory, 37-39, 42, 50, 5l(chart), 115

Index Strain, 97 shear, 97, 98 Stress, 97 compressive, 98 hydrostatic, 98 shear, 97, 98, 109 Subsistence farmers, 5, 14, 77 Subsoil, 72, 81, 103, 124 Sudan, 12 Sugarcane, 72, 84 Sugar yields, 126-127 Sulphate, 53,62,68, 147 See also Ammonium, sulphate Sumatra, 5, 11 Superphosphate, 53, 70, 92 treble, 92 Surface charge, 50, 5l(chart), 114, 137,138,140 net, 143-146 See also Constant surface charge Surface potential, 5l(chart), 109,110 See also Constant surface potential Synthetic oxide systems, 47 Synthetic soils, 21 Taro, 78(chart), 126 Teflon, on plows, 109 Temperate regions, 2, 4, 5,63,92, 133 Temperature absolute, 50, 110 air, 116, 120(table) changes, 11 and plants, 119-120 soil, 97,116-125 Tennessee Valley Authority (TVA),91 Tensile stress. See Stress, compressive Texas, 12 Texture, 7 Thailand,64(table) Thermal conductivity, 8, 120, 122 Thermal convection, 120

Index

Thermal diffusivity, 122, 123-124 Thermal radiation, 120 Tillage, 97, 100, 109 pans, 126 Tirs. See Vertisols Titanium oxide, 43 Titrations, 143 Topsoil, 103, 124 Torrox, 106(chart), 108(chart) Toxicity, 82, 125 Toy, J. T., 116 Trace elements, 74, 82 Trafficability, 97, 99 Transport coefficients, 105 Transport phenomena, 97, 103, 105-127 Tropical soils aggregated, 109 defined, 1-2 and drying, 103 and erosion, 101-102 features of, 13, 59-60 fertile, 11 and oxic horizon, 132-13 3 and phosphorus deficiency, 74-75 study of, 4-5 and variable charge minerals, 63,72 See also Permanent charge minerals; Temperature, absolute; Temperature, soil; Weathered soil Traffic pans, 126 TV A. See Tennessee Valley Authority Uehara, Goro, 21, 23,47, 52, 53, 70, 112, 114 Ultisols, 18, 19(chart), 28, 133, 135 Uruguay, 12 Valency, 49, 50,60,67 van Raij, B., 39, 50, 56, 143 van Wijk, W. R., 84

169

Variable charge minerals, 3,4,5, 32,43-50, 5l(chart), 57, 74, 103,137 classes of, 19, 20 density, 30, 140 dependence of, 37, 55(chart), 141 and laboratory analysis, 14-15 measurement of, 141-143 mixed, 30,53, 57,63-66, 69(table), 129, 134(table) and pH, 66-68, 69, 141 and phosphorus deficiency, 75 and soil classification, 129-133, 134(table), 13 5 in soils, 15-16, 18, 25 specific surface of, 151-152 and water, 106, 115 See also Colloidal soil, variable charge of; Non-crystalline materials, and variable charge model Vermiculite, 20 Vertic materials, 131, 132, 133-134 Vertisols, 12, 17(chart), 18, 19(chart) cation exchange capacity of, 28, 30 ferritic, 21 kaolinitic, 21 permanent charge, 131 silica-sesquioxide ratio of, 21 specific surface of, 28 sugar yields of, 126-127 Virgin Islands, 57 Viscosity, 23, 97, 98 See also Liquid limit Volcanic ash, 53, 75, 102 Volumetric heat capacity, 8, 14, 119,122,123-124 Volumetric water content, 60, 62 Wann, S. S., 47, 52, 53, 70, 112, 114 Water desorption, 106 diffusivity, 106

170

flow, 97, 99, 103, 105-109 -holding capacity, 7, 8, 14, 97,100,106,109,131. See also Gravimetric water content; Volumetric water content infiltration, 97 replacement, 43, 44(chart} Weathered soil, 2, 3, 10, 11, 13, 14,23,45,58,59,62,75,106, 109, 112, 132 See also Feldspar, weathering of; Isoelectric weathering; Rock

Index

weathering; Specific surface, and weathering Wheat, 77 X-amorphous minerals, 4 X-ray diffraction, 3-4, 21, 22, 24(chart} Yams, 78(chart} Young's modulus, 98 Zaire, 11 Zeolites, 22