Soils: Nature, Fertility Conservation and Management

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Soils: Nature, Fertility Conservation and Management

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

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

Copyright © 2004 Lulu, Inc.

All rights reserved.

First published in July 2004 Second impression, May 2006

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

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

AMS Publishing, Inc., 2004

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TABLE OF CONTENTS Chapter / Contents Pages Preface ------------------------------------------------------------------------------------------------------------- iv Introduction --------------------------------------------------------------------------------------------------------1

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

iii

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).

iv

The King of kings

v

vi

INTRODUCTION

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

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

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

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

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

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

ROCKS AND THEIR WEATHERING

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

Mineral content and physical properties of typical rocks On the basis of their chemical composition alone, rocks can be grouped into: (i) The more basic rocks (those containing relatively high proportions of the basic metallic cations) and (ii) The more acid rocks (having an increasing proportion of the total composition as silica).

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SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

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

T yp ical E xam p les

M ineral C o ntent

1.

Ig neo u s

G ranite

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

2.

Ig neo u s

D io rite

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

3.

Ig neo u s

B asalt

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

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

4.

S ed im entary

S and sto ne

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

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

5.

S ed im entary

S hale

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

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

6.

S ed im entary

L im esto ne

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

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

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P hysical P ro p erties

ROCKS AND THEIR WEATHERING

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

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SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

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

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ROCKS AND THEIR WEATHERING

Oxygen and Carbon dioxide, which attack weathering rock by oxidation and the formation of carbonates, are of major importance. Various organic acids derived from the decay of plant and animal materials can also be added to the soil water as it seeps downwards. The solution that can attack exposed rock fragments in the soil and penetrates into massive un-weathered rocks along cracks and joints. The various mechanisms of chemical weathering include: (i) Solution: Water as a universal solvent can dissolve easily soluble minerals present in rocks. Alkali metals such as Ca and Mg are easily solubilised while Fe, Si, Al are not. (ii) Hydration: The simple combination of water with another substance such that the substance formed is not very much different from the original form e.g. hematite can be hydrated to form limonite. 2Fe2O + 3H2O = 2Fe2O3.3H2O (Hematite) (Limonite) (iii) Hydrolysis: The reaction of a substance with water while hydrogen serves a catalyst. It is essentially a decomposition reaction because the water molecule displaces any cation present in the minerals. e.g. KAlSi3O8 + H2O = HAISi3O8 + KOH (iv) Oxidation: the taking up of oxygen from the atmosphere by an element or a compound e.g. the conversion of iron to ferric iron. 4FeCO3 + O2 2Fe2O3 + 4CO2 (v) Carbonation and related acid forming processes: This is the formation of carbonates and bicarbonates. Carbon dioxide can dissolve in water to form carbonic acid, which can dissolve marble and other carbonates. H2O + CO2 = H2CO3 H2CO3 + 2CaCO3 = 3CaHCO3 (vi) Reduction (vii) Attack by acid and alkaline solutions (viii) Removal of the soluble products liberated

From the above, it is obvious that chemical weathering is a complex process, the details of which vary according to the soils, rocks and climate involved. The following five descriptive stages have been recognized in the development of tropical soils: (i) Initial stage – the un-weathered parent material (ii) Juvenile Stage – weathering has started but much of the original materials is still un-weathered (iii) Virile Stage – easily weatherable minerals have largely decomposed: clay content has increased and a certain mixture is discernable. (iv) Senile stage – decomposition arrives at a final stage and only the most resistant minerals have survived (v) Final stage – soil development has been completed and the soil is weathered out under the prevailing conditions. The weathering of Igneous Rocks One of the minerals in igneous rocks is often more easily attacked than the others. The softening and breaking down of such easily attacked minerals usually result in the disintegration of the rock and the separation of the remaining mineral constituents that are then more exposed to further attack. Granites (one of the commonest groups of crystalline rocks) contain quartz, feldspars and a third mineral either mica or hornblende. The feldspars (K, Na or Ca, Al silicates) are the first to weather as the metallic bases they

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SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

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

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

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CHAPTER 2 SOIL COMPOSITION AND FORMATION

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

SOILS: NATURE, FERTILITY CONSERVATION AND MANAGEMENT

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

SOLID PART Air 20 – 30%

Mineral 45 %

AIR / PORE SPACE

Water 20 – 30%

5% Organic Matter

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

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SOIL COMPOSITION AND FORMATION

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