Advances in agronomy volume 14

79 confined the term ‘laterite” to four principal forms of sesquioxide-rich material that either are hard or that harden upon exposure: 1 soft mottled clays that change irreversibly to

A D V A N C E S I N

AGRONOMY VOLUME 14

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ADVANCES IN

AGRONOMY

Prepared under the Auspices of the

AMERICAN Socm-ry OF AGRONOMY

COPYRIGHT @ 1962, BY ACADEMIC PRESS INC

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CONTRIBUTORS TO VOLUME 14

C ROY ADAIR, Research Agronomist, Crops Research Division, Agricul- tural Research Service, United States Department of Agriculture, Beltsville, Maryland

L T ALEXANDER, Chief, Soil Survey Laborato y, Soil Conservation

Semice, United States Department of Agriculture, Plant Industy Station, Beltsville, M a ylund

H M BEACHELL, Research Agronomist, Crops Research Di&on, Agri- cultural Research Service, United States Department of Agriculture, Beaumont, Texas

R L BERNARD, Research Geneticist, United States Regional Soybean Laboratory, Crops Research Division, Agricultural Research Service, United States Department of Agriculture, Urbana, Illinois

D R BOULDIN, Soil Chemist, Tennessee Valley Authority, Muscle Shoals, Alabama

J G CADY, Soil Scientist, Soil Survey Laboratory, Soil Conservation

Service, United States Department of Agriculture, Plant Industry Station, Beltsville, Maryland

J L CARTTER, Agronomist-in-charge, United States Regional Soybean Laboratory, Crops Research Division, Agricultural Research Service, United States Department of Agriculture, Urbana, Illinois

MARLIN G CLINE, Professor of Soil Science, Department of Agronomy, Cornell University, Ithaca, New York

E E HARTWIG, Research Agronomist, United States Regional Soybean Laborato y, Crops Research Division, Agricultural Research Service, United States Department of Agriculture, Stoneville, Mississippi

HERBERT W JOHNSON, Research Agronomist, Crops Research Division, Agricultural Research Service, United States Department of Agri- culture, Beltsville, Maryland

DON KIRKHAM, Curtiss Distinguished Professor of Agriculture and Pro- fessor of Soils and Physics, Iowa State University, Ames, Iowa

RAYMOND J KUNZE, Assistant Professor of Soils, Department of Agron-

m y , Iowa State University, Ames, Iowa

vi CONTRIBmoRs

M D MILLER, Extension Agronomist, Agronomy Department, University

of California, Davis, CaZifornia

S SNARAJASINGHAM, Assistant Chemist, Soil Surceys, Department of Agriculture, Peradeniya, Ceybn

DWIGHT D SMJTH, Assistant Director for Water Management, Soil and Water Conservation Research Division, Agricultural Research Serv- ice, United States Department of Agriculture, Beltsoilk, Maryland

GILBERT L TERMAN, Agronomist, Tennessee Valley Authority, Muscle Shoals, Alabama

FRANK G VETS, JR., Chief Soil Scientist, Northern Plains Branch, Soil and Water Conservation Research Division, Agricultural Research Service, United States Department of Agriculture, Fort Collins, Colorado

J R WEBB, Associate Professor of Soils, Department of Agronomy, Iowa State University, Ames, Iowa

WALTER H WISCHMEIER, Research Investigations Leader for Water Erosion, Corn Belt Branch, Soil and Water Conservation Research Division, Agricultural Research Service, United States Department

of Agriculture, Purdue University, Lafayette, lndiana

PREFACE

The eight chapters in this volume fall into the general pattern established for this series, which is to include reviews of research progress in soil and crop science and developments in agronomic prac- tice The central theme is the soil-plant relationship Some European reviewers of this series have expressed the view that the range of sub- jects covered is far too wide to justify the implied suggestion that they are all branches of one science and that the literature reviewed

is predominantly American Essentially this criticism hinges on the definition of the word “agronomy” which in European usage and par- ticularly British usage does not have the same connotation as in the U.S Indeed one British reviewer states that “In England, little would

be left for agronomy when the claims of chemistry, entomology, plant pathology and so on had been stated-perhaps the study of green manuring, seed rates and sowing dates.” As understood in the United States there is, however, a professional field of agronomy in which the above and many other disciplines have a part There is a professional organization of agronomists with upwards of 4,000 members trained

in a variety of disciplines which they bring to bear on a great diversity

of problems relating to the soil, and its efficient use in the production

of economic crops Much of the science involved is international; but there are aspects that are regional and must be so For example, in this issue there are two extensive reviews dealing respectively with the genetics of soybeans and the management of the soybean crop Some sixty percent of the world soybean production is located in the United States An even higher percentage of the total scientific work on this fascinating crop plant is carried on in the United States, and it is in- evitable, therefore, that the literature should be predominantly Ameri- can Much the same applies to the article on rice production in the United States, where man hour per acre have been reduced to an astonishingly low figure

In contrast, attention should be drawn to the authoritative review on the subject of laterite by Sivarajasingham, Alexander, Cady and Cline, which reflects the world-wide distribution of the investigators of laterites rather than the distribution of lateritic soils

Greater fertilizer usage accounts in part for the steady yield in- creases recorded in most countries in recent years In the development

of fertilizers considerable attention is being directed towards new and unconventional materials, the evaluation of which presents challenging problems Some of these are discussed by Terman, Bouldin and Webb

vii

viii PmFACE

Viets, on the other hand, considers the involved relationships between fertilizer usage and the water requirement of crops, a very important issue in many areas of the world where rainfall is erratic and water reserves inadequate

The remaining articles deal directly with soil properties Kirkham and Kunze discuss some of the applications of the use of isotopes and radiation to problems in soil physics, and Smith and Wischmeier the physical principles of soil erosion by rain

A G NORMAN Ann Arbor, Michigan

July, 1962

CONTENTS

Puge

CONTRIBUTORS TO VOLUME 14 v

PREFACE vii

LATERITE BY S SIVARAJASJNGHAM L T ALEXANDER J G CADY AND M G CLINE I The Term “Laterite” 1

I1 The Nature of Laterite 5

I11 The Environment of Laterite 14

V Formation of Laterite 26

VI Geomorphic Relationships 53

VII Softening of Laterite 55

References 56

IV Profiles Containing Laterite 20

RICE IMPROVEMENT AND CULTURE IN THE UNITED STATES BY c ROY ADAIR M D MILLER AND H M BEACHELL I Introduction 61

I1 Rice Culture in the United States 68

I11 Rice Field Pests 85

IV Origin, Botany and Genetics of Rice 92

Rice Breeding and Improvement in the United States 96

References 104

V RAINFALL EROSION BY DWIGHT D SMITH AND WALTER H WBCHMEIER I Introduction 109

I1 Mechanics of Rainfall Erosion 113

I11 Basic Factors Affecting Field Soil Loss 123

IV Soil Loss Prediction 137

References 144

ix

SOYBEAN GENETICS A N D BREEDING

BY HERBERT W JOHNSON AND RICHARD L BERNARP

I Introduction 149

I1 Reproduction 152

111 Genetics of Qualitative Characters 157

IV Genetics of Quantitative Characters 172

V Breeding 199

References 218

FERTILIZERS AND THE EFFICIENT USE O F WATER BY FRANK G VIETS JR I I1 I11 IV V VI VII VIII IX x XI XI1 I 11 I11 IV V Introduction

Definition of the Problem

The Effects of Fertilizers on the Relationship of Evapotranspiration and Yield

Fertilizers and Water-Use Efficiency in Terms of Applied Water Fertilization and Water-Use Efficiency with Limited Moisture Validity of Evapotranspiration Data

Supply

Fertilization and Moisture Extraction by Roots Other Practices for Increasing Water-Use Efficiency

Is Maximum Water-Use Efficiency Desirable?

Fertilizers and the Infiltration of Water

Fertilization Crop Maturity and Water Use

Conclusions

References

223 226 228 233 246 246 252 254 256 257 259 260 261 EVALUATION OF FERTILIZERS BY BIOLOGICAL METHODS BY G L TERMAN D R BOULDIN AND J R WEBB Introduction 265

Chemical and Physical Characteristics of Fertilizers 266

Concepts of Fertilizer Evaluation 280

Methods Used in Fertilizer Evaluation Tests 295

Conclusions 316

References 317

CONTENTS xi ISOTOPES METHODS A N D USES I N SOIL PHYSICS RESEARCH

BY DON KIRKHAM AND RAYMOND J KUNZE

I

I1

I11

IV

V

VI

VII

VIII

IX

X

1

I1

I11

IV

V

VI

VII

VIII

IX

X

XI

XI1

XI11

Introduction

Soil Water

Soil Aeration

Soil Particle Movement

Transformation of Soil Materials from One Form to Another

Soil Density and Soil Structure

Soil Temperature

Soil Profile Formation and Dating

Disposal of Radioactive Waste

Proposed Future Work

References

THE MANAGEMENT OF SOYBEANS BY JACKSON L CARTTER AND EDCAR E HARTWIG Introduction

Soil and Climatic Adaptation

Time of Planting and Varietal Adaptation

Planting Methods and Equipment

Rotation Practices and Erosion Control

Weed Control

Seed Quality and Seed Treatment

Nutrient Requirements

Water Requirements and Utilization

Growth-Regulating Chemicals

Harvesting

Seed Storage

Discussion

321 322 342 347 348 348 350 352 353 354 355 360 365 372 378 383 386 389 390 401 403 404 406 407 408 AUTHOR INDEX 413

SUBJECT INDEX 427

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LATERITE

S Sivarajasingham, L T Alexander, J G Cady,

and M G Cline Department of Agriculture, Peradeniya, Ceylon, United States Department of Agriculture,

Belhville, Maryland, and Carnell University, Ithaca, New York

Page

I The Term “Laterite” 1

11 The Nature of Laterite 5

A Physical Characteristics 5

B Chemical Characteristics 7

C Mineralogical Characteristics 111 The Environment of Laterite

A Climate 14

15

C Parent Material 16

D Topography

E Age 20

IV Profiles Containing Laterite 20

A Soil Material Overlying Laterite 21

B Laterite within Soil Horizons 22

V Formation of Laterite 26

A 28 B Development of Microstructures 38

C Hardening of Laterite 39

D Development of Laterite in Place without Enrichment from Out- side Sources 45

E Enrichment from Outside Sources 45

C Horizons or Layers Beneath Laterite 24

Weathering as a Preconditioner of Material of Laterite

F Principal Processes Involved

VI Geomorphic Relationships 53

VII Softening of Laterite 55

References

1 The Term ”Laterite”

The term ‘laterite” was originally coined by Buchanan (1807) for a ferruginous, vesicular, and apparently unsbatified material occurring in immense masses over the country rock of Malabar in India The freshly

1 This review constitutes Agronomy paper No 542, Cornell University, Ithaca,

New York

1

2 S SIVARAJASINGHAM ET AL

dug material, as described by Buchanan, was soft enough to be readily cut into blocks by an iron instrument, but upon exposure to air it quickly became as hard as brick and remarkably resistant to the action

of air and water Since this material was used as building brick, and was called “brickstone” in several of the indigenous languages, Buchanan aptly called it ‘laterite” after later-the Latin for brick

Prescott (1954, pp 1-2) has reported that Buchanan did not use the term “laterite” in his journals between 1807 and 1814 but used the term

“brickstone” and that Babington (1821) was the first to use ‘laterite” in formal scientific literature According to Prescott and Pendleton (1952,

p I ) , Buchanan used the word “brickstone” only in his later travels

(1807-1813) through what is now Bihar to describe occurrences in the Rajmahal hills: “He noted the similarity of the Bihar occurrences to those of Malabar, but was puzzled by the fact that the former masses of material while still in the ground and excluded from the air retained their stony form.” At first in Malabar, he had used the terms ‘laterite” and

“brickstone” interchangeably in his descriptions of the soft materials that harden; his later use of the term ‘8rickstone” in Malabar may have been out of a desire to reserve the term “laterite” for the soft ferruginous material that hardens

Though some of Buchanan’s immediate successors ( Voysey, 1833;

Stirling, 1825) used “iron clay” as an alternative term, the word ‘laterite” gradually came into wide use in India Detailed morphological de- scriptions of laterite, still considered to be among the most vivid, were given by Newbold (1844, 1846)

Interest in laterite was stimulated in other parts of the world by the publication of a chapter by Blanford (1879) in the first Manual of the Geology of India, with which the name laterite became finally confirmed (Fox, 1936) Before the end of the nineteenth century, laterite as a surficial or shallow formation was identified on the basis of physical characteristics in many widely distributed areas of Australia, Africa, and South America

Fermor (1911) considered use of the term only for soft materials that could be cut into bricks, a severe restriction, though such use appears to conform with the intentions of Buchanan Blanford (1859)

had mentioned that in some cases the lithomarge underlying laterite becomes hard on exposure, and Harrison (1910) also recorded the occurrence of mottled, creamy white and dark red sesquioxide-poor deposits that harden on exposure Thus, the property of hardening as a criterion of laterite became controversial Later this aspect was further confused when Talbott in Australia (Prescott, 1931) extended the term

LATERITE 3

to include not only hard ferruginous surface formations, but also siliceous and travertine crusts, designating them ferruginous, siliceous, and calcar- eous laterite, respectively Prescott, however, confined “laterite” to the ferruginous and aluminous forms and cited the earlier suggestion of Woolnough (1927), who introduced the term “duricrust” to cover the other kinds of crusts

Though many earlier observers had suggested the ferruginous and even the aluminous nature of laterite (Mallet, 1883), the fundamental chemical character of laterite was first established by Bauer (1898) His analyses revealed the low content of silica and the high contents of alumina and iron oxide in samples collected from the Seychelles Sub- sequent investigations of samples from many different parts of the tropical region gave similar results (du Bois, 1903; Warth and Warth,

1903; Holland, 1903)

Great interest developed because of the possible use of laterite as an ore for aluminum (Holland, 1905) and, in some cases, for manganese (Fermor, 1909) Consequently, much of the early work was confined to chemical analysis of bulk samples that were selected for high aluminum content This prompted a reviewer (Bull Imp Inst 1909, vii, p 133) of

Harrison’s work to suggest that the term laterite be restricted to products

of weathering containing free alumina As a result, controversy developed

among geologists regarding the chemical properties of laterite

Fermor (1911) finally abandoned the physical property of hardness

of a material in its natural state or on exposure as a criterion of laterite and developed a comprehensive system of nomenclature of lateritic materials on the basis of chemical composition, though he rejected the presence or absence of alumina in large quantity suggested by Crook

(1909) He subscribed to the views of Evans (1910) that “though the chemical composition of laterite varies within wide limits, one feature remains constant-the small amount of combined silica in proportion to the alumina present It is in this respect that laterites differ from clays, which also occur as tropical decomposition products.” Fermor, con- sequently, based his classification on arbitrary limits of the ‘lateritic” constituents, which he defined as the oxides of iron, aluminum, titanium, and manganese

Meanwhile Walther ( 1889,1915,1916) had erroneously assumed that the term “laterite” had been chosen to signify red color and “proposed that the word should be used for all red-colored alluvia” (Prescott and Pendleton, 1952, pp 35-36) Ultimately, any tropical red earth came to

be called ‘laterite” or “lateritic.” As studies of tropical soil progressed,

attempts were made to standardize the use of these two terms on the

4 S SIVhRAJASINGHAM ET AL

basis of chemical composition The silica-alumina ratio and, later, the silica-sesquioxide ratio were used to classify soils into “laterite,” “lateritic,” and “nonlateritic” (Martin and Doyne, 1921, 1930; Joachim and Kandiah,

1935) The terms received even wider connotation with the adoption

of “laterite” and “lateritic” as the names of Great Soil Groups by the Lhited States Soil Survey Staff (Byers et al., 1938; Baldwin et al., 1938)

Pendleton (1936) strongly urged that the term “laterite” be restricted

to the original concepts of Buchanan, restated nearly 100 years later by Oldham ( 1893) As a consequence of this rigorous definition of “laterite,”

Pendleton and Sharasuvana (1946, p 434) defined a “laterite” soil as

“one in which a laterite horizon is found in the profile.” They considered

a “lateritic” soil to be “one in which there is an incipient or immaturely developed laterite horizon, and in which it is believed a true laterite horizon will develop if the prevailing conditions persist long enough.” The definitions put fonvad by du Preez (1949) for ‘laterite soil” and

“lateritic soil” are essentially similar to those of Pendleton and Sharasu-

vana His definition of laterite, like that of Pendleton and Sharasuvana,

fails to recognize the importance of alumina, and while he covered some morphologicaI aspects of laterite comprehensively, he ignored the soft variety that hardens on exposure Mohr and van Baren (1954) defended use of the terms ‘laterite” and “lateritic” for soil on grounds of similarity

of weathering products that produce soil as well as material that hardens Kellogg (1949, p 79) confined the term ‘laterite” to four principal forms of sesquioxide-rich material that either are hard or that harden upon exposure: ( 1 ) soft mottled clays that change irreversibly to hard- pans or crusts when exposed, ( 2 ) cellular and mottled hardpans and crusts, ( 3 ) concretions or nodules in a matrix of unconsolidated material,

( 4 ) consolidated masses of such concretions or nodules The Soil Survey Staff of the United States Department of Agriculture (1960, p 62)

proposed a new term, plinthite (Gk plinthos, brick), for essentially the same concept, defining it as “the sesquioxide rich, humus poor, highly weathered mixture of clay with quartz and other diluents, which commonly occurs as red mottles, usually in platy, polygonal, or reticulate patterns; plinthite changes irreversibly to hardpans or irregular ( hard) aggregates on repeated wetting and drying, or it is the hardened relicts

of the soft red mottles.” The term “plinthite” was introduced to avoid the confusion arising from use of the word “laterite” without precise definition for many widely divergent materials

In this paper, the term laterite is retained as a term that would be recognized by most readers, though its use is restricted to material that

conforms generally with the definitions of laterite by Kellogg (1919)

and of plinthite by the Soil Survey Staff (1960)

LATERITE 5

II The Nature of Laterite

The term laterite is restricted in the remainder of this paper to highly weathered material ( 1 ) rich in secondary forms of iron, aluminum,

or both; ( 2 ) poor in humus; ( 3 ) depleted of bases and combined silica;

( 4 ) with or without nondiagnostic substances such as quartz, limited

amounts of weatherable primary minerals, or silicate clays; and ( 5 )

either hard or subject to hardening upon exposure to alternate wetting and drying The term as used implies no restrictions, other than those inherent in the properties defined, on size or shape of the masses, on their internal organization, on the processes by which diagnostic proper- ties have developed, or specific conditions of place or time as factors essential to such development In this sense it includes Buchanan’s laterite and hardened equivalents of it In addition, it includes certain highly weathered material in sesquioxide-rich humus-poor nodules2 that are hard or that harden upon exposure, though they may be surrounded

by earthy material that does not harden, as well as masses of such nodules cemented together by sesquioxide-rich material It excludes ( a ) sesquioxide-rich earthy material, which has been called “laterite”

or “lateritic soil,” that does not harden upon exposure; ( b ) iron- rich masses or nodules with significant amounts of humus, which are characteristic of certain podzols; ( c ) hard masses cemented by silica, carbonates, or substances other than sesquioxides, though highly weathered sesquioxide-rich fragments or nodules within such masses might be included; and ( d ) certain hard pellets or “shot” found in slightly weathered material

A PHYSICAL CHARACTERISTICS Laterite occurs in various morphological forms Pendleton and Shara- suvana (1946, p 438) recognized two distinct physical forms, vesicular and pisolitic, with many types intermediate between the two Du Preez

(1949, p 57) has defined laterite as “a vesicular, concretionary, cellular, vermicular, slaglike, pisolitic or concrete-like mass.” The description of a vesicular laterite by Newbold (1844) is quoted in part here after Prescott and Pendleton (1952, p 5)

“The laterite , generally speaking, is a purplish or brick red, porous rock, passing into liver brown perforated by numerous sinuous

2 The term “nodule” is used not only in the sense defined by Bryan (1952) to include “rounded lumps” of a variety of compositions whether formed by accretion

or by centripetal enrichment, but also to include rounded fragments of laterite inherited from a laterite crust

6 S SIVARAJASINGHAM ET AL

and tortuous tubular cavities either empty, filled, or partially filled with

a greyish-white clay passing into an ochreous, reddish and yellowish brown dust; or with a lilac-tinted litheomargic earth The sides of the cavities are usually ferruginous and often of a deep brown or chocolate color; though generally not more than a line or two in thickness, their laminar structure may frequently be distinguished by the naked eye The interior of the cavities has usually a smooth polished superficie, but sometimes mammillary, and stalactiform on a minute scale The surface masses of the softer kinds present a variegated appearance The clay and lithomarge exhibit lively colored patches of yellow, lilac, and white, intersected by a network of red, purple, or brown The softness

of this rock is such that it may be cut with a spade; hardening by exposure to the sun and air, like the laterite of Malabar.” (Omissions are

by the present authors.) Vesicular laterite may be soft or of varying hardness and commonly has earthy material in the cavities It usually occurs near the surface

Cellular slag-like laterite is a scoriaceous mass The many empty cavities are separated by ferruginow material similar in appearance to that which separates the earthy substance in vesicular laterite Cellular laterite is usually dark colored and may have a dull or lustrous surface

It is of varying hardness and is brittle, being usually easily shattered

when struck a sharp blow with a hammer According to Fox (1936),

cellular laterite is formed by removal of kaolin and other earthy material from the cavities in vesicular laterite when the latter is exposed to

erosion and leaching at the surface Falconer (1911), from his observa-

tions in nothern Nigeria gave a similar explanation, though he avoided using the term laterite for “surface ironstone.”

Nodular laterite consists of individual concretions, pisolites or other crudely round masses, usually the size of a pea but commonly larger or smaller; it is generally ferruginous The nodules may occur as a super- ficial covering or as a component in one or more horizons in the soil, varying in concentration from low or insignificant amounts to very high amounts The nodules vary in hardness; some can be readily cut by a knife but most are hard and brittle

When the nodules of a layer are cemented together, hard “pisolitic”

or “concrete-like” laterite is formed It occurs mainly at or near the surface The individual nodules may either be joined directly to one another or be discrete entities in a cementing matrix of similar, but usually less ferruginous, material

Recent studies by Alexander and Cady ( 1962) present enlightening detail on the physical arrangement of discrete components Though various specimens exhibit a great variety of micromorphological features,

LATERITE 7

certain structures are common to many, but not necessarily all, varieties Commonly under magnification in thin sections, tiny bodies ranging from perfect spheres to oblong rounded forms may be seen embedded in a matrix of fine particles; the matrix may be either very dense or sponge- like The rounded bodies may be individual units or, commonly, may be aggregates of smaller spherical units closely packed Such rounded bodies may be widely spaced or closely packed in the matrix Their boundaries may be smooth and definite or irregular and indefinite in various speci- mens The matrix may be unorganized, may have a gridlike rectangular

or reticulate network of oriented material, or may be largely oriented Ori- ented material commonly lines pores and may appear as skins on the nod- ules Crystalline oriented material is common as pseudomorphs after pri- mary minerals, as porefillings, and as discrete bodies ranging from barely visible units to relatively large homogeneous masses Rock structure may

be preserved or may be entirely absent Quartz particles may be included, and in some specimens weatherable minerals encased in a protective covering of weathered material have been observed

B CHENICAL CHARACTERISTICS Materials identified in the field as laterite have a wide range of chemical characteristics A prominent feature common to all laterites, nevertheless, is a high content of either iron or aluminum or both relative to other constituents (Alexander et al., 1956) This is clearly illustrated by the following analyses, which are thought to be typical examples (Table I )

Bases are almost completely absent Combined silica is generally

Fe7.03

7.06 23.88 21.54 11.05 9.10 H,O (loss on ignition)

99.99 100.00 99.54 97.53 99.42

Site: 1, Coolgardie, Australia (Simpson, 1912) 2, Satara, Bombay, India (Warth and Warth, 1903) 3, Bagru Hill, Bihar, India (Fox, 1936) 4, Cheruvannur, India; Buchanan’s original site (Fox, 1936) 5, Djougou, Dahomey; laterite on granite (Alexander and Cady, 1962)

ND, not determined

8 S SIVAFUJASINGHAM ET AL

low (sites 1, 2, and 3, Table I ) , but some varieties, such as the original laterite of Buchanan (site 4, Table I ) , may have significant amounts This is probably largely in the form of kaolin, which has been found in recent work by Alexander and Cady (1!362) to be the principal or only identifiable silicate clay mineral in samples from Africa Alumina may

be the principal sesquioxide (site 3, Table I ) , but more commonly iron oxide (site 1, Table I ) or iron oxide and alumina together (sites 2, 4,

and 5, Table I ) are the major constituents Combined water, determined

by loss on ignition, is appreciable but is generally higher in aluminous

than in ferruginous varieties, as is shown in Table I Titanium is also common in significant amounts in most varieties and may be a major constituent (site 3, Table I ) Vanadium and chromium are found, but rarely in appreciable quantities

Quartz may be absent or present in only limited amounts, but on rocks high in quartz it is commonly a significant or major component, as

on the granite of site 5 of Table I, for which petrographic studies showed that much of the total silica was contributed by quartz Quartz is also common in laterite over nonquartzose rock, where it appears to be derived mainly from wind-blown or detrital material from outside sources Ten samples of detrital laterite from various parts of India had an average

of ,2074 quartz (Warth and Warth, 1903) Pendleton and Sharasuvana

(1912, p 10) have emphasized that differences in amount of quartz commonly contribute to major variation in SiO, among samples, even

in the same profile

The impoverishment in combined silica and bases and concentration

of sesquioxides during weathering and laterite formation on a dolerite

is illustrated in Table 11 The “primary laterite” of Harrison is not to be confused with ‘laterite” as used in this review It was a weathered earthy product that lay between the surficial hard laterite crust and the un- weathered dolerite rock Major differences in proportions of iron and aluminum and in amount of combined water between weathered material and laterite presumed to have formed in similar material are common, as

in Table 11, but it is rarely possible to be certain that the laterite crust has

indeed formed in material like that of the underlying weathered product

No consistent relationship seems to exist between the relative amounts

of silica, iron, and alumina and the degree to which the physical proper- ties of laterite are developed The shortcoming of any chemical classifi- cation was shown by Fox (1936) from the analyses of laterite samples

from Buchanan’s original sites (site 4, Table I ) These would have been

called ‘lateritic lithomarge” in Fermor’s ( 1911) classifkation because of the high content of combined silica, though the material was vermicular and was being quarried for building purposes

LATERITE 9

The analyses considered so far refer to bulk samples of massive laterite without distinction between segregated nodular material and the matrix The nodular material is, however, found either to be similar in composition to the matrix or to contain less combined silica and more

TABLE I1

Chemical Composition ( ”/o ) of Dolerite, “Primary Laterite,” and Associated Laterite

Ironstone at Eagle Mountain, British Guianaa

H,O (loss on ignition)

2.86 0.50 46.80 23.64 2.50 22.96 0.69 Nil Nil Nil Nil Nil 99.95

-

0.14 0.62 10.54 74.43 0.65 9.60 3.91 Nil Trace 0.02 Nil Nil 99.91

ferric oxide Site 5 of Table 111 illustrates the latter in soft nodules of a ground-water laterite of the Congo Prescott and Pendleton (1952, p 21) believed that nodules usually contain less free alumina than the more massive forms and that they are low in manganese

10 S SIVARAJASINGHAM ET AL

Nodules studied by Alexander et al (1956) were high in sesquioxides

and low in silica Other workers have, however, reported appreciable contents of both quartz and combined silica (Joachim and Kandiah, 1941; Waegemans, 1954) The data in Table 111 illustrate the wide range of silica, alumina, iron, titanium, and manganese in nodules from different places The work of Bennett and Allison (1928) also reveals variations in the composition of nodules in different soils

C M W ~ L O G I C A L CHARAC~ERISTLCS

Chemical analysis alone is not sufficient to reveal the nature and

origin of laterite ( Harrison, 1910; Campbell, 1917) Laterites having similar physical properties, such as hardness or morphology, may differ greatly in chemical composition, and, conversely, laterites having similar chemical compositions may have greatly different physical properties Petrographic studies of thin sections ( Harrison, 1910, 1933), adsorption

of dyes (Hardy and Rodrigues, 1939), differential thermal analysis (Humbert, 1948; Bonifas, 1959), and X-ray analysis (Alexander et al.,

1956; Bonifas, 1959) have been used to supplement chemical determi- nations

Free alumina is mostly in the form of gibbsite (A1203.3H20), as boehmite ( A1203.H20), or as an amorphous hydrated form which has been called cliachite and a variety of other names (Hanlon, 1944; Palache d aE., 1944) Iron is found in the form of goethite ( F e O - O H ) , hematite ( FeeOa), and as amorphous oxides or unidentifiable coatings on

other minerals (Alexander et al., 1956) Free silica is mostly inherited

quartz (Alexander et al., 1956), though Harrison ( 1933, p 40) reported

PLATE 1 Photomicrographs illustrating features of weathering and laterite formation

A Weathering diorite, North Carolina Crossed Nicols The lath-like forms are gibbsite pseudomorphs after feldspar Some dark areas are allophane and some are iron oxide

€3 Soft laterite from granite, Nigeria Crossed Nicols The light areas of the crystal aggregate (upper left) and of the filled channel (lower right) are gibbsite formed upon weathering of kaolinite Dark areas are iron-impregnated clays and iron oxides, which are isotropic or have a very low birefringence

Hard laterite from granite, Nigeria Plain light The dark areas are im- pregnated with iron by local redistribution from the light areas The higher popula- tion of quartz grains (white areas) in the part that has lost iron indicates that these parts have collapsed

Hard laterite from granite, Kigeria Crossed Nicols The yellow parts are crystalline goethite which forms a continuous network, especially on the walls of the small channel at the left The dark streak through the center is a former channel filled with fine-grained hematite White spots are quartz grains

C

D

This Page Intentionally Left Blank

LATERITE 13 secondary quartz in laterites presumed to have been derived from basic igneous rocks containing only small amounts of quartz Chalcedony and opaline forms have been observed also Presence of colloidal silica has been suspected Besides quartz, other resistant minerals like magnetite, zircon, sphene, anatase, and ilmenite have been found

Combined silica is known to occur in kaolin and comparable silicate clay minerals that can be identified but is present in amorphous or subcrystalline forms as well Montmorillonite and illite types of clays have not been identified in significant amounts

The distribution and form of constituents within the laterite has special significance Recent studies by the authors have shown that the finely divided matrix is commonly largely unoriented and unidentifiable

by petrographic techniques, though identsable portions range from none

to most of the material Grains and large patches of oriented aggregates ranging from kaolin slightly stained with iron oxide through kaolin highly impregnated with iron oxide to almost pure goethite or hematite are among the more prominent identifiable constituents Variable optical density within the matrix is related to the degree of iron impregnation (Plate 1, C and D) Hardness of the mass appears to be related to the degree of crystalinity and continuity of the crystalline phase of the impregnating iron, which is largeIy in the form of goethite Tiny spherical bodies comparable to incipient nodules of centripetal enrichment de- scribed by Bryan (1952) are largely unoriented earthy material having

a higher degree of iron impregnation than the surrounding matrix Some have films of crystalline goethite on the surface or as concentric shells within the body Other spherical bodies embedded in the matrix may

be concretionary or pisolitic forms of gibbsite, boehmite, goethite, or hematite Gridlike networks of oriented materials are found in the matrix

of many specimens and are composed principally of goethite (Alexander

et al., 1956) or hematite ( Sivarajasingham, 1961)

Gibbsite pseudomorphs after feldspar (Plate 1A) and goethite pseudo- morphs after ferromagnesian minerals are common constituents These appear to be most abundant in young laterites whereas concretionary or pisolitic nodular forms of the same minerals are more conspicuous in older varieties (Alexander et al., 1956) Gibbstite and boehmite are

found commonly as fillings in cracks and pores, both in the matrix and

in nodules Pores and cracks may also be lined with oriented kaolin having varying degrees of iron impregnation, or even with oriented goethite (Plate 1D) In hard laterite crusts, hematite may be found as pore linings, as bands, or as discrete masses (Plate 1D) In varieties derived from quartzose material, quartz grains are generally distributed randomly through the matrix (Plate 1C) and earthy nodular bodies

14 S SIVARAJASINGHAM ET AL

Quartz, apparently derived from outside sources, may also be present in laterite over nonquartzose rock (Alexander et aZ., 1956)

111 The Environment of Laterite

Laterite is widely distributed in Africa, Australia, Southeast Asia, and South America; Prescott and Pendleton (1952) have given a compre- hensive review of its geographical distribution The observed distribution

of laterite relative to present environment, however, is not necessarily a criterion of conditions under which laterite forms (Hallsworth and Costin, 1953, p 25; Pendleton, 1936, p 107)

A CLIMATE Pendleton (1941) considered that an effective rainfall capable of supporting a forest is necessary and that laterite probably never develops

in a climate that would support a savannah-type climax vegetation

Maignien (1958, p 15) concluded that the Sudano-Guinea climate of

80 or more inches of rain per year with 2 to 4 months relatively dry is optimum for mobilization, accumulation, and induration of iron in laterite Humbert ( 1948, pp 281-282) concluded that continuously wet conditions do not favor laterite formation The occurrence of a period of drought, which is observed in many areas where laterite occurs, was though to be a requisite by Maclaren (1906) This was disputed by Scrivenor ( 1909), however, who declared that laterite occurs in Malacca, where there is no such alternation of seasons Campbell (1917) and Humhert (1948, p 282) considered that a regular alternation of wet and dry seasons is not necessary if there are periods of wetness and dryness, even though irregularly distributed Mohr and van Baren (1954,

pp 69-71) have suggested, on the basis of effective rainfall and evapo-

transpiration, that a monthly rainfall not exceeding 60 mm would con- stitute an adequately “dry” season to provide no excess of rain over evapotranspiration in tropical regions Simpson ( 1912) thought that laterite was forming under the semiarid conditions of western Australia

It appears clear that some minimum amount of water is necessary for weathering, the removal of bases and combined silica, and segregation

of iron so evident in the resultant material It also appears reasonable that periods of drying should favor the crystallization of goethite or similar minerals, which appear to be associated with hardening Observa- tions by the authors suggest that approximately equal wet and dry

seasons favor crust formation and that some degree of alternating wet and dry season is probably essential for this process It is not clear whether the material may or may not be conditioned for this final segregation

and crystallization in permanently wet conditions To some degree,

LATERITE 15

conflicting views may be the result of two confounding factors: (1) the occurrence of laterite under climates that are unlike those under which the laterite formed, and ( 2 ) differential effectiveness of climatic wetness

or dryness in combination with differences of topography, parent ma- terial, and time

Laterite is found where temperatures are warm or are believed to have been warm at the time of formation This does not preclude the possibility, however, that time, not temperature, is the controlling factor If the principal effect of high temperature is to accelerate rates of reactions, the time interval necessary for laterite formation in temperate regions may exceed the period of time that appropriate landscapes have been stable Similarly, observations of Oldham ( 1893), Joachim and Kan- diah (1935), and Prescott and Pendleton (1952) that laterite has not formed under the cool temperatures of highlands in the tropics may be due less to cool temperature than to landscape instability Nevertheless, field observation generally agree with the hypothesis that warm tempera- tures favor the formation of laterite

B VEGETATION

A forest vegetation was considered necessary for laterite formation by Glinka (1927, pp 33-34) More detailed observations have shown that, while laterite occurs in regions with a rain-forest vegetation, well- developed laterite is most commonly found under a low forest and that hard surficial laterite is a very common feature of the open savannah adjacent to the forest Maignien (1958, p 16) concluded that laterite is most extensive and strongly expressed at the boundary of forest and savannah DHoore (1954,1957, p 55) found iron mobilized to a greater degree and to greater depth under tropical grasses than under forest Buchanan’s type of soft laterite is found in Ceylon in the forested areas

of the southwestern lowland

Humbert (1948) suggested that laterite forms in a climate that has a wet and a dry season, and his descriptions and illustrations indicate that the laterite he observed was in an open savannah that was gradually

replacing forest, a common condition where a dry season is prominent

The change from soft laterite to the hard form within a few years after man has cleared the forests has been reported by Alexander and Cady ( 1962) in French Guinea and other parts of West Africa Blackie’s (1949)

description of soils of Fiji and Aubert’s (1950) description of Dahomey also confirm the hardening of laterite following a change from forest to savannah caused by man’s activities It would appear from the literature that laterite is most extensive in areas of savannah but that laterite forms under forest though its hardening is favored by lack of forest cover

Marbut (1932, p 76) has described laterite outcropping on the banks of the Amazon, presumably in alluvium, in association with a ground water table Maignien (1958) has shown that fermginous laterites may develop

on a variety of materials providing there i s a source of iron either in the parent rock or in adjacent higher-lying areas from which water may introduce ferruginous material

The laterite layers may be in material unrelated to the underlying bedrock In some places, laterite clearly has formed in residuum of

rocks that have weathered in place, as evidenced by features such as a constant proportion of iron oxides to alumina with depth or by a continuation of quartz veins from the rock into the laterite It is also found in colluvial deposits; examples are described by Maignien ( 1958)

Often laterite is associated with material that has been transported locally; recently recognition of stone lines in surhial mantles of this character has directed attention to the frequency of such conditions (Ruhe, 1959) Though such surficial deposits may not be directly related to the underlying bedrock, they may not be very different from

residuum of the major rocks in the locality, since transport is often of a

local nature as in cases cited by Nye (1955a) and by Ollier ( 1959) The parent material in which laterite forms may also be derived from material

of different and highly contrasting strata than the underlying rock currently present Whatever the source of material in which laterite forms, an adequate supply of iron appears to be essential Alexander and Cady (1962) have noted that the thickness of laterite crusts is sometimes related to the iron-richness of associated rocks

D TOPOGRAPHY Laterite has been generally associated with a level or gently sloping surface This characteristic was emphasized by Oldham (1893) in his review of numerous laterite locations in India and has been subsequently

LATERITE 17 confirmed by many workers from other parts of the tropics Campbell ( 1917), from his many observations, reported that laterite currently being formed covers the flatter ground and stops where the slopes are steep Holmes (1914) observed that in Mozambique laterite occurs only

on gently undulating plateaus and never on steep slopes Humbert (1948), from his study of laterite in Australian New Guinea, concluded that the best examples of laterite occur almost exclusively on areas of low relief and gentle slopes Prescott and Pendleton (1952, pp 25-26) also emphasized the nearly level nature of the terrain where laterite occurs

The horizontal disposition of laterite deposits is very striking in many relatively arid regions Newbold (1846) observed that in the interior of South India laterite occurs in almost horizontal beds as cappings on the tops of mountains Woolnough (1918) emphasized the significance of relic laterite deposits in western Australia on slightly elevated plateaus Similarly in southwestern Australia, laterite is found in many localities as massive or concretionary deposits forming protective cappings on flat- topped residuals (Mulcahy, 1960, 1962) Laterite is also present in extensive bodies on dissected tablelands (Stephens, 1946)

Laterite is found along river banks and on terraces adjacent to higher ground These masses of “gallery laterite” are believed by DHoore (1957,

p 97) to be formed by enrichment with iron during flood stage followed

by its immobilization when the flood subsides In the flat humid areas,

where it is believed that laterite is forming currently, a high but

fluctuating water table is common The drainage is more or less imperfect, and the ground water at and below the water table is thought to be somewhat sluggish This has led to a common opinion that topography conducive to a high water table may be essential

Soft laterite is found at a shallow depth on low hillocks in south- western Ceylon, generally not much more than 100 to 200 feet above sea level (Prescott and Pendleton, 1952, p 9 ) The slopes are undulating and are certainly not level, Though Oldham (1893) dismissed these occur- rences as “a more or less ferruginous subsoil which never passes into laterite,” the descriptions of Joachin and Kandiah (1935) indicate that these materials, locally called “cabook,” conform to Buchanan’s concept

in every way It should be noted, however, that the underlying rock in this region is charnockite, a quartzo-feldspathic gneiss or granulite with hypersthene, and that the rainfall is over 100 inches per annum but with dry intervals High rainfall and high iron content of the parent rock may favor the formation of laterite even where the surface is not level but is stable owing to the nature of the weathered material and forest cover Mulcahy (1960, p 222, 1962) emphasized that peneplain condi-

18 S SIVARAJASINCHAM ET AL

tions of low relief are not essential, though low available relief conducive

to stable land surfaces for long periods is most favorable

Laterite on areas of low relief, nevertheless, is the most common

Blanford (1879) classified the Indian laterites into high-level and low- level varieties, depending on the mode of occurrence This distinction was at first intended only to signify two contrasting positions, high-level laterite capping the summits of hills and plateaus on the highlands of central and western India and low-level laterite covering large tracts in the coastal regions (Holland, 1903)

Subsequent workers observed that low-level laterite is generally the more fenvginous and commonly does not exceed 30 feet in thickness

It also commonly contains inclusions of sand and pebbles, which indicate

a multicycle or detrital origin (Oldham, 1893) Low-level laterite, how- ever, does not everywhere contain such foreign inclusions Since it was once thought that laterite can form only at or near the water table, the low-level laterite on areas with a high water table was called “live” laterite The laterite on ground above the reach of the water table was thought to have been a product of an earlier period when the ground- water fluctuation reached the laterite zone Hence, this was called “ d e a d laterite ( Campbell, 1917)

The high-level laterite is generally more homogeneous and may be relatively thick, as much as 100 to 200 feet according to Oldham (1893), though it is probable that such thickness is a feature of plateau margins Though it was considered “dead” by Campbell ( 1917), Harrison (1933,

p 16) believed that laterite can form on high-level positions, as on the plateau of the Eagle Mountain range in British Guiana, under conditions

of heavy rainfall though the water table is low As it is presumably being

formed currently, this would be called “live” laterite Laterite in high- level position may include pebbles from even higher surfaces, indicating detrital origin (Ruhe, 1954, p 18)

The laterite found on distinct slopes adjacent to higher ground may

be called foot-slope laterite to indicate its topography I t is often detrital, being formed by the consolidation of fragments of the laterite of the higher level that have moved down the slope as erosion has advanced (Oldham, 1893) The second member of Greene’s ( 1945) “ironstone catena” would be this class “Terrace” laterite occurs on terraces adjacent

to high ground or along the banks of streams It is thought to be formed

by the deposition of dissolved material where ground water moving laterally and down the adjacent slope encounters the oxidizing conditions

of the surface horizons (Campbell, 1917) Maignien (1958, 1959) em- phasized the occurrence of laterite crusts at the borders of natural drainage areas, such as piedmonts, river banks, dissection forms, and

LATERITE 19 other abrupt breaks in slopes where the profile of a saturated zone approaches the surface Another type recognized by Fermor (1911) is

“lake” laterite formed in marshy areas by water flowing from the surrounding higher land, either along the surface or by seepage

Lake (1890) used the accompanying tabulation to classify the laterites

in Malabar, India, according to topographic position, character, and origin:

Group Nature of the laterite Origin Plateau laterite Vesicular Nondetrital Terrace laterite Pellety Detrital

Valley laterite Partly vesicular Partly nondetrital

Partly pellety Partly detrital

More elaborate schemes of classification, based on other factors, have been published by DHoore (1955), Maignien (1958), and du Preez (1949)

Four physiographically distinct landscapes with which laterite is commonly identified in the literature are: (1) high-level peneplain remnants, ( 2 ) colluvial footslopes subject to water seepage, ( 3 ) low-

level plains having high water tables or receiving water from higher land, (4) residual uplands other than peneplain remnants The first three are illustrated diagramatically in Fig 1

Hlgh-Level Plateou

wlth Loterite Cap

Laterits Detritus

Laterite Enriched

FIG 1 Relationships among physiographically distinct landscapes with which

Laterite on high-level peneplain remnants may be in residuum of rocks weathered in place or may be in transported material deposited prior to peneplain dissection Its present position is a consequence of landscape inversion such as that described by Bonnault (1938); it once occupied a low-level position comparable to that illustrated in Fig 1 With uplift or with lowering of base level, areas protected by laterite have remained as erosion lowered the surrounding areas Such areas now occupy the highest positions and are being reduced slowly by

laterite is commonly associated

E AGE

Many existing laterites are clearly relics of geologic antiquity Those

of Queensland, Australia, are reported to be products of two humid periods of the Pliocene (Whitehouse, 1940) In Ceylon, of the crusts described most are thought to be products of the Pleistocene and some,

of Pliocene or earlier periods (Fernando, 1948) Laterites of Nioka (Ituri), Congo have been related to mid- and late-tertiary surfaces by

Ruhe (1954) Though laterite may be forming currently on some ancient peneplain remnants, many high-level crusts are considered to be “ d e a d products of the age of peneplain development Their existence may commonly be considered a factor in the preservation of the land forms

on which they occur, and the period since the lowering of base level is

a measure of the time required for landscape inversion under given

apparently may form rapidly Obviously, formation of laterite from solid unweathered rock can be no more rapid than the time required to attain

a high degree of weathering In unconsolidated weathered material subject to enrichment in iron from outside sources, however, rate of development may be relatively rapid Hardening of the soft pre- conditioned material may take place in a few years upon exposure (Alexander and Cady, 1962)

IV Profiles Containing Laterite

Though either nodular or vesicular laterite may lie within, and may

be genetically related to, the solum of the modem soil, as defined by the Soil Survey Staff of U.S.D.A (1951), many laterites appear to be

LATERITE 21 unrelated genetically to present soil horizons Consequently, the Soil Survey Staff (1960) has defined “plinthite” independently of soil horizon definitions, though they use the presence of soft “plinthite” within the solum as a criterion of soil classification This discussion is concerned with horizons or layers in the entire weathered section, which commonly

is much thicker than the part that would be considered “solum.”

A SOIL MATERIAL OVERLYING LATERITE

At sites where laterite is thought to be forming, it is generally found

as a shallow but not surficial layer Prescott and Pendleton (1952)

reported laterite in Ceylon at various depths, averaging about 2 feet, and

in Thailand from a fraction of a foot to 6 feet Humbert (1948) described

“red to yellow loam” 2 to 6 feet thick over laterite in Queensland Some Iaterite layers have been described by Alexander and Cady (1!362) at a depth as great as 13 feet in Sierra Leone but others were found near or

at the surface Similar observations are very numerous in the literature and indicate that soil material over laterite is mainly less than 10 feet thick Laterite crusts at the surface are very widespread ( Oldham, 1893; Maclaren, 1906; Simpson, 1912; Walther, 1915; Prescott and Pendleton,

1952, among many authors), but such exposure is generally attributed to erosion Though hardening of laterite is thought to be a phenomenon favored by surface position, the literature implies that the initial development of material that will harden most commonly occurs at some depth below the surface Hardened laterite crusts as thick as 200 feet (Oldham, 1893) suggest that development can proceed to this depth, but such an extreme may be only at the edge of peneplain rem- nants where the material is affected by vertical exposure A crust 30 feet thick has been reported by Alexander and Cady (1962); Campbell (1917) observed that laterite seldom exceeds 30 feet in thickness

Mohr (1944) considered laterite to be essentially a soil horizon of sesquioxide accumulation to which the overlying soil material is related genetically This concept has been elaborated by Mohr and van Baren (1954, pp 300304) and was accepted by Pendleton (1936, p 106) Marbut ( 1932) postulated a comparable genetic relationship between laterite and soil material above it In all these cases, the authors have dealt with restricted conditions, comparable in many respects to the original laterite of Buchanan The work of Maignien (1958, 1959), Mulcahy (1960), and D’Hoore (1954, 1957) and observations by the authors (Alexander and Cady, 1962) show clearly that the presence of

a suficial mantle of soil over laterite is no assurance that the soil is related to the underlying laterite genetically, or that if genetic relation- ships are involved, they may be quite unlike those postulated

22 S SIVAEWJASINGHAM ET AL

The overlying soil material may be from sources different than the material in which the laterite has formed Stone lines marking erosion surfaces (Ruhe, 19.59) are very common in the tropics and may mark major discontinuities of material vertically Ollier ( 1959), however, has

reported extensive areas in Uganda where the stone lines appear to be the result of sorting by termites, leaving coarse fragments below and moving fine earth to the surface Nye (1954) also emphasized the activities of termites but reported substantial sorting and downslope creep of the sorted material Berry and Ruxton (1959) also have emphasized an upper zone of migration Resistant minerals, such as quartz, in soil over laterite from quartz-free rock are common and indicate at least contamination of the upper layers of material from outside sources

In some cases surface material can be demonstrated to be residual

from the same rock as that from which underlying laterite has formed and either predates or is contemporaneous with the laterite There are also cases known to the authors in which a surficial soil mantle has developed by disintegration of the upper part of a laterite crust Such soils are commonly thin over the laterite and contain pieces of the disintegrating laterite

Thus, a great variety of soils may overlie laterite Where such soil horizons are residual, they are composed of highly weathered material high in sesquioxides with or without kaolin and with some component of whatever highly resistant minerals may have been present in the parent

rock They may be uniform in character with depth, like Latosols, or may have genetic A-B horizon sequences not unlike Red-Yellow Podzolic Soils Commonly, the first laterite encountered with depth is in the form

of individual nodules within the soil horizons These may reach a maximum with depth below which they decrease without being joined

into masses as in examples given by Nye (1954, 1955a) and Radwanski

and Ollier (1959), or they may pass into masses of nodular or vesicular laterite, as in profiles described by Joachim and Kandiah (1941) and Ollier (1959)

B LA- WITHIN SOIL HORIZONS

Many soils of the tropics have laterite within genetic horizons These may be detrital nodules or fragments from adjacent higher-lying land- scapes containing laterite ( Greene, 1945; Ruhe, 1954), relics of disinte- grating laterite crusts in which soils are forming (Mulcahy, 1960, 1962),

or units developing concurrently with the modem soil (Ollier, 1959) The “murram” used for surfacing roads in India (Prescott and Pendleton, 1952) is mainly nodular laterite and may be in horizons of the modern

LATERITE 23

soil The authors have observed profiles comparable to both Red-Yellow Podzolic Soils and Latosols, both containing nodular laterite in various horizons, in widely separated areas of East and West Africa and in the Philippines The Tifton series, a Red-Yellow Podzolic Soil of south- eastern United States, contains laterite nodules in both Az and B horizons The nodules in the B horizon of Tifton are intact and either forming or stable, whereas those in the A2 are apparently dissolving, leaving quartz grains protruding from the iron-rich matrix Similar conditions in soils

Nodular

Mottled Sandy cloy Loam

3

h r k Sandy Loom

:a\/Cortkm Nodular 1.r - Mottled Red

Though laterite nodules are common in soils that have no obvious high water table ( Raychaudhuri, 1941 ) , such nodules commonly increase progressively downslope on a given land form Figure 2 shows this

relationship at a site near Ibadan, Xigeria where proximity of a zone of saturation to the surface appears to be a controlling factor It is believed that these are developing concurrently with the modem soil and are analogous to the forms described by Nye (1954, 1955a) in Ghana

In the “Ground-Water Laterite” soil described by Kellogg and Davol

24 S SWARAJASISGHASL ET AL

(1949), the laterite is considered to be a genetic horizon of the modern soil In this profile, laterite nodules are present at a depth of 8 inches, increase in numbers with depth, and occupy a major part of a weakly cemented horizon from 23 to 45 inches, which rests on soft massive laterite that hardens upon exposure to air Mohr (1944) and Mohr and van Baren ( 1954 ) have postulated progressive soil development involving ( I ) laterite-free profiles in which an impervious substratum forms, ( 2 ) stages having horizons containing nodular laterite, and ( 3 ) a final stage involving massive laterite

C Homoss OR LAYERS BENEATH LATERITE

Obviously, detrital laterite fragments and nodules come to rest capriciously on whatever material may be present on footslopes of dissection forms, and in such areas of landscape inversion (Bonnault, 1938) the detrital laterite and the underlying material may be unrelated The laterite detritus, however, commonly contributes iron to enrichment

of adjacent layers (Maignien, 1958, 1959) A great variety of unrelated

layers may be found under laterite zones in such positions

Where the laterite zone is apparently residual, however, the literature reveals some measure of consistency of kinds of underlying layers Though Holland (1903) reported that laterite may rest on unaltered bedrock, most descriptions show highly weathered, commonly thick, earthy layers between the laterite zone and bedrock In dry areas or where conditions contribute to good aeration, as on some high-level positions, the underlying material may be high in chroma (bright colored) though commonly variegated in color ( Kellogg and Davol,

1949, p 52) More commonly, especially on low-level positions, the laterite zone is underlain by either a mottled zone, a light-colored layer,

or both, suggestive of poor aeration, reduction of iron, and possible lateral leaching of sesquioxides

U‘alther ( 1915) introduced the terms “mottled zone” and “pallid zone” for comparable parts of profiles of western Australia, which contained

the following layers: (1) ironstone crust, ( 2 ) mottled zone, ( 3 ) pallid

zone, and (4) parent rock

Simpson (1912) described similar sequences on both granite and greenstone schist in the same region Though Maclaren (1906) used different terms in describing a lSfoot section in India, his sequence, hard laterite-soft laterite-reddish buff sandy clay-white grit-de- composed biotite and quartz schist, is very similar Marbut’s (1932, p 74)

idealized description of typical “Ground-Water Laterite” of the Amazon

Valley and Cuba reveals the same layers: (1) soil, ( 2 ) iron-oxide layer,

p 302) described similar layers in idealized profiles on volcanic ash

in which laterite is formidg in Indonesia:

A - Redearth

B3 - Layer of mottled clay differentiable into an upper layer

of Fe203 (incipient laterite) and a lower gibbsitic layer (mottled zone?)

B2 - Spotted white clay (pallid zone)

B1- Layer with siliceous cement

C - Unaltered ash of the basic suite

The numbering of “ B horizons by superscripts upward indicates the hypothesis of development upward above the slowly permeable silica- cemented B1 layer, which supports a perched water table

Mulcahy (1960, 1962) described thick pallid zones in Australia under ancient laterite in high-level positions where mean annual rainfall is now less than 20 inches per year, their thickness decreasing from 60 or

80 feet under 20 inches of rain to about 10 feet under 13 inches Jessop (1960) described pallid zones from 60 to 200 feet thick in the south- eastern part of the Australian arid zone under a silicified cap on plateau remnants and concluded that the pallid zone is a relic of an ancient profile from which the ferruginous material has been stripped From observations in Africa and Australia, Alexander has concluded that some pallid zones are consequences of exclusion of air by overlying laterite; they appear to be actively forming even on high-level positions in relatively dry climates when they lie beneath a crust dense enough to permit little access of air and where the zone is saturated for some period during a rainy season A profile by Kellogg and Davol (1949, p 52) in the Congo suggests that on relatively dry sites a thick (4-foot) layer of unmottled soil, comparable to laterite-free soils of the locality, may lie between a relic laterite crust and a mottled zone

Commonly immediately above unweathered rock is a soft layer that has undergone major chemical change while retaining the structural character of the rock from which it was formed This was called “zersatz”

by Harrassowitz (1926, 1930) This may lie beneath a mottled or pallid zone, as in the “Ground-Water Laterite” of Kellogg and Davol (1949)

26 S Sn‘AEMJASINGHAhl ET AL

may be separated from massive laterite by only a zone of concretionary laterite, as in a profile described by Humbert (1948), or may occur in profiles without laterite

In the more arid regions of western Australia, an additional horizon

of siliceous nature has been recognized by Whitehouse (1940) in profiles

containing laterite Layers of silica-cemented sandy material, known as

‘billy,” or clayey material, known as “porcellanite,” were described beneath laterite and over sedimentary rocks Jessop ( 1960) has described silicified layers within the pallid zones of profiles presumed to have once contained laterite and has concluded that they were formed in the manner postulated by Mohr and van Baren for silica-cemented layers under laterite in volcanic ash Though such materials have been recog- nized in both western and southern Australia, their relationship to laterite, if any, has not been clearly demonstrated They may be the result of accumulation of silica removed from iron-rich layers above in at least some cases

In summary, profiles containing laterite may have, from the surface downward, some combination of layers of the following sequence: ( 1 ) soil with or without nodular laterite in some horizon, ( 2 ) laterite, ( 3 )

“mottled zone,” ( 4 ) “pallid zone,” ( 5 ) silica-cemented layer, ( 6 ) “zer- satz.” Descriptions of profiles with these layers in India have been presented by Satyanarayana and Thomas ( 1961 ) All may be present, and one or all apparently may be related genetically to the laterite None except the laterite itself appears to be essential to development of laterite in all environments, however Accurate appraisal of relationships among such layers is enormously complicated by the common occurrence

of laterite as relics of a former contrasting environment, the variety of geomorphic relationships involved in landscapes on which laterite is currently found, and the fact that the enrichment necessary for laterite

formation can occur in a number of different ways

V Formation of Laterite

The early theories of laterite formation were as varied as the nature

of the material and the conditions under which it is found Lake (1890)

has given an excellent summary of these to 1890 in the appendix to his report of the geology of South Malabar Briefly, one group including Babington (1821), Benza ( 1836), Blanford ( 1859), Buist ( 1860), Clark

(1838), Kelaart (1853), McGee ( 1880), Wingate ( 1852), and others considered it to be a residual product of weathering in place Blanford

(1859), Cole (1836), King and Foote (18641, Newbold (1844, 1846),

Theobald ( 1873), and Wynne ( 1872) recognized detrital, sedimentary,

LATERITE 27

or other origins not dependent upon residual weathering, though several

of these authors recognized residual forms as well Voysey (1833) and Carter ( 1852), among several, proposed volcanic origins with subsequent weathering Issues of the Memoirs of the Geological Survey of India, starting with Volume 1 in 1859, the Madras Journal of Literature and Science during the first half of the nineteenth century, and the Record of the Geological Survey of India, starting with Volume 1 in 1868 are rich

in articles on laterite

The various theories proposed were not without reasons The slaglike character of laterite and its occurrence as horizontal masses over basalt flows of the Deccan plateau led Voysey (1833) to suggest a volcanic origin Oldham ( 1893), possibly influenced by Mallet’s ( 1881) com- parison of Indian laterite to certain ferruginous sedimentary beds of Ulster, postulated a lacustrine origin Thickness as great as 200 feet locally, occurrence as extensive sheets overlying rocks as unlike as basalt and gneiss, high concentration of iron in laterite over rocks low in iron, and similar apparent anomalies led him to reject hypotheses of weathering

in place advanced earlier by Newbold (1844, 1846), Foote (1876), and Lake (1890) Holland (1903) conceded that weathering of sorts is involved but concluded that a simple hypothesis of chemical weathering could not explain the abrupt transition from laterite to weathered rock nor the absence of laterite in regions having warm summers but cold winters He postulated action by an organism capable of separating alumina from silica in silicates but destroyed by low temperatures-a process to be added to “the long list of tropical diseases against which the very rocks are not safe.”

As DHoore (1955) has indicated, it is evident that the chemical composition of laterite can be a consequence of either or both of two processes: ( 1 ) concentration of sesquioxides by removal of silica and bases; or ( 2 ) concentration of sesquioxides by accumulation from outside sources It is not often possible to decide on the basis of chemical analyses alone whether one or the other or both are involved Commonly the nature of the parent material is not known with certainty, as the laterite may be in unidentifiable transported surficial deposits or may occupy positions that could receive iron-bearing water from adjacent areas Even where the laterite is clearly in residuum of rock weathered

in place, resistant index minerals or chemical constituents that can be used to reconstruct absolute changes in composition from rock to such highly weathered material are commonly not present in amounts or forms that would make such calculations unquestionably reliable It is not surprising, therefore, that many conflicting theories have been propounded, even in recent years