Soil Nature Fertility Conservation and Mangement

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Soils: Nature, Fertility

Conservation and

Management

Ezekiel A. AkinrindeAgronomy Department, University of Ibadan, Ibadan, Nigeria

AMS Publishing, Inc. 2004

Tel: +00921 231 13333, Fax: +00921 231 13334

Vienna, P. O. Box 1123, Austria


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Copyright 2004 Lulu, Inc.

All rights reserved.

First published in July 2004

Second impression, May 2006

No part of this publication may be produced or transmitted in any form by any means, electronic or

mechanical, including photocopying, recording, or any information storage and retrieval system, without

permission in writing.

Address requests for permission to reproduce materials from the book or for further information to:Akinrinde E.A., Agronomy Department, University of Ibadan, Ibadan, Nigeria

Comments and observations can also be directed to the editors:

Prof. Victor Chude,National Programme for Food Security, PCU Headquarters, Federal Ministry of Agriculture and Water

Resources,Near VIO Office MABUSHI District, Abuja, Nigeria.

&

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

AMS Publishing, Inc., 2004


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iii

TABLE OF CONTENTS

Chapter / Contents Pages

Preface ------------------------------------------------------------------------------------------------------------- ivIntroduction --------------------------------------------------------------------------------------------------------1

1. Rocks and their Weathering------------------------------------------------------------------------------3Types of rock and their minerals --------------------------------------------------------------3

Rock weathering ---------------------------------------------------------------------------------8

2. Soil Composition and Formation - ---------------------------------------------------------------------11Soil Components -------------------------------------------------------------------------------11

Factors influencing soil formation ----------------------------------------------------------- 19

3. Soil Profile and Properties------------- ------------------------------------------------------------------21Soil Profile Study -------------------------------------------------------------------------------21

Soil Properties -----------------------------------------------------------------------------------27

4. Soil Fertility Conservation and Management----------------------------------------------------------39Introduction -------------------------------------------------------------------------------------39

Chemical dynamics of mineral soils ---------------------------------------------------------44

Measures of soil chemical dynamics ---------------------------------------------------------51

General principles of soil management ------------------------------------------------------55

Soil erosion, desertation problems and Control -------------------------------------------- 60

5.

Soil Biology and Fertility---------------------------------------------------------------------------------67Soil Biology ------------------------------------------------------------------------------------ 67

Soil Fertility ------------------------------------------------------------------------------------ 72

Fertilizers --------------------------------------------------------------------------------------- 77

6. Soil-Water-Plant Relations-------------------------------------------------------------------------------- 83Water use by crop plants ----------------------------------------------------------------------- 83

Irrigation and Management of Irrigated Soils ------------------------------------------------87

Principles of land Drainage --------------------------------------------------------------------- 93

References ------------------------------------------------------------------------------------------------------------ 97

Appendix ------------------------------------------------------------------------------------------------------------- 101

Subject Index ---------------------------------------------------------------------------------------------------------113


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iv

PREFACE

The soil is a very crucial factor in food production. Its impact can result to food crises. The most important

problem of tropical agriculture is the inability of the land to sustain annual food crop for more than a few

years at a time. Soil science as a discipline is represented by the sub divisions of soil physics, soil

chemistry, soil mineralogy, soil microbiology, soil fertility, soil genesis, soil morphology, classification and

survey, soil technology and soil conservation. These sub divisions generally aim at providing the basis and

idea of maintaining or improving the productivity of farmlands. Recognizing this situation, agricultural

establishments (State and Federal Ministries of Agriculture, Agricultural Research Stations and Colleges or

Schools/ Departments of Agriculture) are putting increased emphasis on the research into and the teaching

of Soil Science. It is an obvious fact that a potential agriculturist should be well educated on the basic

principles of soil science.

For quite a long time, the need for a comprehensive but concise introductory textbook on soil science for

undergraduates has been felt. This book is intended to provide basic but yet thorough introduction to the

study of soil science that is involved in the new course content provided under the minimum standard

created for universities in Nigeria. It is based on each topic of the course content with some additions to

suit the undergraduates and graduate students in the university system.

The topics have been subjected to daily classroom teaching. As such students difficulties have been taken

into consideration. Indeed, efforts have been made to treat some interesting topics which most students

seem to find difficult in such a way that learners can follow up without either Teachers guidance or

reference to other textbooks. In most cases, graduate students making use of this book will need to make

very few references to other advanced textbooks in order to have thorough conception of the discipline. The

author wishes to put on record his gratitude to Messrs Akinpelu, Iyiola and Gbadamosi for their respective

assistance in typing the original manuscript, and encouraging the printing of the book. Finally, it is a

pleasure to express our gratitude to God Almighty with whose support the efforts have been successful.

Ezekiel A. Akinrinde (July 2004).


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The King of kings


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INTRODUCTION

Agricultural development is crucial to the survival of mankind in as much as the provision of foodshelter and clothing is closely associated with it. Food, in particular, is necessary for growth, energy

production for good health and normal; development of the populace.All living things (Plants and animals) depend on their environment for survival to remain alive, thrive and

reproduce their kinds As could be expected, nearly all green plants including our farm crops (having their

roots fixed in the soil) depend on the fertile and productive soils that provide anchorage and conduciveenvironment on one hand and supply all the essential materials which they need for their growth. Since

animals, in turn, depend on plants, it becomes obvious that all agricultural activities directly or indirectly

depend on the soil. It is from the soil that plants obtain their food (called nutrients) and water. It also

contains air needed for respiration of the roots. Plants are able to stand upright because their roots arefirmly held by the soil. Certain organisms that may affect the growth of the plant are also found in the soil.

Thus, soil is far from being a simple substance. It is a mixture of several things mineral matter, humus,water, air, animals and unicellular plants including bacteria. Physically, the soil is a mixture of mineralparticles of varying sizes coarse and fine. It can also be taken as a natural body on the surface of the

earth, which supports the growth of plants.

In present day Agriculture, considerable emphasis is given to the inorganic nutrition of the plant in somecases with seeming disregard for massive role of carbon dioxide and light. Keeping the latter two factors in

perspective, however, it is appropriate to discuss mineral nutrition. The mineral elements are criticaindeed, and facet of the environment is one readily changed by the agriculturist through soil management

and fertilizer application practices.

There is no doubt, the need for a more intensive crop production to feed the ever-increasing humanpopulation. As such, yields of genetically improved crop varieties should be further enhanced by optimum

plan nutrition the process by which living organisms obtain their food materials from their environmentA soil may be regarded as fertile when it supplies adequate plant nutrients. Absence of any one of the

ESSENTIAL NUTRIENTS acts as a limiting factor and thus affects normal growth of the plant. The plantshave the ability of assimilating large amounts of certain elements out of proportion to their abundance in

the soil. Plants usually take in simple materials and build them into more complicated substances, which

can be used as human/animal food. Such materials are H2O, CO2 and mineral salts (e.g. NO3, SO4 andPO4). From these they build up carbohydrates, oil and protein. The process of building up of chemical

substance from simpler substances is known as SYNTHESIS.

The two basic criteria for establishing the essentiality of an element are:(i) If the plant (when grown in a medium devoid of that element) fails to grow and to complete its

life cycle, whereas in the presence of a suitable concentration of that element it grows and

reproduces normally. In this wise, an indirect or secondary beneficial effects on some otherelements, do not qualify an element as essential.

(ii) If the element is shown to be a constituent of a molecule which is known as an elementmetabolite.

It is important to keep in mind that the quantities of nutrients taken from the more readily available supplyin the soil. Furthermore, the quantities removed by a single crop may seem rather small in some instances

but when the quantities contained in all the crops of a rotation are summed or when the amounts removed

by crops for several years are considered, the necessity of supplying plant nutrients in the form of fertilizersand manures to maintain soil fertility is apparent.

Before a farmer applies fertilizer to his farm for replenishing of nutrients, he has to know the deficient

elements and at which quantity should it be used to produce optimum yield because


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SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

2

different quantities for its optimum production. Appropriate fertilizer types should be applied to the soil so

as to avoid chemical imbalance because non-availability of others to the plants while it is possible for the

same thing to happen if one element is in excess in the soil. Hence, soil fertility evaluation is like drawingup a nutrient balance sheet of crop soil relationship in effort to produce at optimum level and yet maintain

the integrity of the soil for many years.

Since human survival depends so much on productive and fertile soil, preservation and conservation

methods must be ensured to avoid soil mineral losses through various degradation processes. Soils shouldnot be over used and they should be kept at an optimum productivity level if supply of food and fibre forthe ever-increasing human population will be maintained. Furthermore, high yields are necessary for

farming to be economic and to raise world food production. For these reasons, it is highly essential and

desirable for agriculturists to be knowledgeable in SOIL SCIENCE the study of soil physical, chemical

and biological properties. Indeed, the study of agriculture logically begins with the study of the soil andproper understanding of soil leads to its wise management.

The study of the soil as a science involves the knowledge of the more basic sciences (geology, chemistry,

physics and biology). Hence, soil science is the application of the science of the theoretical basic sciences.The focus in the first section of this book is on the following: Soil components, Types of rock and minerals,

Soil formation and weathering of rocks, Factors influencing soil formation, and Properties of soil (type,

texture, structure, aeration, temperature, pH). Subsequently, attention is given to Nutrient cycling andMaintenance of soil fertility.


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CHAPTER 1

ROCKS AND THEIR WEATHERING

Types of Rocks and their Minerals

The knowledge of rocks that form the earths crust as the soil parent materials is essential to thestudy of soil formation. Such a body of knowledge is termed GEOLOGY. Similarly of great importance in

soil formation is the knowledge of landscape forming processes that have resulted in the relief and the

formation of secondary deposits, which are also parent materials. The study of the landscape formation isGEOMORPHOLOGY (Physical geology).

A rock may be described as an igneous or stratified mineral constituent making up the earths crust. It is the

base on which the sub soil and the soil parent material immediately lies. The classification of rock

involves the placing of the rock in the right category according to their origins (the ways in which theywere formed), colours, texture, shapes of crystals, hardness, reaction, to HCl, presence of fossils, presence

of metals and concentration of sand and clay particles. In this way tow major types of rock have beenidentified.

1. Original or Primary Rock.This is the rock from which the others are ultimately formed. It is otherwise known as IGNEOUSROCK. This name was originally formed from Latin word Ignis which means, fire. The rock is

formed by heat from molten magma. When it cools down it hardens. This means that rocks are derivedfrom an original molten material or magma transferred from the lower regions f the earths crust to

layers near the surface. The rock can either be intrusive i.e. formed in situ in the earth or pushed up

even to the surface (in the case of Extrusive Igneous rocks). As the cooling occurs, crystals combine toform the rock. Differences in this type of rock are due to the method and the speed of the cooling

process. The slower the cooling of the magma, the larger would be the crystals since slow rate ocooling permits growth before the rock become hard. If, however, sudden cooling occurs, it gives rise

to very minute crystals of individual minerals. They may be minute making it difficult to be seen withthe naked eye and such rocks appear to be uniform and without individual mineral crystals except when

viewed through a microscope. Examples of Igneous rock include granites, diorites, basalts and gabbros.

2. Secondary rockThere are two categories of this type of rocks. When the original rocks are exposed at the surface, they

can be weathered and eroded and the detached materials transported and later deposited as sedimentwith the aid of wind or water. Rocks derived from such sediments are known as SEDIMENTARY

rocks. The way in which such rocks are built up layer gives rise to the characteristics stratified nature.

Examples of rocks so formed by the consolidation of sediments that are accumulated by wind or waterat the surface level are sandstones, shale, limestone and conglomerate.Occasionally, previously existing rocks, (igneous or sedimentary) can be greatly affected and changed

by heat and pressure to form the second category of secondary known as METAMORPHIC rocks

Examples are Gneiss, Slate, Marble, Quartzite and graphite.

The Inorganic Framework of Rocks

Rocks differ in their mineral contents. They also vary in the size, arrangement and chemicalcomposition f the constituent minerals. A mineral is naturally occurring substance having a fairly

uniform chemical composition and a regular well defined crystalline structure, though a particular


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mineral can vary slightly in its exact chemical composition as a result of the substitution of one element

for another in the crystal structure.

In most cases, the minerals are silicates combination of silicon and oxygen with other elements.

Knowledge of the structure of a mineral helps in comprehending how easily it can weather and what

elements it is likely to release. The basic structural unit is, however, very simple silica tetrahedron or

pyramid (a four sided unit) in which one relatively small silicon atom at the centre is linked (bybonding) to four much larger oxygen atoms that surrounds it and which form the four corners of theregular tetrahedron.

The silicate minerals can thus be classified on the basis of the way the fundamental tetrahedron units

have linked up to form the mineral. The four different types are:

A. Nesosilicates: Silicate minerals in which the silica tetrahedron remains from each other with noshared oxygen atoms, but is linked by intermediate cations. This is the reason why the name was

coined from the Latin word nesos, which means, Island. Thus in the olivine group of minerals,the silica tetrahedral are linked by divalent magnesium (Mg

2+) and iron (Fe

2+) ions. Olivine (termed

ferro magnesium silicate mineral) is easily weathered since the ferrous iron and magnesium

cations are exposed at the edge of the crystals and can oxidized or hydrated to cause thedisintegration of the mineral. Some other neso-silicates that contain other cations can be more

resistant. It can be concluded, therefore, that Olivine is an example of a group of minerals that is

rich in iron ad magnesium and weathers relatively easily and releases magnesium and iron. It is

usually dark in colour ad may include minerals of other structural.B. Inosilicates: Coined from the Latin word inos meaning fibre, these silicate minerals have their

silica tetrahedral joined to form chains. Those that occur in single chains are members of the

pyroxene family of minerals while those in double chains are the amphiboles. Both types have Ca2+

and Mg

2+cations as the link of the chains. With hornblende as the most important member of the

group, pyroxenes and amphiboles are, thus, calcium silicates. Due to isomorphism, however, other

cations like Fe2+

, Mn2+

or Na+

can exist in the crystalline structure to give rise to different minerals

within the family. They are known to be dark colored minerals that weather relatively easily andexpectedly release large amounts of Ca and Mg to the soil.

C. Phyllosilicates: These include both the common rock forming minerals (the micas) and thesilicate clay minerals. The silica tetrahedra share three of their oxygen atoms to form flat sheets of

tetrahedral. The sheets are tied to each other (above and below) by linking cations. Since they

appear as leaves the name was taken from the Latin word phyllon, meaning, leaf.

D. Tectosillicates: In this extreme case of linking up each silica tetrahedral shares all of its fouroxygen atoms with other ones above, below and on the sides of it. With every oxygen atom shared

by two adjoining tetrahedral, there are half the oxygen atoms in relation to silicon compared to the

case in nesosilicates where the tetrahedral are all separate and non of the oxygen atoms are share.The general composition for the tectosilicate is SiO2 compared to SiO4 for the nesosilicate. A very

typical example of a tectosilicate is quartz - a mineral s\consisting if silica tetrahedral and ofnothing else. This simple and regular structure of quartz makes it extremely resistant to chemicalweathering. Another very important group of tectosilicates are the feldspars having a more complex

formula and less regular structure than quartz due to isomorphism, during rock formation, of one

ion in the crystal lattice by another of approximately the same size. Though isomorphism means

same shape to imply that the ions introduced are approximately of the same size as those beingreplaced, in practice they are either a little smaller ad may also have a different valency. For an ion

of a slightly different to be fitted into a crystal, the structure becomes imperfectly regular to the

extent that the extra stresses lattice and cause decomposition or weathering more rapidly than whatoccurs for a more regular one. In the same vein if a cation substitutes another one of a different


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valency (e.g Al3+

,replacing Si

4+or Mg

2+replacing Al

3+), there will be net negative charges from the

oxygen left unsatisfied. For electrical neutrality, an additional cation (such as Na+

or K+) has to be

introduced. Such additions modify the structure.

The relevance of isomorphous substitution can be further illustrated by comparing two groups of common

minerals, the micas and the feldspars, in which isomophous substitution occurs to a considerable degreewith quartz in which the simple structure (without the possibility of isomorphous substitution) gives themineral a very high degree of resistance to chemical weathering. Micas are sheet silicates (phyllosilicates)

having many years, each consisting of two sheets of silica tetrahedral held together by a layer of aluminium

and hydroxyl ions (in the case of white mica). The silica aluminium layers are held to each other

relatively weakly by potassium ions and can be separated easily to give the micas their characteristicscleavage that permits them to be separated very thin sheets. White mica (muscovite) is therefore a

potassium aluminum silicate KAl2 (AlSi3O10) (OH)2 one quarter of the silica tetrahedral have had the

silicon atom at the centre replaced by aluminium and in each case this has been balanced by bringing in onepotassium ion. Biotite (black mica) is formed when the aluminium in white mica is replaced by iron or

magnesium, so that (Mg. Fe)3 replaces Al2 in the formula, Biotite is therefore, the richer of the two types

of mica as regards plant nutrients and is also more easily weathered than the relatively resistant muscoviteBiotite is also one of the ferro magensian minerals with the typical dark color. Sericite is a form of white

mica, usually but not necessarily of muscovite composition, occurring as flakes and is often a constituent of

the metamorphic rocks.

Feldspars being a typical example of tectosilicates, have a three dimensional block structure whereby allthe silica tetrahedral share oxygen atoms with all adjacent tetrahedral and each oxygen atoms is therefore

shared between two tetrahedral (as in quartz) since a proportion of the central silicon ions (valency of four)

have been replaced by Al3+

there is an excess negative charge to be satisfied by a cation. A potassium ionis introduced to satisfy the excess charge. The potassium feldspar (KAlSi3O8) is thus formed when a

potassium ion is introduced to satisfy the excess charge. The potassium feldspars are of two type

(orthoclase and microcline) with similar composition but simply different crystalline forms because of

different temperatures or formation. It is, however, possible for the excess negative charge not to besatisfied by a single cation but by a combination of cations (K+ and Na+ for alkali feldspars or Na

+and

Ca2+

for plagioclase feldspars). It is, therefore, evident that feldspars are not fixed composition though inphysical appearance they are known o be similar having whitish, grey or pink color. Calcium feldspars are

believed to be grey or pink color. Calcium feldspar is believed to be the most easily while potassium

feldspar is the least.

Quartz (SiO2), also a tectosilicate (with the same 3 dimensional block structure as the feldspars) consistsof silica tetrahedral and nothing else. In essence, there is no isomorphous substitution. It is extremely

resistant to neither chemical weathering since there is neither a basic control to be attacked nor

isomorphous substitution to weaken the simple regular structure of the mineral. As such, it usually merely breaks down physically to smaller particles and may accumulate in soil after other minerals have been

broken down. It is usually hard, transparent and the sole component of the sand fraction of the soil.

Mineral content and physical properties of typical rocks

On the basis of their chemical composition alone, rocks can be grouped into:

(i) The more basic rocks (those containing relatively high proportions of the basic metalliccations) and

(ii) The more acid rocks (having an increasing proportion of the total composition as silica).


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All silicate minerals (except quartz) contain silica. Acid rocks are known to contain more than 66% silica.

The other recognized types include intermediate rocks (52 66% silica), basic rocks (45 52% silica) and

ultra-basic rocks (with less than 45% silica).

Rocks can also be sub-divided into:

(i) Alkaline cations predominated rocks (containing K+ and Na+)(ii) Calcic elements predominated rocks (containing Ca++ and Mg++)

The basic elements predominated rocks are believed to be more common. The more basic rock is, the more

its content of the ferromagnesian minerals. On the other, the more acid a rock is, the more its content of

feldspars and quartz. As a result, basic and ultrabasic rocks have olivine and pyroxene plus some

hornblende or biotite. The intermediate rocks contain hornblende, biotite and plagioclase feldspars whileacid rocks contain quartz and feldspar (usually mainly orthoclase) and some biotite. The essential

components and physical properties of some typical examples of the major types of rocks are presented in

Table 1 below.Metamorphic rocks are usually derived from sediments rocks or from the metamorphosis or pre-existing

igneous rocks. The following are typical examples of the transformations:

(i) Gneiss Metamorphosed granite(ii) Slate metamorphosed shale(iii) Marble metamorphosed limestone(iv) Quartzite metamorphosed sandstone(v) Graphite metamorphosed coal

Table 1: Ro ck types and their prop erties

Ro ck Typical M ineral Physical

Type Examples Content Properties

1. Igneo us Gra nite Do minant minerals are Light in colour. Have coarse toQu artz and Felspars med ium particle sizes.

They are some micas,Amphiboles and iron oxides.

2. Igneous Diorite Little or no quartz but rich Grey to dark coloured. Coarse toRich in felspars, amphiboles, med ium textured .M icas and iron o xides

3. Igneous Basalt No quartz but there is l it t le Dark/Black coloured. Dense to

Felspars, pyroxene and iron fine grained.Oxides.

4. Sedimentary Sandstone Have quartz and some Light to red coloured and

Cements CaCO 3 , FeO 2 granular or porou s in structureand c lays.

5. Sed imentary Shale Have clay minerals, some Light to dark coloure d and withquart z and som e orga nic thinly laminated structur e.matter

6. Sed imentary Limestone Hav e calcite and do lomite Light or green in colou r. FineWith some iron oxides, grained and compact.Clays pho sphate and

Organic matter.


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Heat, Pressure and chemical changes are attendants to the metamorphosis of rocks at some depth within the

earths crust. The folding of the earth, other earth movement as well as contacts between rocks and

intrusions of molten magma can subject rocks to great heat and tremendous pressure. The result is theformation of a new structure and change of the components into new (secondary) crystalline minerals. The

size of such crystals varies from very fine (microscopic) to coarse. Another characteristic of metamorphic

rocks is the orientation of the constituents to produce a banded effect. Coarse-grained rocks showing only a

rough banding are grouped as banded gneiss.


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ROCK WEATHERING

Originally, the earth consisted of nothing but rocks, some of which are still exposed today. Most of theelements of the earths crust have combined with one or more other elements to form compounds called

minerals. The minerals usually exist in mixtures to form the rocks of the earth. Gradual processes have

formed soils formed from the rocks by erosion. The origin and development of soil is known as soil

genesis. Soil is derived from decomposition of mineral particles of rocks as well as plant and animalresidues. The product of the wearing away (weathering) of rock particles in the absence of organic matter istermed Crust of weathering. Soil is formed only if the weathering of minerals occurs in the presence of

organic matter. When formed as a result of deposits (by streams and rivers) of more or less weathered and

sorted material the soil is called alluvium while the term colluvium: refers to the soil forced as a result

of the movement of materials down a slope largely under the influence of gravity. When a soil appears tohave been developed from materials similar to the underlying rocks, it is referred to as sedentary or

residual soil. The material (hard rock or any unconsolidated deposit) in which soil develops and in which

a soil profile begins to form is termed parent material while the parent material that are rocks are knownas parent rocks.

Rock Weathering

Rocks may, in the process of soil formation, be acted upon and broken down by the action of rain,

running water, frost, wind, action of micro and macro organisms (such as bacteria, fungi, protozoaearthworms, etc.) interactions of various chemical substances and numerous other agents to form soils. If

limestone is the rock material exposed, the agents enumerated above and a lime can break it down rich soil

will be produced. This is also true of sandy and clayey soils, the former being formed from sandstones andthe latter form shale, granite or similar rocks.

The weathering of rocks is known to be a combination of two processes:

(i) Destruction and(ii) Synthesis

Destruction involves the breakdown of rocks to give the parent material while synthesis is the changing of

the parent materials into new materials such as silicate clays and very resistant products like iron andaluminium oxides. Associated with the two processes of rock weathering are the major forms of weathering

itself physical and chemical weathering? Physical weathering ensures the disintegration or destruction

process as rocks are merely broken down by mechanical means to smaller and smaller particles withouttheir chemical composition being changed, though the fragmentation may make chemical attack easier

later. This is so because it causes the exposure of inner and larger surfaces to water and other agents for

further breakdown. This predominates in dry climatic zones as in deserts where temperature changescause contraction (shrinking) and expansion and hence cracking and breaking up of rocks. These processes

happen because the rocks are aggregates of minerals with different coefficients of expansion. Physical

forces (e.g. winds, expansion of roots in rocks crevices and water) may also break up rock particles byrolling impact and so on. On the other hand, during chemical weathering, the rock is decomposedchemically to liberate the constituents of that it is composed and such are either removed to form new

substances as in the formation of clays. This form of weathering is prevalent wherever rainfall is moderate

to heavy as in most parts of West Africa. The main agent of chemical weathering is the percolating soilwater. Rainwater dissolves some quantities of atmospheric constituents such as nitrogen oxide, sulphur

dioxide, oxygen, and carbon dioxide and perhaps traces of ammonia, sodium chloride and other

compounds. Nitrous, nitric and sulphuric acids aid the chemical breakdown of rocks.


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Oxygen and Carbon dioxide, which attack weathering rock by oxidation and the formation of carbonates

are of major importance. Various organic acids derived from the decay of plant and animal materials can

also be added to the soil water as it seeps downwards. The solution that can attack exposed rock fragmentsin the soil and penetrates into massive un-weathered rocks along cracks and joints. The various

mechanisms of chemical weathering include:

(i) Solution: Water as a universal solvent can dissolve easily soluble minerals present in rocks.Alkali metals such as Ca and Mg are easily solubilised while Fe, Si, Al are not.

(ii) Hydration: The simple combination of water with another substance such that the substanceformed is not very much different from the original form e.g. hematite can be hydrated to form

limonite.

2Fe2O + 3H2O = 2Fe2O3.3H2O

(Hematite) (Limonite)(iii) Hydrolysis: The reaction of a substance with water while hydrogen serves a catalyst. It is

essentially a decomposition reaction because the water molecule displaces any cation present in

the minerals. e.g. KAlSi3O8 + H2O = HAISi3O8 + KOH(iv) Oxidation: the taking up of oxygen from the atmosphere by an element or a compound e.g. the

conversion of iron to ferric iron. 4FeCO3 + O2 2Fe2O3 + 4CO2

(v) Carbonation and related acid forming processes: This is the formation of carbonates andbicarbonates. Carbon dioxide can dissolve in water to form carbonic acid, which can dissolvemarble and other carbonates.

H2O + CO2 = H2CO3

H2CO3 + 2CaCO3 = 3CaHCO3(vi) Reduction(vii) Attack by acid and alkaline solutions(viii) Removal of the soluble products liberated

From the above, it is obvious that chemical weathering is a complex process, the details of which vary

according to the soils, rocks and climate involved.The following five descriptive stages have been recognized in the development of tropical soils:

(i) Initial stage the un-weathered parent material(ii) Juvenile Stage weathering has started but much of the original materials is still un-weathered(iii) Virile Stage easily weatherable minerals have largely decomposed: clay content has increased

and a certain mixture is discernable.

(iv) Senile stage decomposition arrives at a final stage and only the most resistant minerals havesurvived

(v) Final stage soil development has been completed and the soil is weathered out under theprevailing conditions.

The weathering of Igneous Rocks

One of the minerals in igneous rocks is often more easily attacked than the others. The softening and

breaking down of such easily attacked minerals usually result in the disintegration of the rock and th

separation of the remaining mineral constituents that are then more exposed to further attack. Granites (oneof the commonest groups of crystalline rocks) contain quartz, feldspars and a third mineral either mica or

hornblende. The feldspars (K, Na or Ca, Al silicates) are the first to weather as the metallic bases they


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contain are removed and the remaining silica and alumina combine to form kaolin. The less easily

weathered mica and the very resistant quartz may remain as part of clay. The order or weathering of the

common mineral constituents of igneous rocks has given as follows:1. Olivine most easily weathered.2. Calcium feldspar3. Pyroxenes and amphiboles (hornblende)4. Sodium feldspar5. Black mica (biotite)6. Potassium feldspar7. While mica (muscovite)8. Quartz most resistant to weathering.

The Weathering of Sedimentary and Metamorphic Rocks

Secondary materials (already weathered, transported and deposited) lead to the formation of sedimentary

rocks. As such, sedimentary rocks often contain very resistant residues. Thus, sandstone that is largely

quartz sand will break down on weathering to the original sand or a poorer sandy soil. If some feldspar orsand (other than quartz sand) is contained in the sandstone, weathering may result in the formation of some

clays and the soil will be less lights-textures. It is known that sedimentary rocks are much less likely tocontain crystalline silicates, which can weather to give nutrients to the soil that in the case of igneous and

metamorphic rocks.

It is very difficult to make general statements on metamorphic rocks both in respect of the rate ofweathering of their minerals and the release of plant nutrients. This is because of their tremendous range of

characteristics. At one extreme, quartz-schist (obtained from quartz sand) is usually nearly sterile and

breaks down to more quartz sand. At the other extreme, certain base-rich metamorphic rocks resemble the

more basic igneous rocks and give rise to very fertile soils.


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CHAPTER 2

SOIL COMPOSITION AND FORMATION

THE CONCEPT OF THE SOIL

Soil is difficult to define precisely. Yet, different people have different ideas about the soil which isone of the natural resources with which mankind is endowed. The geologists and mining engineers are

concerned with the rocks and minerals below. The soil may be of little interest to them. In fact, it is a

nuisance and must be disposed off in order to get at the mineral wealth that must be dug out. To thehighway engineer, the soil is the material for the construction of roads. If the properties are suitable, the soi

is useful. If not, the soil must be removed and gravel put in place.

The farmer or the soil scientist is not usually concerned with what is deep down in the soil except in as

much as it helps him to understand the formation and parent rock of the soil itself. He is interested in thatpart of the earths covering, which supports plants and animal life. The soil scientist can this define the soil

as being that natural covering of the earths surface in the soil as a habitat for plants and animals. He makeshis living from it. Hence, it is more than useful; it is indispensable being the major source of the nutrients,air and water for the growing plant apart from giving mechanical support to it. Other definitions of the soil

by soil-scientists include the following:

(i) Soil is the collection of natural bodies that have been synthesized in profile form from a variablemixture of broken and weathered minerals and decaying organic matter which cover the earth.

(ii) Soil is a thin layer that covers the earth; supplies mechanical support and sustains plants whencontaining proper amounts of air and water.

(iii) Soil s the collection of natural bodies (on the earths surface), which supports the growth ofplants and is the principal source of mans food and clothing.

(iv) Soil is a loose surface of the earth as distinguished from solid rock(v) Soil is an unconsolidated material derived from rock weathering which has been acted upon byclimate and vegetation.(vi) Soil is a natural body of loose, unconsolidated material, which constitutes a thin layer over

several meters deep of the earths crust.

It is evident from the various definitions that although the soil can be studied in may ways some of more

practical value than others soil scientists are mainly interested in aspects of the soil influence on plantgrowth.

Soil ComponentsSoil is a heterogeneous material and may be considered as consisting of the following three major

components:

(a)Solid phase(b)Liquid phase and(c)Gaseous phase.

All the three phases influence the supply of nutrients to plants roots. The solid phase is the main nutrientsreservoir. The inorganic particles of the solid phase contain cationic nutrients elements such as K, Na, Ca

Mg, Fe, Mn, Zn and Co while the organic particles of this phase provide the main reserve of N and to a

lesser extent also of P and S. The liquid phase of the soil (the soil solution) is mainly responsible fornutrient transport in the soil to plant roots. Nutrients transported in the liquid phase are mainly present in

ionic forms, but Oxygen and Carbon dioxide are also dissolved in the soil solution. The gaseous phase of

the soil mediates in the gaseous exchange, which occurs between the numerous living organisms of the soil


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(plant roots, bacteria, fungi and animals) and the atmosphere. The percentage composition of each of the

three phases is given in Figure 1.

The natural bodies in soils can also be classified into organic matter (mostly the remains of plant andanimal tissues), inorganic matter (mostly minerals), living forms (micro and macro flora and fauna), air

and water.

Mineral salts are compounds that normally release nutrients for plants absorption while microorganisms

make the decaying processes possible in soils. Both the organic materials and mineral particles areintimately associated in the topsoil. If the organic material is removed or destroyed, the mineral particleswill remain. Microorganisms play

SOLID

PART

Air Mineral 45 %

20 30%

AIR / PORE

SPACE

Water 5%

20 30% OrganicMatter

Figure 1: SOIL COMPOSITION

an important role in the uptake of plant nutrient elements from soil. Shortage of energy substrates makes it

unlikely for n fixers in the soil microbial population to fix significant amounts of nitrogen. Yet, Mg and Feuptake by plants can be altered by microbial activity while non-nutritional bacteria effects can also

influence growth of plants.Air is a mixture of gases such as oxygen, carbon dioxide, nitrogen, etc. Oxygen is required for thegermination of seeds as well as for respiration by roots of plants and the soil macro and micro organisms.

Carbon dioxide is usually a product of respiration.

Water plays a major part in almost all the physical, chemical and biological processes in the soil. It is

involved in most forms of mechanical weathering, redistributes materials throughout the soil profile, andcarries away both soil particles and solute and transports nutrients to plants. Soil water is variable in

quantity over time and space.


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Classification of soil waterAs a matter of convenience, various forms of soil water can be recognized as illustrated Figure 2. These

are:(a)Run off water(b)Gravitational / Percolation water(c)Capillary water(d)Hygroscopic water(e)Water of Crystallization/Structural water

Water enters the soil through rainfall or irrigation. It infiltrates the soil by moving through the air/pore

spaces. The rate of infiltration depends on the intensity of water supply and the amount and state of pores in

the soil. If already saturated by previous rainfall of irrigation water, infiltration is reduced. Infiltration is

also adversely affected when the soil surface is compact and dense, or the pores are small and few innumbers.

If water cannot infiltrate the soil, it tends to run off over the surface (especially on steep slopes). This is

referred to as RUNOFF WATER. This form of soil water usually flows to meet rivers, streams oceans, seasand other large bodies of water. It is not available for plants use because it runs off from and does not reach

the plants roots. Although on rough ground or low angle slopes, its movement is also and it may be stored

in latter case, gullies may develop and considerable losses may occur.The water that enters the soil pores is affected by GRAVITY and MATRIC (CAPILLARY) forces

downwards and is only effective in very large pores or macro-pores (> 0.06mm in diameter). The matirc or

capillary forces are responsible for the retention of water in the soil since they lead to the attraction of water

under the influence of force of gravity is called GRAVITATIONAL OR PERCOLATION WATER. Itsinks so freely such that no plant root can absorb it within the micro-pores (


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Some of the water in the micro-pores (called HYGROSCOPIC WATER) is held so tightly to the soil

particles to the extent that it can be assumed immobile. In fact, it is adsorbed on to the soil particles by anelectrochemical bond such that it can only be removed by heating or prolonged drying. In a similar fashion,

a small proportion of water is also bound up within the structure of the soil particles e.g. within the crystal

lattice of the clay minerals. This is called WATER OF CRYSTALLIZATION or STRUCTURAL WATER.

It can only be released by destruction of the clay particles. The hygroscopic and structural forms of soilwater are unimportant in terms of processes of water movement and availability to plants but they aresignificant when the moisture content of the soil is being measured.

Summary:

In this section, soil components have been discussed. They include:(a)Air(b)Water(c)Organic matter / humus(d)Micro organisms and(e)Mineral salts.

Laboratory Techniques for Quantitative Determination of Soil Components

There are several experiments that can be performed to show the presence of each of these components in

the laboratory. In some cases, quantitative assessments may also be involved. For this introductory book on

soil science the following simple methods have been carefully selected for ease of comprehension andperformance in the laboratory.

A. Soil Air:

Materials: Two Transparent beakers, Soil sample, Water.

Procedure: Carefully collect soil from a farm with minimum disturbance.i. Put some soil in a transparent glass beakerii. Pour in some water contained in the other beaker

.

. Water

.....

. .. Beakers. . .. . .. .. Air Bubbles

. . . . . . .. .. . . .. Soil Samples. . .. . .. ..

Figure 3: Identification of Air as Soil component

Observation: Bubbles of air could be seen escaping from the soil as water enters the soil.

Conclusion: The air that escapes should have been in the soils pore spaces that did not contain moisture

initially.On adding water to the soil, the molecules of water replace the air.


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B. Soil Water

MATERILAS: Evaporation basin, stirring (glass) rod, Balance, Soil sample, Bunsen burner, Tripod stand,wire gauze.

Procedure: Determine the mass of an empty evaporating basin following the steps:

i. Collect some soil from a depth of about 10cm in a farmii. Put a sample of the soil into evaporating basin and weighiii. Then heat up using Bunsen burner flame to a temperature of about 1050C.iv. Occasionally stir the soil with a glass rod, weigh the evaporating basin and content again and

after about 1 hour when all the moisture contained in the soil should have been evaporated.

v. Repeat this process until a constant weight is obtained.

Glass Rod

Evaporating basin

with soil sample

Wire Gauze

Tripod Stand

Bunsen Burner

Figure 4: Quantitative and qualitative assessment of soil moisture.

Results and Calculation:

If mass of the empty evaporating basin = A gramsAnd mass of the basin and fresh soil = B grams

Therefore, mass of the fresh soil = (B A) grams

If mass of the basin and heated soil = C gramsMass of the heated soil = (C A) grams

Loss in Weight (Mass of water driven off) = (B A) (C A) gramsTherefore, % Moisture content = (Mass of moisture driven off x 100) %

Mass of fresh soil 1

= [(B - A) (C A) x 100] %

(B A) 1


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Conclusion: The boiling point of water is 1000C. Hence, at 105

0C water turns into vapour which when

driven off reduces the weight of the soil.

C. Soil Organic Matter / Humus

Materials: Evaporating basin, dried soil sample, Balance, Tripod stand, Wire gauze, Bunsen burner, Stirring

rod, Desiccators.Procedure: Place the evaporating basin with the dry soil in the last experiment over the Bunsen burner andheat strongly while observing the change in the appearance of the soil.

Keep the soil well stirred but take care not to loose any of its particles.

Continue to heat for about half an hour

Cool the basin and its content in desiccators and weigh.Repeat the process until a constant mass is obtained

Calculation: Mass of evaporating basin and soil after ignition = X gramsMass of ignited soil only = (X A) grams

Mass of organic matter / humus

= Mass of dry soil Mass of ignited soil

= [ (C A) (X A)] grams

% Organic matter in dry soil sample = [(C A) (X A) x 100] %

(C A) 1

Conclusion: Continuous heating of dry soil removes the humus / organic matter in soil by converting it into

gases which escapes into the air. Thus, there would be reduction in the mass.

D. Soil Micro OrganismsMaterials: Two 250cm

3conical flasks, Rubber corks, two pieces of fresh meat, fresh soil sample, Sterilized

soil, 2 strings / threadsProcedures: Set up the apparatus as shown in Figure 5 below.

i. Put some fresh soil sample in conical flask A, suspend a piece of fresh meat in it with the aid ofa string and cork it.

ii. In conical flask B, Put some sterilized soil sample to serve as control, suspend another piece offresh meat and cork. Leave the experimental materials for four days.

Observation: A bad odour similar to that of rotten eggs resulted from flask A when opened on the fourthday because the fresh meat has started decaying. In the conical flask B, no decaying was observed.

Discussion:

The decaying of meat in A is an indication that microorganisms are present in the fresh soil.This is because microbes are known to be agents of decay resulting in bad irritating odour.


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

in a sterilized soil should have been rendered inactive.

E. Soil Mineral Salts

Materials: A round bottom flask, delivery tube, Rubber Cork, Dilute HCl, Dry soil, Conical flask.

Procedure: Set up the apparatus as shown in Figure 6. Pour some soil in the round bottom flask andcarefully pout HCl on to it.

Observation: As soon as dilute HCl reaches the soil effervescence occurs resulting in the production of a

gas whish led through the delivery tube to the conical flask contains limewater.Subsequently, the limewater turns milky.

Discussion: The gas that can turn limewater milky is carbon dioxide, which can be produced when dilute

HCl reacts with carbonates of Ca, K, and Na etc. The observation in this experiment indicates the presenceof carbonates (which are mineral salts) in the soil. The resulting reaction can be represented as follows:

Rubber Cork

String

Fresh Soil Sterilized Soil

Figure 5: Identification of microorganisms as soil components.


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2CaCO3 + 2HCl CaCl2 + 2H2O + 2CO2

Dilute HCl

Thistle funnel

Delivery tubeRubber cork

Round

Bottom flask

Co nical flask

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

. . . . . . .. .. .. . . . Dry Soil Lime. . . . . . . Water

Figure 6: Identification of mineral salt as soil com pone nt


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FACTORS INFLUENCING SOIL FORMATION

The characteristics of a soil are as a result of the influence of five main groups of soil-forming factors:

(i) The parent material from which the soil is developed.(ii) The climate (past and present) of the area(iii) The vegetation supported by the soil (influenced by climate) and the soil fauna (bacteria, othermicro-organisms, worms and termites) that live the soil and(iv) The type of relief associated with the soil and(v) Time i.e. the length of time during which the other factors have been influencing soil

formation.The factors of soil formation can therefore be summarized as climate, living organisms (biotic factors)

topography, parent materials and time. Climatic and biotic factors are termed active or causal factors

while topography and parent materials are passive period when climatic and biotic factors act on the parentmaterials under a given topographic condition to produce soil.

Climate exerts the most important effect on soil formation. Rainwater is one of the major agents of

chemical weathering. Rainwater is one of the major agents of chemical weathering. It facilitates the

washing down of the products of disintegration deeper into the soil.Rainfall and temperature can also influence the type of vegetation that grows, dies and decays to form part

of the soil. Heat aids chemical reactions to the extent that the greater the heat, the faster the rate of

reactions. Wind is involved in the transportation of soil particles. The greater the wind velocity, the morethe particles transported.

Living organisms help in one way or the other in the disintegration of rocks. Microorganisms may secrete

certain substances that can help in various chemical reactions in the soil. When finding their ways intosmall cracks in the rocks, roots of plants widen the cracks and cause greater breakdown of the rocks. The

larger animals also help to break the rocks into smaller particles through their activities on the surface of

the earth. In the same vein, the roots of the plants penetrate parent materials and open up channels for air

and water circulation at death. On decomposition, they add many nutrients to the soil and become part of

structure of soils.The chemical composition of the soil parent material can give an indication of the type of soil formed.

Thus, a parent material having only quartz will form a very poor sandy soil, whereas if micas and feldsparsare composed in the parent material, the soil will contain some clay. Furthermore, the rate of soil profile

development is faster in parent material that are permeable to water than in the case of parent materials that

are impermeable.Topography affects the soil formation as a result of its relation with water movement. It, thus, influences

erosion, temperature as well as the composition and density of vegetation. Rains falling on steep on steep

slopes tend to run off and collect in depressions. As a result of this the rate of soil profile development onsteep slopes is less than in depressions since the latter receive more water and too rapid run off (on steep

slopes) tend to delay soil profile development.

Time as a soil formation factor indicates the period that it takes a soil to develop fully at the instance of thefactors already discussed. It is well known that soil development under warm, humid, forested conditionswill occur faster than under cool, dry and scantily vegetated area. Similarly, soil developing under bedrock

will take a longer time to develop than ones developing under disintegrated type of parent material.

The earths crust is composed of elements; the major ones in their order of predominance are oxygensilicon, Aluminium, iron, calcium, sodium, potassium, magnesium, titanium, hydrogen, carbon, phosphorus

and sulphur. These elements combine in various proportions to form the rocks of the earth, which are the

parent materials. The parent materials differ in structure, composition and rate of decomposition. Thesedifferences are due to the difference in the proportions and types of elements that form the parent material.


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The parent material disintegrates to form the soil whose properties will be greatly influenced by the

properties of the parent material.

A sixth soil forming factor is man who uses the soil and causes important changes in the process.

S un

Cloud

Rain Burrow ing earthworm Plant roots growing

Rain-water Inside rock

action on a crevice

R O C K

Figure 7: So me so il form ing factors


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CHAPTER 3

SOIL PROFILE AND PROPERTIES

SOIL PROFILE STUDY

The following are some of the uses to which the soil can be put.

(i) As a medium for plant growth (Farmers view)(ii) As a structural material in the making of highways, dams foundations for building or for other

engineering purposes (civil engineers view)

(iii) In manufacturing bricks and tiles (Mason)(iv) For waste disposal systems (sanitary engineering)

The suitability of soil for the various uses man can put them is highly dependent on their physical

properties. It is important and beneficial for any one involved with the use of the soil to know what extentand by what means its properties can be changed.

In knowing the use to which a soil should be put, one may consider the following:

(i) The rigidity and supporting power of the soil(ii) Wet and dry drainage of the soil(iii) Moisture storage capacity of the soil(iv) Plasticity of the soil(v) Ease of penetration of the soil by roots(vi) Aeration of the soil(vii) Retention of plant nutrients in the soil

All the above factors are closely associated with the physical condition of the soil and it is the considerationif such properties that indicate the type of soil found in any location. The physical properties of the soil that

are of prime importance to the soil scientist are:(i) Texture(ii) Structure(iii) Weight, pore-space and air-relationships(iv) Colour(v) Temperature


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SOIL SAMPLING TOOLS

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

profile pits.

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

It is used for digging inspection holes in soil survey.

Chisel end


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Square- pointed spade is used in collecting soil samples,

especially after the preliminary excavations has been

made either with a digger or a chisel

Square pointed spade Post-hole Spade

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

be limited on gravely soils.

Screw auger

The ordinary screw auger is 100 to 150cm long, with provisions for adding extra lengths for deep boring.

The screw or worm part should be about 16.5cm (7ins.) long, with the distances between flanges about

the same as the diameter. If the distances between flanges are narrower than the diameter, it will be difficulto remove the soil with the thumb. The soil sample is clogged within the screw of the auger. It is

convenient to have a scale marked on the shaft of the auger from the tip. The screw auger is very useful inprobing soils to depths of 150cm and beyond.

Typical Soil Profile

In determining and describing the properties of a soil, it is useful to examine both the top few centimetres,

the part which most affects plant growth and the entire soil profile (Fig. 8) (Showing the variouslayers/horizons). This is usually done for sufficient information on the soil. A SOIL PROFILE is a vertical

cross section through the soil showing the various horizons of which the soil is made up. The horizons

represent the different zones or layers of soil material that together form the entire soil. Each of thehorizons is different in some uppermost few centimetres of the soil is called TOP SOIL and is made up of


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plant materials and humus as well as minerals and other inorganic matter. The layer is dark coloured due to

the presence of humus, which varies in amount with the vegetation and other factors. Good humus topsoil

is typically dark and greyish-amorphous and glue-like substance derived from plant remains (leaves,flowers and branches) that fall on to the soil surface. It can also be derived from dead and decomposing

roots and the bodies of microorganisms and other soil fauna. Apart from giving the soil particles the dark

colour, humus helps to bind them together such that they are frequently more or less loosely combined to

form soil crumbs. It usually decomposes or mineralizes to release plant nutrients. It, thus, has greatinfluence on the physical and chemical properties of the soil and expectedly on the soil fertility and productivity. A very thin layer (THE LEAF LITTER LAYER) remains continuously thin on the soil

surface because decomposition of the great quantity of materials (leaves, twigs, flowers, fruits, branches

and other pant parts) is rapid that there is very little time for their accumulation. The attack by bacteria,

fungi, termites, worms and other animals and insects starts the decomposition as soon as a leaf falls. It is,however, possible for the leaf litter layer to be thicker in cooler parts of the world than in tropical soils, as

decomposition rate is lower. Climatic factors (particularly high temperature) evidently speed up bacterial

activity and it therefore responsible for the rapid decay of leaf litter in tropical climates. To examine thetopsoil, the following steps should be used:

(a)Collect a small handful of the topsoil and view it with a hand lens.(b)Feel the soil by pressing it lightly between thumb and first finger.(c)Rub it between fingers(d)Take it apart carefully; examine the structure, the pores and natural spaces in it.

The topsoil may contain gravel stones and a large number of very small roots. This is because the topsoil is

the home of the feeding roots of grasses, herbs, shrubs and trees usually having dense but shallow rootmats.

The second major horizon called SUB SOIL consists mainly of inorganic materials with colour markedly

changed from that of the topsoil above it. The subsoil colours are more striking and varied (brown, brownish yellow, red, grey etc.) than relatively uniform colours of humus stained topsoil. The red and

brown colours are associated with the occurrence of iron compounds in the subsoil. The subsoil may also

be relatively compact, with fewer roots, pores and channels than the topsoil, which is often more opening,

porous and easy to work. The subsoil horizon could, therefore, be said to be much thicker than the first(topsoil) and its colour varies according to its parent material and the amount of organic matter present.

The subsoil merges into a third horizon, the weathered substratum consisting of parent materials. Thetransition between the second and the third horizon is usually a gradual one and the boundary between them

is very diffuse, irregular and difficult to see. It may take considerable experience to separate the two

horizons accurately and their differences depend on the type of soil ad the differences depend on the type of

soil and the nature of the present rock below. The weathered substratum may in turn merge into hard, freshrock below, usually referred to as BED ROCK, which may or may not be similar to the rock from which

the upper parts of the profile were ultimately derive. It may occur at such great depths that it cannot often

be seen in a soil pit.


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Geologists hammer

A geologists hammer, or small hand pick, one end of which can be used as a hammer, cut along roadsides.

For most soils and those containing many wooded root, a chisel pointed hammer is better, whereas fordry soils a sharp pointed hammer is better.

A more detailed consideration of the typical soil profile than the simple view presented above is given inFigure 8. The entire profile can be divided into two portions the Solum and the Regolith. The regolith is

composed of the parent material and the bedrock while the other upper sections form the solum. There are

the O and A horizons, usually referred to as zones of elluviation (where materials are removed, washed or

leached away). The Bhorizon of illuviation, which acquires the materials, moved or leached away from Oand A horizons such that the material accumulates in this zone. An inclusion of deep feeders in a crop

rotation scheme ensures that such plants tap


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the nutrients that have accumulated in the zone of illuviation. This is the advantage of crop rotation tonutrient recycling.

The zone of elluviation (Top soil) can be classified into:

(a)Organic horizon. This can be subdivided into H, O1, O2, O3, etc. it is essentially the horizon inwhich the original forms of most plant and animal matter cannot be recognized with the naked eyes

due to decomposition.

(b)Mineral horizon: This can be subdivided into A, B and C sub horizons. The A horizon consists oforganic matter formed or forming adjacent to the surface. It is also an horizon that has lost clay, iron

or Al and having a resultant concentration of quartz or other resistant minerals of sand and silt size.The B-horizon is one in which the dominant feature is an illusion concentrate of silicate clay, Fe, Al or

humus alone or in combination. The C-horizon is a mineral layer excluding the bedrock. It is either likeor unlike the material from which the solum is formed.

Methods of Soil Profile StudyThere are the following major techniques of studying the soil profile:

(a)Using a soil auger to bore a deep hole into the soil. The vertical sample of materials extracted isthen carefully laid on a clean sheet of white cardboard in the order brought out. If one needs toreach the bed rock, however, a well-like hole (a typical soil profile) needs to be dug.

H Ho rizon Organic

TopMaterial

O - H orizonZone ofElluviation

M ineral or A H orizonTop soil

Sub soil or B - Ho rizon(Zo ne o f illuviation)

Parent material or C - Ho rizon

Bed rock or R - Horizon

Figure 8 : A T ypical soil Profile.


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(b)Observing an exposed vertical cutting such as road cuttings, quarries, mines or building sites.

In all cases, the following steps are crucial in the profile study:(i) Take measurements of the various horizons and have accurate scale drawing.(ii) Identify the parent rock and note the different colours of the layer above the rock. The junction

between dissimilar layers should be noted / recoded.

(iii) Study the amount of vegetation and humus on the surface and the amount of gravel that willaffect drainage.(iv) Study the textures, structures, and consistencies of the different layers.(v) Compare the different samples taken from different locations, discuss and record the differences

to allow the development of a soil map of the region concerned.

It is important to stress, here, that in writing a soil profile description, soil scientists focus on soilcolour, texture, structure, thickness and pH.

SOIL PROPERTIES

SOIL COLOUR

The colour of soil serves both farmer and soil scientists, provided that they understand the

causes of the various colours and is able to interpret them in terms of soil properties. Organic mattercontent, drainage and aeration are soil properties related to colour that are of interest. Soil colour is the first

soil characteristic that is observed during profile study and it is often used to describe the soil. Soil colour

has indirect effect on plant growth through its effect on temperature and moisture. Colour under which asoil has been developed or its parent material. In most cases, the productive capacity of a soil can be judged

from its colour. While the topsoil is of similar uniform, dark colour due to the presence of humus, thesubsoil are usually of more striking colours. Practically all colours (white, red, brown, grey, yellow, black,

bluish and greenish tinges, etc.) occur in soils. Predominantly, soil colours are not pure, but mixtures, suchas grey, brown and rust. Pure blue and green are not known to exist in soils. When two or three coloursoccur in patches, mottling is said to take place.

The colour of the soil is usually a composite of the colours of its components. The colloidal material has

the greatest impact on soil colour. Thus, humus is black or brown; iron oxides may be red, rust brown or

yellow depending on the degree of hydration. Reduced iron is blue-green. Quartz is mostly white.Limestones are white, grey, or sometimes olive green. Feldspars have grey, white or red as determined by

the type and the amount of iron coatings. Wet and moist soils look darker than dry soils.

As earlier mentioned, colour can serve to tell much about a soil. Generally, the darker a soil, the higher isits productivity due to the amount of organic matter present. Light colour often results from the

preponderance of quartz, a mineral that has no nutritional value. With some exception, the sequence o

decreasing productivity is black, brown, rust-brown, grey brown, red, grey, yellow, white. In youngsoils, colour is an indication of the parent material. In mature soils, it is an indication of the climate in

which they have developed.

Practically, all soil profiles reveal a change in colours from one horizon to the next. The changes are mostobvious in mature soils, while both in young and very old soils they are less pronounced. This is because,

in young soils there has not been sufficient time for much differentiation, while in the very old ones,

leaching has proceeded to considerable depth and has left only the least soluble components.

The specific colour of the horizons makes it possible to recognize erosion. In many fields, the eroded spotsstand out clearly from the rest of the land.


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In the classification of soils, colour is frequently very helpful. Since colours are good indicators of soil

characteristics, they serve well in the study of the genesis of soils and in arriving conclusion concerning

their best use and management.

The colour of the soil affects other soil conditions through its effect upon radiant energy. Black and dark

colours absorb more than light colours or white. As such, dark soils tend to be warmer than light coloured

soils when the sun shines or when the atmosphere is warm and the soil is able to absorb energy from it.The greater amount of heat energy available to the soil results in higher rates of evaporation. A dark soilwill therefore dry out faster then a light-coloured one under identical conditions. A cover of vegetation or

much will expectedly reduce or even eliminate this on heat balance as it affects temperature and moisture

of the soil and indirectly plant growth microbial of the soil, and indirectly plant growth, microbial activity,

and soil, and indirectly plant growth, microbial activity, and soil structure. It is important to note, however,that it is only the colour of the soil surface that can have an influence on other soil conditions since the

colour that is not exposed cannot be of significance.

The colour of an object depends upon the kind of light, which it is capable of reflecting to the eye.Description of colour of light is usually associated with the measurement of its three principal properties,

hue (the dominant wavelength of colour of the light), value (i.e. brilliance or total quantity of light that

increases from dark to light colours) and chroma (the relative purity of the dominant wave length of lightwhich increases with decreasing proportions of white light).

The three basic factors or components of light (hue, value and chroma) underlie the construction of THE

MUNSELL COLOUR CHARTS. The Munsell notation of colour is a systematic numerical and letter

designation of each of the three variable properties of colour. The Munsell notation for a given soil samplecan be determined by comparing the sample with a standard set of colour chips. The chips are mounted in a

notebook with all the colours of a given hue on one page. Each page then corresponds to a slice through the

colour cube parallel to its front. The pages are arranged in the order of increasing or decreasingwavelength of the dominant colour in order to facilitate the matching of the unknown soil colour with the

colour of the standards. It is important to note that the relationships of the colours to one another can be

shown by use of a solid (e.g. a cube) in which hue, value and chroma are plotted along the three edges.

Each possible colour represents a point in this cube and is completely defined by the three co-ordinates ofthat point, which is its Munsell notation.

Many soil horizons have a single dominant colour. Horizons that exhibit mottled colour condition (whenseveral colours are present in a spotted or variegated patter) have a mixture of two or more colours, as they

are dry during part of the year and wet on the other part of the year.

Generally, it is not economical to attempt to change the colour of the soil surface. In some cases, however,

black or white powders or sand or coloured plastic sheets can be placed on the soil. The plastic is formulching to affect temperature and moisture of the soil and the colour change will be incidental. The

commonest use of a colour treatment is the use of crop residue mulch on top of the ground. When it

decomposes early, it can be dark at the season when it is necessary to absorb sunshine energy. Fresh straw,on the other hand, is usually lighter than the soil.


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SOIL TEXTURE

Texture is the most permanent and important characteristics of the soil. The term soil texture refers to the

fineness or coarseness of the soil. It is the relative size of the soil particles i.e. the relative proportion of thevarious ultimate grain-size fractions (sand, silt and clay) in the soil. It can also be said to relate to the

relative percentages of sand, silt and clay in a soil. The names of the predominant size fractions are

normally used as texture designations (Table 2) while word loam refers to a situation whereby all the

three major size fractions occur in sizeable proportions.Thus, soils having 85% or more particles of sand are called sandy soils. Those with 7 27% clay, 28 50% silt and less than 52% sand are loamy soils while those with 40% or more clay particles, less than

45% sand and less than 40% silt are clayey soils. For greater precision, soils that come between these

categories are described as loamy sand, sandy clay loamy, silt clay etc. The term silt clay describes a soil

in which the clay characteristics are outstanding and which also contains much silt. A silt clay loam issimilar to the silt clay except that it contains more sand and therefore mellower. The articles can be

distinguished as presented in Table 3 below:

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

contents.

Texture Designations % Sand % silt % Clay

Sandy Soil ? 85 < 15


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The three major types of soil are:

(i) Clayey (water logged) soil: In this type of soil air and water do not move easily and the particlestend to form lumps when dried. As such, they are difficult to work as they drain poorly and tendto crack in the dry season. They have a colloidal nature that enables them to hold large numbers

of mineral ions which are however not available to plant roots because of poor drainage. Such

soils can be improved by organic matter and lime additions that allow more effective water

percolation.(ii) Sandy (artificially dried) soils: These are well aerated, light and very easy to work with. They

are however, termed hungry soils because nutrients are easily leached away in them. Due to the

very minute quantity of water that may contained in them, they are referred to as the artificially

died soils.

(iii) Loamy (well drained) soils: These contain a fair balance of clay, silt and sand particles and aresuitable for most crops. Their clay content enables them to retain nutrients and water while the

sand content permits adequate drainage. They can also be referred to as moderately drained

soils.As a guide to the textural classification or identification of soils, the soil-texture triangle (Figures 9 & 10)

having 12 main textures with their compositions is usually employed as a strategic instrument. A glance at

the texture triangle indicates the importance of specific surface. It takes more than 80% of silt to call a soilsilt and more than 85% of sand to call a soil sand but only 40% clay is required to call a soil clay. In

the consideration of the mechanical composition of soil, the terms clay, sand, and silt are used both

for soil separates and for texture designations. The percentage of the individual fractions is calculated on

the basis of organic free, oven-dry soil particles less than 2mm in diameter. Although the organic mattecontent is mineral soil is not indicated in the texture designation, it is of great importance in determining

the value of the soil. Soils containing over 15% organic matter are designated as mucky or muck (for well

decomposed materials) and peaty (for soils with only partially decomposed plant residues).

The knowledge of soil texture is evidently of immense significance for the following reasons:

(i) It is a guide to the value of land since land use capability and soil management techniques aredependent on it. It is widely accepted that the best agricultural soils are those containing 10

20% clay, 5 10% organic matter and the rest divided approximately equally between sand and

silt.(ii) It is helpful in the study of the morphology and genesis of soils as well as for their classification

and mapping.

(iii) It governs the rate and extent of many important physical and chemical reactions in soils since itcontrols the amount of surface on which the reactions can occur.


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Figure 9: Soil Textural Triangle


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Determination of soil texture

There are several techniques for soil texture determination. Some provide approximate or rough estimationswhile others give more accurate descriptions of the texture of soils. The following two major categories of

the techniques are recognized:

(a)Field or Feel Method: This is a rapid method that can be used on the farm although not accurate. Itinvolves moistening the soil sample on the palm and rubbing it gently with the thumb or forefinger.The extent to which it can be shaped is an indication of the texture. Fine textured clayey soils willroll into a long worm than can be bent into a ring without cracks. It will be sticky to touch and

with no feeling of grittiness. Light clay forms a circle with cracks in it. Medium textured loamy

soils will not be mouldable but may be slimy and with a very fine gritty texture. Coarse textured

sandy soil will feel very gritty and will not hold together at all. Sand cannot, thus, be shaped orworked at all and the most that can be done is to heap it up into a pyramid or cone. If the sand

contains sufficient finer material to enable it to be shaped into a ball, it is loamy sand. If the sample

can be rolled into a cylinder, but breaks when bent further, it is a loam (a light loam forms a short,fat cylinder, an ordinary loam one which is full length). If the cylinder can be bent into a U shape

but no further, the sample is a heavy loam.The above practical test is of much value as it gives an indication of how easy or difficult to handlethe soil during cultivation. High clay content makes the soil relatively hard to work with and may be

described as heavy. On the other extreme, sandy soils are usually much easier to dig, hoe or

plough and are referred to as being light.

(b)Mechanical Analysis: The determination of the amount of the various separates present in the soilis called a mechanical analysis of particles size analysis. Mechanical analysis can also be defined as

the relative distribution of the size groups of ultimate soil particles. The three major steps involved in

all types of quantitative mechanical analyses are:(i) Destruction of soil organic matter where necessary

Figure 10: Simplified Soil Textural Triangle


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(ii) Separation of all particles from each other(iii) Measuring the amounts of each size group in the sample.

The various types of quantitative mechanical analysis that permits varying degrees of accuracy are:

(i) Soil Mechanical Analysis by riddle: This involves placing a weighed dried sample in the top section ofa soil riddle, which is a set of sieves with different mesh sizes. The sieves should correspond to the

desired particles size (2mm, 1mm and 0.5mm) out of the largest particles, sand. The next mesh issmaller and it is for separating out the smaller (silt) particles while the third is very fine and allows onlythe smallest (clay) particles to pass through. Soil crumbs should be broken down with hand while the

content of each of the sieves are separately weighed and the percentages found to determine the

respective proportion. For finer materials ( 0.05mm), this method is unsuitable.

(ii) Soil Mechanical analysis by sedimentation techniques:The sedimentation techniques are based on the long known fact that the velocity of fall an object in

a liquid medium is influenced by such conditions as:(a)The viscosity of the medium(b)The difference in density between the medium and the falling object(c)The size and shape of the object.Stokes law (formulated in 1851) describes the rate of setting of spherical particles in a various medium. It

states that the resistance offered by a liquid to the fall of a rigid spherical particle varies with the

circumference of the sphere and not with its surface. Conversely, the force of fall by the particle is

proportional to its weight and consequently to its volume. The components that make up the equation ofstrokes law, thus involve the cause of settling i.e. Cause of settling = resistance of settling.

Thus, Volume of particles x Density difference x Acceleration = Circumference of particle x velocity ofsedimentation

In summary, the sedimentation techniques are based upon the particle that the rate at which a particle

settles from a suspension is proportional to its diameter. Large particles settle more rapidly than small onesand by measuring the time it takes particles to settle from a suspension of water, it is possible to estimate

their diameter. Hence, in reality, the sedimentation techniques (just like sieving) merely subdivide the soilinto several size fractions but do not measure the diameter of individual grains.

Soil Mechanical analysis by fractional sedimentation is a typical and simple laboratory procedure that can

give a detailed analysis and subdivide a soil sample into a large number of fractions and thereby obtain a

very fine and precise estimation of particle size. The following steps are involved:i) Pour water on about 20g soil contained in a glass jar and stir thoroughly. The water becomes dirty

and some soil particles sink to the bottom of the glass jar

ii) Pour away the dirty water into another jar.iii) Add more water to the first jar, stir again and pour the dirty water into the second jar.

iv). The last process is continued until the water remains clean and the first jar contains pure sand.v) Allow the second jar containing the dirty water to stand for abou

Soil Nature Fertility Conservation and Mangement

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