Major soil groups of the world ecology, genesis, properties and classification

The nomenclatures rely systematically and simultaneously on the two most commonly used classifications: Soil Taxonomy and World Reference Base.. He wrote the book Mapping of the Soil, a

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About the Book

Soils of the world exhibit considerable diversity both in their features, properties and in their ages and genetic conditions Thorough knowledge of these characteristics is indispensable for the study, utilization and conservation of the natural environment This profusely illustrated book gives an exhaustive account of the principal types of soils of our planet The

“progressive descent of weathering fronts” model, recognized and used by eminent international scientists is the guiding principle of choice to link the observations and to give the reader a synthetic and coherent view of the differentiation of soils In each case, the introductory reminders summarize the physicochemical and mineralogical principles necessary for understanding the text The nomenclatures rely systematically and

simultaneously on the two most commonly used classifications: Soil

Taxonomy and World Reference Base This reference manual is particularly

directed at students of the Bachelor's and Master's degree courses, but is also intended for workers and scientists in this subject area (geologists, pedologists, agronomists, land-use planners, foresters, etc.) as well as for all those concerned with or interested in protection of the environment

Jean-Paul Legros is an agricultural scientist and has a Doctorate in Science

He has spent almost his entire career in the Institut National de la Recherche Agronomique (INRA) at Montpellier (France) He has also taught for a dozen years as an Invited Professor at the École Polytechnique Fédérale de Lausanne He has been President of the Association Française pour l'Étude

du Sol (2009-2010), and President of the Académie des Sciences et Lettres of

Montpellier (2008) He wrote the book Mapping of the Soil, also from the same

publisher

V.A.K Sarma retired in 1994 as Principal Scientist (Pedology) from the

National Bureau of Soil Survey and Land Use Planning, an institute of the Indian Council of Agricultural Research He had earlier worked on the faculties of the Indian Agricultural Institute at New Delhi, Punjab Agricultural University in Ludhiana, and Government Agricultural College, Thiruvananthapuram, India

About the Book

publisher

About the Book

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Major Soil Groups

of the World Ecology, Genesis, Properties and Classification

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Major Soil Groups

of the World Ecology, Genesis, Properties and Classification

Jean-Paul Legros

Directeur de Recherche (h)Institut National de la Recherche Agronomique (INRA)

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Taylor & Francis Group

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‘Rhazes said, in his book The Physics of Auscultation, that with time rock changes to clay

according to the action of the sun and of rain’.

In: The Book of Agriculture by IBN-AL-AWAN, Arab

author who lived in Seville in the 12th century, citing Rhazes, Arab physician of the 10th century [(from the French translation of

J.J Clément-Mullet (1864)].

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Objectives

The objectives of this book are the following:

• to explain the mechanisms of formation of the soils seen on our Planet;

• for this, to present a unifying viewpoint: the weathering-front model This approach is introduced in Chapter 3; it is developed later in the various chapters of the second part, except in the very last that it does not pertain to;

• to extend the idea to our understanding of nature; we profit from the study of certain mechanisms for explaining what every inquisitive naturalist can observe while strolling and travelling; for example, the appearance of karsts, the phenomenon of will-o’-the-wisp, the red tropical landscapes, etc Soil Science is one

of the gateways to understand better and love our Earth

Readership

This book is addressed to students who have soil science courses in their studies, to geographers and geologists, to agronomists and, lastly, to lovers of nature who wish to know it better The essential prerequisites are limited to some elementary concepts in chemistry

Pedagogic aspects

This book provides a foundation When one has read it, the lary and concepts being included, one should find it easy to tackle all scientific publications, even specialized and difficult, covering the same subjects

vocabu-It starts out with discussion of the very mature soils that required millions of years to form and ends with the soils affected by processes that have to be studied on the scale of a month or even a minute Thus there is a progressive slide from pedogenesis to dynamics Descent of weathering fronts explains the general differentiation of soils but does

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not concern present-day dynamics, for example the rapid movements of oxygen or of salts Thus, the unifying view is no longer mentioned in Chapters 12 and 13 But we revisit it in the conclusion because it is central

to the concepts that underpin this book The order of presentation is also chosen to enable the reader to easily acquire, one by one, the ideas the totality of which is necessary for understanding a chapter further along

in the text Beyond that, the chapters are largely independent Where required, cross-references are made

The content is centered on a dozen major soil groups This is not exhaustive but will suffice to explain the main principles We have wanted to avoid a heavy and useless ‘catalogue’

In the matter of classification, the terminology of the World Reference

Base (WRB) that is accepted in a large part of the world serves as our

language But the terms in Soil Taxonomy are also provided To make

easy the decoding of the latter we have systematically split the terms

into the formative elements For example, we write Verm-ust-oll instead

of Vermustoll (Chernozem) However, in this book, typology is simplified

and indicative It gives only the paths for later tackling the manuals devoted to these questions Moreover, if knowing how to name a thing

is relevant, understanding its dynamics and functions is otherwise exciting…

We have distinguished what has been agreed to by all from what

is poorly understood or still debatable Pedology is a living science Its difficulties are not hidden

Point of view and limitations

In this book, all that touches on humus, biology and organic matter has been kept to the minimum They are treated specially when it

is essential to do so, as in the case of Andosols, Podzols and soils saturated with water Our point of view, therefore, is partial but not

biased When an infant takes its first steps, the psychologist says ‘He

has succeeded in learning that one can live outside his mother’s skirts’, the

physician indicates ‘He now has the mechanical means of fighting against

gravity’ and the neurologist adds ‘Coordination of movement is henceforth established’ All of them are right Fortunately, there is no single point of

view for admiring the Earth and its major soils

Unless otherwise mentioned, the soils studied here are found where there have been no disruption such as colluviation, aeolian deposition, lateral slippage, truncation by erosion, etc In other words, we examine the broad mechanisms but not the infinite variety of cases in which they weaken, particularly on slopes

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Typical plan of a chapter

The following sections are always presented in a highly structured manner:

• prerequisites (e.g., concept of amorphous material in the case of Andosols);

• distinctive characteristics of the soil under study;

• ecology and distribution in the world;

• genesis and dynamics, age;

• paths for an agronomic approach

But the sequence of sections varies slightly from one chapter to another according to pedagogic imperatives

K Djili, J-C Favrot, F Feder, P Herrmann, S Marlet, P Quantin,

P Ross, J.P Rossignol, D Schwartz and, very specially, J-P Party who reread the entire book Thanks to the publishers who believed in me as much for the French version (PPUR) as for the English version (Science Publishers) I am particularly thankful to the INRA (Institut National de Recherche Agronomique), where I spent my whole career, for providing the necessary facilities so that the updating of the French version of this book could be carried out in a good scientific environment And many thanks to V.A.K Sarma who translated the text into English with great care not only with regard to style but also to the content, as he

is a soil scientist himself The exchanges with him were very pleasant

I am grateful to my wife, children and grandchildren, who picked up the habit of supporting me whether present or absent, mind elsewhere, roaming the Earth, keeping fingers poised over a computer keyboard… Thanks to my father whom I neglected too much during all these years devoted to synthesis and writing

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Introduction vii-ix

1 Definitions, Concepts and Tools 1

1.2 Methods for Study of Pedogenesis 5

2 Factors of Pedogenesis 25

2.4 Geomorphology and Surface Formations 43

3.5 Organization of Soils at all Scales 105

4 Classifications: International WRB and U.S 115 Soil Taxonomy

4.2 World Reference Base for Soil Resources (WRB) 118

5 Ferralsols and other Soils of the Hot Regions 147

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5.2 Typical Profile and Differentiation 150

5.5 Evolution of Ferrallitic Environments 169

6.1 Typical Profile and Differentiation 180

6.6 Evolution/Degradation of Vertisols 1956.7 Return to the Weathering-front Model 199

7.6 Utilization of Calcareous Soils 232

8 Cambisols, Luvisols and Planosols 238

8.1 Fundamental Mechanisms of Development 238

8.4 Ecology and Duration of Pedogenesis 254

9 Red Soils of the Mediterranean and Dry Tropical 265 Zones

9.1 Soil Sequences in the Rhône Valley 265

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11 Podzolic Soils 323

11.1 Morphology and Properties of Podzolic Soils 323

11.5 Utilization of Podzolic Soils 352

12 Gleysols, Stagnosols and Histosols 363

12.2 Dynamics of Iron in Presence of Excess Water 37012.3 Evolution of Organic Matter when Excess Water 375

is present

13 Saline Soils: Solonchaks and Solonetz 409

13.5 Evolution and Typology of Saline Soils 43013.6 Elements of Land Reclamation 435

14 Chernozems; General Conclusions 448

Index 455

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Definitions, Concepts and Tools

In this chapter we shall review the definitions, concepts and methods

on which the study of soil formation is based

called horizons because of the more or less horizontal arrangement of

their boundaries These horizons are differentiated by their colour and

by their physical, chemical and biological properties The sequence

of characteristic horizons of a given soil is termed soil profile Rather

incorrectly, specialists say they dig a ‘profile’ when they open a soil pit The

rock that has weathered to form the soil is called the parent material.

By definition, a soil profile does not have defined width It therefore

seems practical to define an elementary volume of soil, the pedon

(Simonson and Gardner 1960) It is a theoretical individual because the soil is a continuum In the vertical direction the pedon corresponds to the profile including a small thickness of the upper part of the unweathered rock In the horizontal plane its width is several decimetres or a few metres and is always small enough for the pedon to be considered free

of lateral variability

Nomenclature of horizons

The idea of sequentially naming the horizons of a soil profile by the first letters of the alphabet, written in capitals, comes from German

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authors who worked in the middle of the nineteenth century The system, revived a little later by Dokuchaev (§ 1.1.2) and his students, has endured and has attained the status of a gradually refined international coding system (Table 1.1) It still suffers from lack of universal acceptance

in some details We turn to Tandarich et al (1994) for the C and D

horizons

Table 1.1 Codification of the major horizons.

O L , O F , O H Highly organic horizons of drained forest environments (with L for litter,

F for the fermenting layer of decomposition (fermentation), H for the humifying layer); for the Americans, all these correspond to a ‘folic’ (with leaves) horizon.

H Highly organic horizons of wet environments (cf histic horizons — see

Histosols).

A Surface organo-mineral layer with living organisms and/or traces of

biological activity.

E Almost totally mineral horizon depleted of iron and organic matter or clay,

often bleached (‘E’ for ‘eluvial’).

(B) or S Mineral horizon representing a stage of limited weathering compared to

the parent material of which some features are still seen (rock structure, minerals, etc.); signs of evolution are: development of a specific colour (expression of iron) and/or a structure of pedological, not geological origin (prismatic, blocky, etc.).

B Horizon in which components such as iron, clay, humus, salts or secondary

carbonates are accumulated, but from which primary carbonates have disappeared.

C Bottom horizon of the soil; it is already subject to the effects of atmospheric

agents; it shows cracking related to alternate wetting and drying cycles, and partial or slight oxidation because of penetration of oxygen On the other hand, if not explicitly stated otherwise, it is devoid of the accumulations typically seen in the B.

D Parent material that has been altered but does not have the indications of

meteoric weathering seen in the C horizon; for example, granitic sand, limestone scree, moraines (see ‘regolith’, Chap 2).

R Corresponds to the unweathered and unfragmented hard rock; it is called

‘parent rock’ when it is the source of the soil above it.

A given soil need not have the entire sequence of horizons described above! For example, we find profiles of the AR or A(B)C type

These horizon symbols can be qualified with suffixes that define their nature better For example, Bh where h indicates the presence of organic matter Unfortunately there is no international accord regarding these suffixes If several horizons of the same type follow one another, they are numbered in sequence, for example: A, B1, B2, C If a given

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horizon has features intermediate between two typical horizons, it is identified by the two letters Thus, there may be an AB horizon or even a BA if it is desired to indicate that it resembles a B horizon more than an A A horizon that locally exhibits very marked characteristics

of an A horizon in some places and distinct features of a B horizon in others will be designated A/B If the soil profile is differentiated in two superimposed geological materials, the corresponding discontinuity is brought out by a Roman two (II) preceding the symbols of the horizons pertaining to the second geological material For example, A, IIB, IIC

Limitations of the horizon concept

French scientists of the IRD (formerly ORSTOM), while studying the ancient soils of the intertropical zone, noticed that, in a trench cut horizontally through the soil on a slope, ‘soil volumes’ not always parallel to the surface were found instead of horizons In shape, they resembled lenses that tapered off laterally (Fig 1.1)

Fig 1.1 Example of the lateral organization of the soil seen in section on a slope in Guyana

1.1.2 The Idea of Pedogenesis

Vassili Vassiliyevitch Dokuchaev (1846–1903), a geologist at the University

of St Petersburg, had been involved in the mapping of Russia in Europe

in various situations between 1875 and 1879 (Legros 2006) To do his work properly as a practical man he apparently traversed 10,000 km on foot Dokuchaev thus gained considerable experience and was the first

to assert, with supporting examples, that soils depend on the following five factors of formation:

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of Darwinism in biology.” The contribution of Dokuchaev enabled the

establishment of pedology We will revert to him on several occasions.

We can now present the formation of soil as follows (Fig 1.2):

Process

Mechanism Mechanism Mechanism

Fig 1.2 Mode of differentiation of soils.

For example, rainfall (factor) leads to decarbonation, acidification and transport of clay (mechanisms); the three follow one another or even work together (process) to create Luvisols In this book, we shall therefore proceed to examine the factors of pedogenesis (Chap 2), the mechanisms (Chap 3) and finally the processes and at the same time the corresponding soils (Chaps 5 to 14)

Dokuchaev did not hierarchize the different factors of soil formation

He noticed that a factor predominant on a given scale could become secondary on another scale For example, at continental scale, variation

in climate is essential for explaining the diversity of soils On the other hand, at field scale, variability in soils is mostly related to slope or to the underlying parent material

In the United States, Hans Jenny (b 1899, Basle, Switzerland; d 1992, Auckland, USA) propagated the ideas of Dokuchaev He tried to quantify the weight of the different factors in the equation (Wilding 1994)

soil = f (climate, parent material, biota, relief, time)

and published his major work Factors of Soil Formation in 1941.

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1.2 METHODS FOR STUDY OF PEDOGENESIS

1.2.1 Field Observations

Description of the soils

In science, everything or almost everything proceeds from observation

It is therefore necessary to learn how soils in nature are appropriately described However, observation does not at all enable understanding and interpretation To demonstrate this, we shall take the following theoretical case, similar to situations actually observed in the field in tropical environment (Fig 1.3)

A clayey layer upslope is associated with a sandy layer downslope

At the boundary between them are seen interpenetrations, as shown

in the diagram In the absence of other data, there are three possible interpretations:

• Things have been in this condition since the start of deposition

of the materials, but such immobility is unlikely considering the strength of weathering phenomena in hot, humid climate

• Or, the clay moves downhill invading the sand, which would progressively transform the pure sand to clayey sand or to sandy clay In this hypothesis we ‘see’ appear, in the sand, clayey volumes or lenses, a kind of advance parties of the invasion

Then the colonization is almost complete at the clay front In the

rear, only a few isolated pockets of sand on the way to reduction survive (in the meanings given to ‘pocket’ and ‘reduction’ in battle!)

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• But at this stage of observation, a third hypothesis is compatible with the facts We can think that the layer of sand is the residue from chemical decomposition, or mechanical removal, of the clay The little sand that the clay contains is concentrated in place until it alone survives In support of this new hypothesis,

it is ‘observed’ that clay has already disappeared in the upper

slope from some pockets In the sandy front it has been almost

completely eliminated Here it only survives in some volumes

on the way to reduction

One may, therefore, hesitate to choose between uphill invasion by the sand and downhill invasion by clay In practice, a lot of other information is collected in order to decide between various hypotheses For example, does the clay contain sufficient sand to explain the possible concentration of the latter? Is it the same sand on both sides of the boundary, and so on? It is clear that morphological observation, essential for posing such problems, is not sufficient for solving them

Be that as it may, the above example shows that there are transformation

fronts in many soils Pedological volumes are formed at the expense of

some others This is observed time and again in the tropical environment where the weathering phenomena are pronounced and last long, which makes them at least visible, if not evident In temperate or cold climate,

as we will demonstrate later, things happen in the same way, but are less evident

Soil-environment relations

As emphasized above, uncertainties are part of observations in situ

Some scientists are detached from reality and only consider laboratory experiments conducted on ‘soil extracts’ as supreme The environment, though, indisputably carries important information, particularly on soil-environment relations For example, to see a thousand times Podzols under rhododendrons (Chap 11) demonstrates better than all research station experiments that this vegetation is favourable for podzolization The field naturalist does not have to be ashamed for his empiricism There are instances when observation is by far the best method… It

is still necessary to know how to do it Soil mapping, which requires understanding of the distribution of soils in order to describe it by accounting for it through a pattern, is a good method for learning to see (Legros 2006)

Sometimes, in nature, we find situations in which all the pedogenetic factors are constant but for one that varies For example, under constant

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climate and uniform vegetation we can study the role of relief in differentiation of the soils We shall see in Chapter 2 some situations that allow these monofactorial studies in the field.

1.2.2 Laboratory Investigations

Thin sections

The thin-section technique was established by geologists The method consists of slicing rocks to sections as thin as a few hundredths of a millimetre, at which thickness they are transparent and can be studied under the microscope It was adapted for soils by Henri Lagatu and Auguste Delage of France These scientists introduced a hardener to prevent crumbling of the naturally soft soil From then on a new world became accessible to the soil scientist The work earned the authors a gold medal at the Paris World Fair of 1900

Following this, micromorphology became a branch of the discipline of

pedology It led to description of the aggregates that combined particles

of various sizes (sands, silts, clays) in soils This in turn gave rise to the publication of numerous papers using a new vocabulary, quite esoteric, sad to say In the twenty years between 1970 and 1990, it was fashionable

to learn to talk jargon by using the new language But descriptions in science are, most of the time, of interest only if they can be interpreted

in terms of soil processes Yet, unfortunately, this has not always been the case Thus micromorphology has suffered a sort of disaffection The pendulum has certainly swung too far It is a pity we are today deprived

of an always enriching direct method of study (Fig 1.4)

Examination of sands

The study of the surfaces of sand grains enables identification of polished faces (flowing water), of dulling (wind), of dissolution and corrosion (soil solution) or even precipitated forms In Rwanda, it was shown that the surface of quartz grains was different at the top, in the middle and at the bottom of slopes in relation to the intensity of dissolution in

accordance with topographic position (Marcelino et al 1999).

1.2.3 Chemical Analysis

Consideration of this question is not in our objectives or resources, although analytical data have to be used in presentations and demonstrations Specialized books on the topic are available (Baize 1993)

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Fig 1.4 Example of a thin section sampled from a weathered granite in the Pilat massif

(Loire, France) The section represents 2 cm × 3 cm The matrix is composed of primary erals such as quartz and feldspars (2) and dark micas (3) A crack (1) shows that pedological weathering has begun The walls are lined with clay coatings that have been deposited in

min-successive layers (4) Photo: author.

1.2.4 Mass-balance Analysis

For understanding the genesis of a horizon of a soil, it is often important

to calculate the quantity of material that has been exported, for example

of iron, aluminium or silicon Let us examine the methods (Legros 1982;

Driese et al 2000) taking as example the losses of iron, whereby we

avoid having to introduce an index showing that every element could

be concerned

The notations are chosen in such a way that they can directly be used in elaboration of a computer program in C, for example Let

and da(i) that of the horizon i studied,

from the present volume because of gains or loss of material),

resistant to weathering and MI(i) the quantity of the same mineral in the horizon i studied (in g per 100 g),

horizon i studied, also expressed in g per 100 g.

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The present masses of the index mineral and of iron in the horizon

i studied are given by:

In the same horizon, the initial masses of the index mineral and of

iron before weathering and pedogenesis were

volor(i).da(r).MI(r)/100 (2) volor(i).da(r).Fe(r)/100

Then the loss of iron in i, except that we do not know volor(i), is

The two terms of the numerator could indeed be reversed But the

proposed method has the advantage of indicating the losses by negative

values and gains by positive values

Again, we have

Loss Fe(i) = vol(i).da(i).Fe(i)

What remains now is to determine the value of volor(i) For this,

there are two methods, presented below

Isovolumetric reasoning

In isovolumetric reasoning, we calculate the losses on the assumption that

the soil has not undergone changes in volume (Fig 1.5)

in the same volume

Fig 1.5 Explanation of isovolumetric reasoning.

In this case,

volor(i) = vol(i)

and the losses in horizon i become, by simplification of equation (3):

Loss Fe(i) = _ Fe(i).da(i)

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We could arrive at the same result more quickly! But the presentation given shows more clearly the connection between the two methods proposed.

It is easy to show that the losses are 0 for the parent material or –1

(equivalent to 100%) if Fe(i) = 0.

This method is very useful for the study of the first stages of weathering (R Æ C) It is applied, for example, to the case of a granite that decomposes and loses resistance and material without disintegrating

(Fig 1.4) It is then transformed into an isaltérite (saprolite), so termed

because the original volume is retained

Reasoning using index mineral (MI)

Figure 1.6 gives the principle of the calculation

Reference

horizon

(R or C)

Horizon

of the substance which would normally have had to accompany the resistant index mineral if it was concentrated in the same proportion

volor(i) = vol(i).da(i).MI(i)

Substituting this in equation (3) and simplifying, we get

Loss Fe(i) = Fe(i).MI(r)

For example, if there is as much of the index mineral in i as in r, the

loss or gain of iron is directly deduced from the corresponding contents

of the element If there is twice as much index mineral in i as in r, but

the iron content does not change, the actual loss of iron is 0.5 or 50 per cent

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This quantity is called strain by scientists.

Following an oft-quoted paper by Chadwick et al (1990), numerous

scientists have used this change in volume for calculating the losses mentioned in equation 6 But it is a roundabout means that leads to

a fruitlessly complicated expression, unfortunately reproduced from article to article It is as follows, using our terminology:

Loss Fe(i) = _ Fe(i).da(i)

It is clear that one can arrive at equation (6), which is equivalent and simpler, without too much effort by substituting in the above equation

varvol(i) by its value from equation (7) In fact, the losses calculated on

the basis of an index mineral do not involve density!

In the context of this book and considering developments that we will conduct at many places, these equations are important

Graphic representation

Since the losses are calculated as a proportion of the original masses, then on a scale of 0 to 1 or 0 to 100 per cent, we can give for each soil profile a composite diagram, all components being shown simultaneously (Fig 1.7)

The index mineral method is more commonly used than the isovolumetric method Indeed, different minerals are quite stable in soils

Quartz has long been used (isoquartz reasoning) in the temperate

region Actually it is tricky to use (Legros 1982): (i) its determination is difficult because it is not the only silica-containing mineral in the soil; (ii) part of the quartz, unattacked, is physically displaced; for example, one finds coatings of quartzose silt on coarser particles in acid mountain soils

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= doubling of the initial quantities)

Level of losses or gains

Iron

GAINS LOSSES

Change of volume

Fig 1.7 Example of the representation of losses and gains during weathering, from the

bottom up in a soil profi le, assuming an index mineral as unchanging (Driese et al 2000).

In the tropical zone, at the time scales generally considered, quartz

is soluble The index mineral elements often used are partly titanium,

Ti, abundant in rutile (TiO2) and partly zirconium, Zr, contained in the silicate zircon (Egli and Fitze 2000) But these elements are scarce and

it is difficult to accurately determine the contents of each and more so their ratio between two horizons Furthermore, Ti can migrate in soils

that have undergone intense weathering (Cornu et al 1999) As for Zr,

it is sometimes of aeolian origin (Stiles et al 2003) The choice of the

appropriate index mineral is, therefore, to be carefully reasoned out in each case

1.2.5 Test Minerals

For investigating the current processes in the soil, small porous-plastic

sachets containing test-minerals can be buried in it These are first

weighed, examined under the electron microscope and analysed before being placed in the field in selected soils At the end of the experiment (season, year, etc.), the minerals are recovered and studied again The method enables us to calculate the rates of weathering or dissolution

in a selected soil horizon under a given climate Locating the buried test-minerals months later is very difficult even if the sachets had been placed in a location precisely defined with GPS, measurements taken,

a sketch drawn and photographs taken of the site We must therefore proceed on the basis of the principle that only one-half of the sachets will

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be recovered and plan the original burying of the sachets accordingly Three specific kinds of test mineral have been employed.

Vermiculite (Ranger et al 1992)

Vermiculite, in which the exchange sites have first been saturated by

a single cation, say calcium, is placed in polyamide bags, made of an

non-degradable material manufactured to have a porosity of 20 µm

The vermiculite may be mixed with pure quartz sand to promote free drainage The mouth of the bag is then heat-sealed Once in position, the vermiculite gives information on the processes in the soil It can, according to conditions,

• be destroyed, that is to say, dissolved slowly (pH<2.5),

• be partially attacked (pH 2.5 – 5.0) It releases Si, Fe and Al

As regards aluminium, two outcomes are possible: it is either released and removed in the drainage water or it is slightly displaced and is trapped in the interlayers of the clay mineral during its degradation This case corresponds to a progression toward aluminous vermiculites,

• exchange its calcium with other cations that are retained instead and then determined (pH >5)

Calcite (Delmas et al 1987)

Crystals of calcite (pure calcium carbonate) are crushed and the lustrous splinters obtained are used as test-mineral The surface of the calcite, the initial state of which is checked with the electron microscope, reacts almost instantaneously to the characteristics of the water that bathes it

Kaolinite

When buried in tropical soils, kaolinite weathers rapidly, which points

to its tendency to disappear naturally from these soils (Cornu et al

1995)

1.2.6 Lysimetry

Lysimeters are field equipment that enable recovery of the waters passing through different soil horizons for analysis and study of what substances are transported in solution or in suspension (Fig 1.8)

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Drains forced into the soil

Apparatus for collecting waters

Vegetative cover

Shored up cutting

B A

C

Soil in situ

Fig 1.8 Installation in situ of lysimeters in a pit with cast walls, covered to protect against

frost and to prevent deliberate or inadvertent damage (Dambrine 1985; Keller 1991).

This approach has been very useful in the context of study of podzolic soils (Dambrine 1985; Keller and Vedy 1991)

At the end of several weeks or months of the experiment, the

author recovered the various fractions obtained: resistate (unweathered fraction), eluviate (decomposition and neoformation in place), leachate

(solubilized and exported fraction) This apparatus allows adjustment

of various settings: (i) number of cycles and, thus, the volume of water that has passed through the sample; (ii) oxygenation, because the treated sample can be totally immersed or go above the siphon; (iii) reaction temperature; (iv) pH, which is controlled by introduction of carbon dioxide or various acids through the top opening of the apparatus These experiments, superbly conceived and interpreted, clarified the mechanisms of weathering of rocks on the surface of the Earth

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Introduction of carbon dioxide

Drainage

Accumulation

of leachate Heat source

Atmospheric zone Phreatic zone Evaporation

R E

Fig 1.9 The Soxhlet apparatus used by Pédro (1964).

The limitations of the apparatus are of two kinds, as reported in work by students of Pédro: (i) when the experiments are continued over

a long period, there is risk of green algae developing especially if the work is done at low temperature; it is then advisable to eliminate light and to sterilize the assembly; (ii) silica being soluble, the glass of the Soxhlet could contribute some of it to the extract

1.3 DATING AND TRACING

1.3.1 Generalities

The chemical elements contain definite numbers of protons, electrons and neutrons Two atoms that differ in their number of neutrons may have different atomic masses but retain the same general chemical

properties They have the same atomic number They are isotopes (e.g

14C and 12C) Two atoms that have the same atomic mass but different number of neutrons and protons have electrons in different amounts and therefore have different chemical properties They are different bodies (e.g 40K and 40Ar) They have different atomic numbers

In soil science, isotopes are used in two ways as discussed below

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• Firstly, if they are unstable and disappear over time (radioisotopes), they can serve as clocks under certain conditions for dating Let us recall that alpha radiation corresponds to loss of two protons and two neutrons or to the emission of one nucleus of helium 4He2+; the atomic mass is thereby reduced by 4 In beta-radioactivity, the nucleus will lose or gain an electron without change in mass, but the atomic number changes and therefore the properties

In gamma-radioactivity there is emission of electromagnetic energy corresponding to a lowering of the excitation level of the nucleus

• Secondly, some substances comprise a fixed proportion

of different stable isotopes Thus C3 plants and C4 plants (photosynthesis types) absorb 13C and 12C in different proportions This constitutes a sort of signature that can be used for ‘isotopic tracing’ For example, we can determine in what proportion the organic matter of the soil found under maize comes from the

decomposition of roots of that crop (Balesdent et al 1987).

1.3.2 Isotopic Dating

Dating with 14C was invented before 1950 by the American physicist Willard Frank Libby and his team at the University of Chicago Libby received the Nobel Prize in Chemistry in 1960 for this work It is not necessary to develop the principle of the method here because it is easily found on the Internet As the rate of decay of 14C corresponds to a half-life of 5734 years, we can get valid periods between a few centuries and about 45,000 years Of course, the sample should contain carbon, which practically limits the use of the method to organic materials Theoretically, carbonates are affected but as the proportion is presumed identical in the material and in the atmosphere, substances resulting from physicochemical precipitation are excluded

The combined use of another dating method (tree rings, see below) has shown, from 1958 on, that the 14C method is associated with a systematic bias: the 14C age is less than the actual age and the difference varies with time It can go as high as 17 per cent The error is linked

to the variation in cosmic rays responsible for the transformation of irradiated nitrogen in the upper atmosphere to 14C It is therefore necessary to introduce a correction obtained from a calibration curve.The analytical technique has long depended on the measurement of radioactivity But although many atoms of 14C are present in the sample,

a few of them decay during the measurement time in the laboratory To

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compensate for this it used to be necessary to count the disintegrations

on a large quantity of material (not always easy to obtain) and for a

long time The situation is different now Nowadays, accelerator mass

spectrometry (AMS) is used and all the radioactive atoms are directly

counted, whether they are decaying or not This allows us to work with a quantity of carbon much smaller than about a milligram The method has become usable for fractions of objects It has ceased being destructive

The case of carbon presented above enables us to understand that every decaying radioactive element is potentially a means of measuring time The conditions are: (i) to make a clock usable in pedology, the decay should not be too slow (billions of years); for example, the potassium-argon method (40K/40Ar) that has a period of 12.9 billion years

is not useful for us; (ii) the elements considered have to be sufficiently abundant in the soil; this is the case of uranium and thorium, the content of which in some granitic soils may be as high as a few tens

of milligrams per kilogram (Evans et al 1997); (iii) the stable element

studied should originate solely from the reaction of decay and not anything else For example, 40K gives 40Ar or 40Ca, but it is only 40Ar that is useful for dating

The example of carbon isotopes 13 C and 12 C

Let us consider an example to illustrate the problem If a forest is cleared for cropping maize or sugarcane, we are replacing C3 plants (with photosynthesis driven by carbon compounds with three carbon atoms) with C4 plants (with photosynthesis that makes compounds with four carbon atoms) Each of these two types of photosynthesis corresponds

to a definite proportion of the carbon isotope 13C To characterize this, it

is customary to calculate the quantity d13C (van Noordwijk et al 1997):

d13C = 1000 × [

13C/12C of sample 13C/12C of standard – 1 ]

The reference standard is the Cretaceous carbonate of the mollusc

Bellemnitella americana from the Pee Dee formation of South Carolina, also

termed Pee Dee Bellemnitella or even PDB.

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Plants with C3 photosynthesis have a d13C of –35 to –20 whereas those with C4 photosynthesis have –17 to –9 Thus we can distinguish the residues from these two types of plants on the basis of their isotope contents and also calculate the proportion of plants of the two types that have given an organic mixture with known 13C content Figure 1.10 gives an example of the application of this method.

Time in years since cropping started

80 years

Fig 1.10 Variation over time of the different types of organic matter, identifi ed by their

proportion of 13C in a soil cleared at time 0 then cultivated to sugarcane (from Scholes et al 1997; van Noordwijk et al 1997; diagrammatic).

At the end of 80 years, the stable carbon deposited under forest had not sensibly reduced On the other hand, the carbon with rapid turnover, also formed under forest, disappeared in about forty years and was replaced by carbon of the same type but characteristic of C4 plants, here sugarcane

The above example has allowed treatment of a problem of time in

a manner similar to what was done in the case of the isotopic dating method But the information on the dates here are not given by the transformation of isotopes It is linked to the knowledge of the history

of the land parcels The two approaches must not be confused But these are composite investigations: the simultaneous use of 12C, 13C and 14C enables calculation of times and identification of the type of organic matter

Other isotopic ratios used

A special issue of the journal Geoderma (vol 82, nos 1–3, 1998) is devoted

to these isotopic methods and their use in soil science The principal examples are given below (Table 1.2)

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Table 1.2 Isotopic tracing methods used in soil science.

Method Examples of application

13 C/ 12 C Dynamics of compounds in organic matter; study of neogenesis of

CaCO3

15 N/ 14 N Origin and fate of nitrogen in soils

Deuterium/H Conditions of clay mineral formation; quantification of evaporation and 18 O/ 16 O and transpiration

44 Ca/ 40 Ca Cycle of basic cations in soils and plants

66 Zn/ 64 Zn Dynamics of Zn; pedogenesis

87 Sr/ 86 Sr Study of the dynamics of the soil/plant/atmosphere system

By way of example, let us examine the third row of the table Clay minerals contain different isotopes of oxygen and of hydrogen Various factors are liable to alter the contents of these isotopes (Savin and Hsieh 1998) It is therefore difficult to interpret the results In the present state

of knowledge, the 18O and deuterium contents of a kaolinite seem to increase with temperature, which could give indications of the climate (cold, temperate or tropical) that reigned in the epoch during which the mineral was formed

1.3.4 Other Dating Methods

Dendrochronology

Tree rings enable counting the years of growth, provided the base of the trunk is considered (for at the top of the tree, the year-old shoot, when cut, will show only one growth ring!) The rings are thicker when the climatic conditions are favourable From one tree to another, it is possible

to find out characteristic growth sequences corresponding to a given group of years, for example, ‘p–p–p–V–p-V-V-V–p’ where p indicates poor growth and V, vigorous growth In USA, using trunks preserved

in a peat bog, researchers were able to splice such ‘barcodes’ from one tree to another and reconstruct little by little a complete sequence for nearly 9000 years Carbon dating of different rings gives the limits of this isotopic method (see above)

Thermoluminescence

Crystals in nature are irradiated by radioactive elements (238U, 235U, 232Th,

40K, etc.) that emit alpha-, beta- and gamma-rays To this irradiation by natural radioactivity is added that caused by cosmic rays Some atoms

of the crystals impacted by such rays are found to be ionized, whereby electrons are liberated to be trapped in defects in the mineral structure

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The irradiation being presumed to be constant, the number of charges accumulated is proportional to time If such a crystal is picked up and heated, the electrons in unstable state are mobilized and ejected They actually combine with positive ions, giving rise to photons and thus to light The amount of light emitted can be measured It is proportional

to the number of electrons liberated, therefore proportional to the charges captured, thus proportional to the time that has elapsed since the epoch of the preceding heating This last heating could be related

to the deposition of the crystal in a basaltic flow, to rise in temperature

of the soil under the influence of fire, or even at the firing of a pottery

if this object is concerned We can thus date such events If the crystal has never been emptied of free electrons, its electron traps could be saturated so fully that the time cannot be measured The method is in demand in archaeology between 50,000 and 200,000 years BP, that is, in

a time interval too long to be studied with 14C and too short to apply the potassium/argon method Its precision in calculated age is as high

as 5 to 7 per cent

Palaeomagnetism

The Earth’s magnetic field changes with time, according to transformations that affect the Earth’s core and probably to certain cosmographic data parameters The changes are also correlated with variation in 14C indicated above

Various iron-bearing minerals susceptible to magnetization are formed or are deposited in this magnetic field that changes with time

in intensity as well as in direction (azimuth, declination) These minerals mark the field by aligning their dipoles with it Some environments (marine and lacustrine sediments) permit us to follow magnetization over long periods and, thus, changes in this field over time by identifying the characteristic reversals or alterations This makes it possible for us

to reconstruct reference chronological sequences that can then serve in dating specific phenomena, for example, the deposition of a volcanic flow

Therefore it is necessary to measure the magnetic susceptibility of the soil at a given depth For doing this, a feeble alternating current is

passed through a coil wound around a sample of soil (de Jong et al 2000)

The magnetic susceptibility of soils is chiefly related to the presence of maghemite (g-Fe2O3) and magnetite (Fe3O4) The susceptibility depends

on that of the parent material and on the type of pedogenesis that will concentrate or destroy in the profile the iron-bearing minerals originally present (Shenggao 2000)

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Actually, much progress remains to be made for obtaining reliable scales, concordant between continents It is also necessary to understand better certain spectacular variations in the magnetic field.

Using these palaeomagnetic techniques in the tropical zone, a rate

of weathering of the base of soils of nearly 11.3 mm per thousand years has been demonstrated (Théveniaut and Freyssinet 1999) Thus in 100,000 years, a metre of rock can be weathered This is only an order

of magnitude, but apparently is typical

Rates of export

This rarely used method is replete with lessons to be learned It consists

of calculating the time that was necessary to give rise to a given soil by studying the export of one constituent Let us consider an example that could be found in tropical conditions

A 50-cm deep soil on a limestone with 10 per cent decalcification residue is studied The specific gravity of the limestone is 2.6 and that of the soil 1.3 Thus a column of this soil with cross-section 1

cm2 weighs 50 × 1.3 = 65 g It had been contained in a rock column

65 × 10 = 650 g, representing 650/2.6 = 250 cm (it should be noted that

the soil is produced by dissolution in situ of 250 cm of the rock and not

of 5 metres as could be presumed if we forget the difference in density This column of rock contains 650 – 65 = 585 g of calcium carbonate Knowing that the current annual rainfall at the site is 200 cm and annual evapotranspiration 100 cm, in a year 100 cm3 of rainwater is left to ensure the evacuation of the calcium carbonate (all calculations are done for a 1-cm2 area of soil) Knowing the mean annual temperature at the site is 20 °C, the solubility of calcium carbonate in rainwater saturated with carbon dioxide at atmospheric pressure is then 10–3.4 moles of Ca++per litre or 0.02 g CaCO3 per litre, which represents 0.002 g evacuated annually by the 100 cm3 of water available Thus 585/0.002 or 282,500 years will be required In short, the soil could be between 200,000 and 400,000 years old

These calculations are very approximate They do not take into consideration climatic changes that possibly affected rainfall as well

as temperature They assume that there had been no erosion, no additions and no lateral circulation of water They also assume that the water passing through the soil has enough time to get saturated with carbonates, which is not always the case (Egli and Fitze 2001) They do not consider the ‘cation pump’ represented by plants Also ignored are the daily and seasonal variations of temperature, humidity and soil CO2

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But the result obtained, although vitiated by error, suffices to show that a long time is needed for a soil to be formed, even if shallow Furthermore,

we know that little time is needed to remove the soil by erosion The soil is a non-renewable resource!

Calculations of age based on rates of export have also been done with iron and aluminium (Tardy 1969; Gac 1979; Legros 1982; Lucas 1989) These elements are very slightly soluble and thus permit estimation of the great ages that concern tropical soils and the degraded old soils of temperate regions A review of studies has been published (Bourrié and Lelong 1994)

CONCLUSION

Scientists are thus equipped with methods for characterizing their soils morphologically and chemically (we have said that biology is outside our field of discussion) They have also tried to calculate the age of soils and have estimated the substances the soils had lost over time They have also reconstructed geochemical transformations in the laboratory

by using experimental approaches In short, they are equipped with the tools for attempting to understand long-term changes that form the subject matter of this book

REFERENCES

Baize D, 1993 Soil Science Analyses A Guide to Current Use J Wiley,

Chichester, 192 pp

a tracer for soil organic matter dynamics studies Soil Biol Biochem

Boulaine J, 1984 L’héritage de Vassilli Vassilievitch Dokoutchaev Science

du Sol, 2: 93-104.

versant In: Pédologie, 2, Constituants et Propriétés du Sol Second edition,

Masson, pp 240-273

Chadwick O, G Brimhall and D Hendricks, 1990 From a black box to

a grey box: a mass balance interpretation of pedogenesis In: PLK

Knuepfer and L McFadden (eds.) Soils and Landscape Evolution Elsevier,

Trang 38

Cornu S, Y Lucas, E Lebon, JP Ambrosi, F Luizão, J Rouiller, M Bonnay and

C Neal, 1999 Evidence of titanium mobility in soil profiles, Manaus,

central Amazonia Geoderma, 91: 281-295.

Dambrine E, 1985 Contribution à l’Étude de la Répartition et du Fonctionnement

Biogéochimique des Sols de Haute Montagne, Massif des Aiguilles Rouges et

du Mont Blanc Thèse de Spécialité, Université Paris VII, 265 pp.

Delmas AB, J Berrier and H Chamayou, 1987 Les figures de corrosion de la calcite Typologie et séquences évolutives In: N Fedoroff, LM Bresson

and MA Courty (eds.) Micromorphologie des Sols Ouvrage AFES, pp

303-308

Demolon A, 1949 La Génétique des Sols Que sais-je?, PUF, 133 pp.

Driese SG, CI Mora, CA Stiles, RM Joeckel and LC Nordt, 2000 Mass-balance reconstruction of a modern Vertisol: implications for interpreting the

geochemistry and burial alteration of paleo-Vertisols Geoderma, 95:

179-204

Egli M and P Fitze, 2000 Formulation of pedologic mass balance based on

immobile elements: a revision Soil Science, 165: 437-444.

Egli M and P Fitze, 2001 Quantitative aspects of carbonate leaching of soils

with differing ages and climates Catena, 46: 35-62.

Evans CV, LS Morton and G Harbottle, 1997 Pedologic assessment of

radionucleide distributions: use of a radio-pedologic index Soil Sci

Soc Am J., 61: 1440-1449.

Gac Y, 1979 Géochimie du Bassin du Lac Tchad Bilan de l’Altération, de l’Érosion

et de la Sédimentation, Thèse, Université L Pasteur Strasbourg, 249 pp.

Jong (de) E, DJ Pennock and PA Nestor, 2000 Magnetic susceptibility of

soils in different slope positions in Saskatchewan, Canada Catena,

Keller C, 1991 Étude du Cycle Biogéochimique du Cuivre et du Cadmium dans

Deux Écosystèmes Forestiers Thèse de doctorat no 916, EPFL, Génie

Rural, Lausanne, 170 pp + app

Keller C and J Vedy, 1991 Apport de la lysimétrie sans tension pour l’étude des transferts de Cu et Cd dans les sols forestiers faiblement pollués

Science du Sol, 29 (2): 107-124.

Labeyrie J, 1976 La datation par le carbone 14 La Recherche, no 73, pp

1036-1045

Legros JP, 1982 L’évolution de la Granulométrie au Cours de la Pédogenèse

Approche par Simulation sur Ordinateur Application aux Sols Acides sur Matériaux Cristallins en Zone Tempérée Thèse d’État Université des

Sciences et Techniques du Languedoc, 436 pp

Legros JP, 2006 Mapping of the Soil Oxford & IBH Publishing, 405 pp Lucas Y, 1989 Systèmes Pédologiques en Amazonie Brésilienne Equilibres,

Déséquilibres et Transformations Thèses, Université de Poitiers, no 211,

157 pp

Trang 39

Marcelino V, G Mussche and G Stoops, 1999 Surface morphology of quartz grains from tropical soils and its significance for assessing soil weath-

ering European Journal of Soil Science, 50 (1): 1-8.

Noordwijk M (Van), C Cerri, PL Woomer, K Nugroho and M Bernoux, 1997

Soil carbon dynamics in the humid tropical forest zone Geoderma, 79:

187-225

Pédro G, 1964 Contribution à l’étude expérimentale de l’altération

géochimique des roches cristallines Ann Agro 15, nos 2, 3 and 4,

Shenggao L, 2000 Lithological factors affecting magnetic susceptibility of

subtropical soils, Zhejiang Province, China Catena, 40 (4): 359-373.

Simonson R and DR Gardner, 1960 Concepts and function of the pedon

Trans 7 Int Cong Soil Sci., Madison, 4: 127-131.

Stiles CA, CI Mora and SG Driese, 2003 Pedogenic processes and domain boundaries in a Vertisol climosequence: evidence from titanium and

zirconium distribution and morphology Geoderma, 116: 279-299

Tandarich JP, RG Darmody and LR Follmer, 1994 The pedo-weathering profile: a paradigm for whole-regolith pedology from the glaciated

midcontinental United States of America In: Whole Regolith Pedology

SSSA Special Publ 34: 97-117.

Tardy Y, 1969 Géochimie des Altérations Etude des Arènes et des Eaux de Quelques

Massifs Cristallins d’Europe et d’Afrique Thèse d’Etat Mémoires du

Service de la carte Géologique d’Alsace et de Lorraine, no 31, 199 pp.Théveniaut H and Ph Freyssinet, 1999 Paleomagnetism applied to lat-eritic profiles to assess saprolite and duricrust formation processes:

the example of Mont Baduel profile (French Guyana) Paleogeography,

Palaeoclimatology, Palaeoecology, 148 (4): 209-231.

Veillon L, 1990 Sols Ferrallitiques et Podzols en Guyane Septentrionale Relations

entre Systèmes de Transformations Pédologiques et Évolution Historique d’un Milieu Tropical Humide et Forestier Thèse Université Paris VI, 194

pp + app

Wilding LP, 1994 Factors of soil formation: Contributions to pedology In:

R Amundson, J Harden and M Singer (eds.) Factors of Soil Formation:

a Fiftieth Anniversary Retrospective Soil Sci Soc Am Special Publ No

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We shall follow the same plan for each factor Firstly, we shall make observations showing what acts on the soil Then we shall attempt

to understand its mode of action without going into details of the mechanisms Lastly, we will examine the range of consequences in regard to the types of soils and their geography

2.1 CLIMATE

2.1.1 Observations

Dokuchaev, wanting to work objectively, demarcated the soils in Russia

with equal humus content—the isohumic soils—and published his map

in 1883 The master of pedology established that the soils richest in humus, the black earths, the famous Chernozems (Chap 14), extended east–west Dokuchaev knew how to generalize and realized that the great groups of soils were organized by latitude, on the scale of Greater Russia, according to the types of vegetation: soils of the tundras, of coniferous forests, of broad-leaved forests, of steppes and of deserts Obviously the soils depended on the climate Dokuchaev then thought

that the same zonality had to be found in the mountains, because the

climate there varied from the bottom to the top He verified this roughly

by organizing expeditions in the mountain ranges (Caucasus, Altai, etc.)

Tiêu đề	Major soil groups of the world
Tác giả	Jean-Paul Legros
Trường học	Taylor & Francis Group
Chuyên ngành	Soil Science
Thể loại	Sách
Thành phố	Boca Raton

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