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UNIT 1 NATURE AND FORMATION OF

Structure

1.1 Introduction Objectives 1.2 Soil and its Importance 1.3 Characteristics of Soil Profile Soil Horizons 1.4 Soil Genesis: Origin and Formation of Soil and Rocks Weathering and Soil Formation Factors Affecting Soil Formation 1.5 Soil Types of India 1.6 Summary 1.7 Terminal Questions 1.8 Answers

1.1 INTRODUCTION Evolution of Earth took place five to eight billion years ago when it was a red hot magma (molten rock). It took millions of years for the magma to cool down and during this process it got transformed in many ways. In the initial stages there was no water on Earth’s surface, it rained for some million years and filled up valley like places to make oceans and seas. In these early stages of evolution, Earth conditions were quite drastic in which formation of many silicates, like orthoclase, took place. These in contact with water slowly hydrolysed to give open structured silicates called minerals. This transformation took another million years. Sometimes in between, life came into existence in water and evolved. This evolution was accompanied by the evolution of soil organisms which helped in the formation of soil. The process of soil formation, factors affecting the soil formation, soil morphology, and the common soil types are discussed in this unit. The next unit deals with different characteristic parameters of soil which determine the nature and quality of the soil. To begin with, it is important to understand what is soil and how is it important for all living beings. We will discuss this in the next section.

Objectives After studying this unit you should be able to: • define soil and explain its significance, • describe the soil profile and soil horizons, • identify different types of soil horizons, • define rocks and minerals, • explain the process of weathering, • describe the factors which affect soil formation, and • describe soil taxonomy in general and the different types of of India.

1.2 SOIL AND ITS IMPORTANCE Everyone knows about soil as a resource which fulfils the basic requirement of human The noun soil is derived from the latin solum , which kind by supporting variety of plants and other vegetation. It is difficult to give a means floor or ground. unique definition for it, as its observation is quite subjective in nature. In a very broad As a transitive verb the manner, soil may be visualised as a “thin layer” of Earth’s crust which serves as a word 'soil' means "to make natural medium for the growth of plants. However, it has different meanings to dirty" as in the case of different people. To a farmer, soil is that portion of Earth’s surface which he can soiled dishes or clothing. 7 Soil plough and grow crops to fulfill needs of family and animals. For a civil engineer, soil is the foundation for all construction activity like roads, buildings, embankment of canals and drains etc. For an oil technologist, soil clays and clay minerals are the sources of petroleum cracking agents. Thus, soil is a vital natural resource which needs proper understanding and management to maintain and improve its productivity, which in turn, helps to maintain a healthy and green environment.

The definition of soil has changed a great deal from a thin “outer layer of Earth’s crust” to “a collection of natural bodies on the surface of the Earth ”. The currently accepted definition of soil given by Staff of United States Department of (USDA) is an elaborate one and is beyond the scope of this course. The operational part of the definition visualises the soil as “the collection of natural bodies in the Earth's surface, in places modified or even made by man of Earthy materials, containing living matter and supporting or capable of supporting plants out-of-doors”.

To be soil, a natural body: • must contain living matter, • should be capable of supporting plants.

Having defined soil, let us look into why does one need to study about soil and what is Why study soils? its importance? You would be aware that, from the dawn of agriculture, cultivators - People and society were attracted to fertile soils of river valleys. Early Greek and Roman writers have depend on soil - Soil is a vital natural described farming systems that involved leguminous plants, use of ashes and sulphur resource as soil supplements. German chemist Justus von Liebig (1840) found that crop yields - Soils are the major were increased by adding minerals to soils. He proposed that the elements in source of food, fiber the soil added as manures and fertilizers are essential for plant growth. According to and renewable or reusable resources him certa in factors are essential for plant growth and if any one of these factors is (biomass etc.) limiting (controlling) plant production would be reduced. Soils are studied today to ascertain which of these factors is below desired level and how its limitation to plant growth can be removed.

Pedology focuses on soil formation (from nature In the last two centuries of scientific study, two concepts or approaches to the study of source), its classification soil have evolved. One called treats soil as a natural entity, a biochemically and composition. weathered and synthesised product of nature and deals with its classification and description as it occurs. The other named , treats soil as a natural Edaphology deals with for plants and studies it in terms of its productivity and means of conserving and soil productivity, its conservation and, improving it, the production of food and fibre being their ultimate goal. We however management (for are going to follow a combined approach. Having understood about soil and its efficient production of importance let us study about the general features of soil. Before that try to answer the food and fibre). following SAQ.

SAQ 1 How is pedology different from edaphology? ………………………………………………………………………………………… ………………………………………………………………………………………… …………………………………………………………………………………………

1.3 SOIL MORPHOLOGY

Parent material: The Soil morphology is the description of the soil body and its general characteristics. The primary material from morphology of the soil is expressed by number, kinds and arrangements of the which the soil is formed. different layers constituting it and their observable and measurable characteristics. The development of soil from the parent material is the result of soil forming 8 processes occurring under the influence of soil forming factors (discussed in next section). These processes take a long time- may be a few thousand years to create a Nature and Formation soil. As the time passes, the soil matures and generally becomes deeper and develops of Soil distinct layers called horizons. A may be defined as “a horizontal layer of regolith, approximately parallel to the soil surface, and possessing relatively Regolith: the loose Earth homogeneous physical, chemical, and biological properties produced by soil forming material above solid rock. processes”.

In a vertical section of the soil (as can be seen in a freshly exposed pit) several characteristic horizons can be identified. Such a section is called as a profile or soil profile. It is commonly conceived as a plane at right angles to the surface. The upper part of a soil profile above the parent material in which processes of soil formation occur and within which most plant roots and soil animals are found is called solum. A study of the soil profile refers to the identification and description of all the horizons as completely as possible. Different horizons are differentiated on the basis of the characteristics observable in the field, though sometimes laboratory data is also needed for complete description. In practice, however, a description of a soil profile includes soil properties that can be deter mined only by inspecting volumes of soil.

The characteristics of soil do not change vertically downwards only, there are lateral Pedon: Greek word variations also as soil is a three dimensional body. Therefore, a three dimensional meaning soil or Earth. sample within a soil, called a pedon , is chosen as a unit to represent the nature and The area of a pedon arrangement of horizons and variability in the properties of soil. A group of similar 2 pedons that are surrounded on all sides by “non-soil” or by pedons of unlike character ranges from 1 to 10 m . is called a polypedon. A schematic sketch of a soil pedon in relation to the soil, polypedon and soil profile is shown in Fig. 1.1. In the following subsection you will study about the characteristics of profile of a given soil. It is important for the classification of the soil as well as in putting the soil to an appropriate use.

Polypedon

Soil pedon O A

B Soil profile

Soil horizons

C

Fig. 1.1: A pedon in relation to the soil, polypedon and soil profile.

1.3.1 Characteristics of Soil Profile Colours are caused by iron oxides and The profile characteristics studied in the field consist of locating the soil hor izons organic matter that (based on its colour description that includes the colour name, the Munsell notation), coat the surface of soil the water state, and the physical state. The physical state is recorded as broken, particles. Clays tend rubbed, crushed, while the water state of a sample is given either as "moist" or "dry". to be orange - red due The Munsell notation is a soil colour system devised originally in the USA and now to redox reactions, whereas darker colours widely accepted. It is based on the three variables of colour viz., hue, value and are caused from chroma. The notation is recorded in the form: hue value/chroma e.g., 7.5R 7/3. organic matter.

9 Soil Hue is the dominant spectral colour (rainbow) and is related to the wavelength of light. In Munsell system there are five principal hues; red (R), yellow (Y), green (G), blue (B), and purple (P) and five intermediate hues representing midpoints of each pair of principal hues. The intermediate hues are yellow -red (YR), green-yellow (GY), blue-green (BG), purple-blue (PB), and red-purple (RP). Thus a total of 10 hue names each with four numerical segmentation i.e., 2.5, 5, 7.5, and 10 are used to describe the notation, e.g., 2.5Y, 5Y, 7.5Y and 10Y etc. are the possible notations for principal hues and 2.5YR, 5YR, 7.5YR and 10YR etc. represent the same for intermediate hues.

Value refers to relative lightness or darkness of a colour and is a measure of the amount of light that reaches the eye under standard lighting conditions. On a neutral grey scale, the value ranges from (0/) for pure black to (10/) for pure white. Extremes are usually not met in any soil.

Chroma is the relative purity or strength of the spectral colour and its numerical value extend from (/0) for neutral colo urs to (/8) for the strongest expression of colo ur.

Munsell notation for a given soil sample can be determined by comparison with a Munsell system colour book (Fig. 1.2(a)) which consists of different colo ured cards, or charts, systematically arranged according to their hue as defined above. A card of particular hue has a series of chips arranged vertically to show equal steps from the lightest to the darkest shades (value) of that hue as shown in Fig 1.2(b). For a particular value colour chips are arranged horizontally in the order of their increasing chroma from left to right. The three properties are always given in the order hue, value and chroma, for the example quoted above (7. 5R 7/3) 7.5R is the hue, 7 is the value and 3 is the chroma.

Fig. 1.2: a) The Munsell colour book b) A soil colour chart from Munsell colour book

To make precise differentiation among soil horizons and among the soil groups, the field data on soil morphology are supported with laboratory measurements of selected soil properties, viz. mechanical composition, , pH, electrical conductivity of saturation extract of soil, organic carbon, mineralogical composit ion(primary and secondary minerals) etc. All these data are used in interpreting the soil forming processes which have acted over many years in shaping the characteristic soil horizons. We would like to emphasis e that profile studies are the first step in understanding soil genesis and also the basis of soil classification. In the next subsection we shall discuss about the designation of different horizons. Before that try to answer the following SAQ.

10 SAQ 2 Nature and Formation of Soil a) What is the significance of soil colour? ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… b) The term ‘2’ in the colour description, 5YR 6/2 refers to: i) Value ii) Hue iii) Chroma ………………………………………………………………………………………… …………………………………………………………………………………………

1.3.2 Soil Horizons In a profile, different horizons or layers are identified by certain designations assigned after comparison of the properties of the layer with those of the soil material. The soil material taken for comparison is the one which has not yet got affected by soil forming factors. Three kinds of symbols are used in various combinations to designate horizons. These are capital letters, lower case letters, and Arabic numerals. Capital letters are used to designate the master hor izons; lower case letters are used as suffixes to indicate specific characteristics of master horizons; and Arabic numerals are used both as suffixes to indicate vertical subdivisions within a horizon and as prefixes to indicate discontinuities.

Designations for Horizons Of the several horizons, the master horizons are the results of the fundamental soil humification: forming processes, viz. humification, eluviation and illuviation. The master horizons accumulation of humus (decomposed organic and layers of soils are represented by the capital letters O, A, E, B, C, and R. Here, O matter) represents organic horizon at the surface; A, E, B, and C are the mineral horizons while the symbol R is for the hard bedrock. These soil horizons are not necessarily eluviation: of present in every soil. organic matter and salts

illuviation: deposition of ‘O’ is the organic horizon formed from the organic litter derived from plants and clays animals and fresh or partially decomposed organic material.

‘A’ is the mineral horizon consisting of organic matter accumulation and has lost clay, iron, or aluminium with resultant concentration of quartz or other resistant minerals of or size.

‘E’ is the mineral horizon in which the main feature is the loss of silicate clay, iron, aluminium, or some combination of these, leaving a concentration of sand and silt particles. These horizons exhibit obliteration of all or much of the original rock structure.

‘B’ horizon is characterised by an accumulation of silicate clays, iron and aluminium oxides, gypsum and calcium carbonate. These materials come from the upper layers or formed through a weathering process.

‘C’ is a mineral horizon or layer, excluding bedrock, that is either like or unlike the material from which the solum is presumed to have formed, relatively little affected by pedogenic processes and lacking properties diagnostic of O, A, E or B horizons.

11 Soil Subordinate Distinctions In addition to the above mentioned designations, master horizons are further characterised by specific properties such as distinctive colour or the accumulation of materials such as clays and salts. These subordinate distinctions are identified by using lower case letters that designate specific characteristics. A list of these subordinate distinctions and their meaning is given in Table 1.1. The word "accumulation" is used in many of the definitions in the sense that th e horizon must have more of the material in question than is presumed to have been present in the parent material. A Bt horizon is a B horizon characterised by clay accumulation (t); likewise in a Bk horizon carbonates (k) have accumulated. When more than one suffix is used, the following letters, if used are written first: a, e, i, h, r, s, t and w.

Table 1.1: Lowercase letter symbols used to designate subordinate distinctions within master horizons.

Letter Distinction Letter Distinction

a organic matter, highly p ploughing or other Please note that the information given in decomposed disturbance Table 1.1 is for reference only. There b buried soil horizon q accumulation of silica is no need to memorise these c, cn concretions r weathered or soft bedrock details. d dense unconsolidated s illuvial accumulation of materials silicate clays

e organic matter of intermediate t accumulation of silicate decomposition clays

f frozen soil (permanent ice) v plinthite (high iron, red material due to repeated drying and wetting)

g strong greying (mottling) w distinctive colour or reduction of iron and other structure compounds and development of grey colours due to poor drainage

h illuvial accumulation of x fragipan (brittle with high organic matter bulk density)

i organic matter, slightly y, cs accumulation of gypsum, decomposed calcium sulphate

k, ca accumulation of carbonates z accumulation of soluble salts m strong cementation or indurations

n accumulation of sodium

o accumulation of Fe and Al oxides

12 A good examination with optical aid reveals more detailed features which are helpful Nature and Formation in understanding . This is known as soil micromorphology. This can be of Soil done by hand lens to some extent but rigorous examination needs preparation of a thin Pedogenesis: Origin and section of a selected ped. This ped is impregnated with resins which on drying formation of soil. becomes as hard as rock and can be cut to thin sections for examination under a petrographic microscope. A good deal of information, viz. origin of parent material, Petrographic microscope exogenic processes, soil forming processes and management etc. are is used to examine thin deciphered. sections of rocks and minerals. After getting familiar with the main soil horizons and their notations let us try to understand the development of soil by various processes. Before that try to answer the following SAQ to check your understanding regarding notations used for soil horizons.

SAQ 3 Explain the following horizons: i) Oap ii) Btk ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………..……….

1.4 SOIL GENESIS : ORIGIN AND FORMATION OF SOIL Soil - a product of evolution, is constantly changing as the landscape changes. It is a product of the interaction of five factors: parent material, climate, organisms, topography, and time. The climate and living organisms act on the parent material, while the topography (or local relief) exerts a modifying influence and time allows the soil-forming processes to take place. There are many degrees or variations of soil- forming factors, and the potential for creating different kinds of soil is enormous. However, wherever all elements of the five factors are same under similar environments in different places, soils are similar. A study of soil genesis leads to an understanding of, how soils develop? Why soils differ in their properties and productivity and how soils can be managed for various uses?

The development of soil takes place in two stages. In the first stage there is formation of parent material which is generally consolidated and consists of weathered mineral and/or organic matter etc. The physical and chemical weathering processes discussed in subsection 1.4.2 erode the rocks and into mineral particles and dissolve minerals in solution. Part of these weathered minerals are transported away by volcanoes, wind, water, ice, and waves etc. These ver y factors transport the weathered minerals from other weathered rocks and organic matter on the decayed rocks to create the so called parent material from which the soil is formed. In the second stage the soil forming or pedogenic processes develop the soil over a period of time. The soil genesis can be schematically represented as given in Fig.1.3.

Thus soil genesis embodies two distinct phases. One is weathering that involves disintegration and decomposition of rocks and minerals, the other is the depos ition involving development of soil by pedogenic factors and soil forming processes. It is important to study about rocks and rock minerals to have a better understanding of the weathering process. You will study this in the following subsection.

13 Soil ROCK

Decomposition by Decomposition by Physical Processes Chemical Processes

Weathered Rock Stage-I and Minerals

Transport to other weathered or weathering rocks

Transport from other Organic matter from weathered or plants and animals weathering rocks Parent Material

Stage -II

Soil Forming Pedogenic Processes Processes

SOIL

Fig. 1.3: Schematic representation of the stages in soil genesis.

1.4.1 Minerals and Rocks A mineral is an inorganic body, formed by the process of nature, usually having a definite composition. If formed under favourable conditions, it has a crystalline form and other physical properties. Soil scientists, however, often include inorganic, amorphous (non crystalline) components found in soil within the term soil mineral. Plant scie ntists also use minerals to connote those essential plant nutrients derived from the soil. Based on origin, minerals are classified into two groups, primary and secondary.

The minerals originally formed when once-heated magma (molten rock) cooled and formed solid igneous rocks (explained below) are called as primary minerals e.g., quartz, mica, feldspar, etc. These minerals have gone through little change since they were formed and are found in igneous and metamorphic rocks (explained in the following paragraphs). The Earth's crust continues to form in this manner. While primary minerals are formed at temperatures and/or pressures higher than that normally encountered at the Earth’s surface (one atmosphere, and <100°C), secondary minerals are formed under conditions of temperature and pressure found at the Earth’s surface by the weathering of pre-existing minerals. In this process, involving water as a common participant, elements are released into solution. Some of these elements are leached out of the soil material and end up in ground water, streams and eventually, ocean while some of the released elements are adsorbed and retained on the surface of the fine soil particle and serve as a readily available source of plant nutrients. Other released elements recombine, along with water, to form secondary minerals. Minerals such as silicate clays and iron oxide, which are formed by breakdown and weathering of less resistant materials, are the examples of secondary minerals. Some important primary and secondary minerals are listed in Table 1.2. 14 Table 1.2: Primary and secondary minerals found in soils. Nature and Formation of Soil Primary Minerals Secondary Minerals Name Formula Name Formula

Quartz (SiO2) Geothite (FeOOH) Muscovite [KAl3Si3O10(OH)2] Hemaetite (Fe 2O3) Gibbsite (Al2O3.3H2O) Orthoclase [KAlSi3O8] Clay minerals Aluminium silicates Biotite [KAl(Mg,Fe)3Si3O10(OH) 2] Dolomite [CaMg(CO3)2] Calcite Alibite [NaAlSi3O8] CaCO3 Gypsum Hornblende CaSO4.2H2O [Ca2Al2Mg2Fe3Si6O22(OH)2] Augite [Ca2(Al,Fe) 4(Mg , Fe) 4Si6O24] Anorthite [Ca Al2Si2O8] Olivine [(Mg, Fe)2SiO 4] Mica [K2(Si6Al2)Al4O20(O H)2]

A mass of mineral matter is called rock. It may be composed of one or more minerals. Some rocks are hard and compact e.g. granite and basalt, while others are loose and feebly aggregated such as and loose sand. Each rock possesses certain properties like colour, structure, specific gravity, cleavage and mineralogical make up. These properties can be used to identify a rock-mineral. Rocks are divided into three main classes. These are discussed briefly in the following paragraphs.

Igneous rocks: As mentioned above, igneous rocks are formed by the cooling of magma. Erupting volcanoes produce igneous rocks such as granite, pumice, and obsidian. The igneous rocks constitute nearly 95 per cent of the Earth’s crust and are about sixteen kilometers thick. These massive rocks are mostly crystalline in nature and are referred to as hard rock. These rocks consist mainly of primary minerals of which quartz, feldspars, amphiboles, pyroxenes , olivines, and micas are the essential minerals. Under the right environmental conditions, igneous rocks can change into sedimentary and metamorphic rocks.

Sedimentary rocks: Sedimentary rocks are formed from the breakdown of igneous rocks. These rocks are broken apart by plant roots, ice wedges, and Earth movements and are transported by glaciers, waves, currents, and wind to far away places. These transported particles are cemented together by substances like silica, iron oxide or lime in a consolidated form to give rise to new rocks. In some cases these rocks are formed of substances which were at one time in solution in water and were deposited as rock masses either by cooling, evaporation or by direct chemical precipitation. Sandstone, limestone, and shale are types of sedimentary rocks that contain quartz sand, lime, and clay, respectively. Many of these rocks are deposited in layers or strata and hence they are known as stratified rocks. Some of the more important sedimentary rocks and the minerals commonly dominant in them are given in Table 1.3.

Metamorphic rocks: Igneous and sedimentary rocks that have been altered from their previous condition through the combined action of heat and pressure below Earth's surface are called metamorphic rocks and the process is called metamorphosis. The changes so brought about are both physical and chemical in character, which alter the structural features of rocks. Many times both the structural features and composition of original rocks are affected simultaneously. In some cases the metamorphism is so pronounced that the new rock looks quite different from the original. The action of water tends to remove some material from, or to introduce new material like silica, lime or iron oxides. A loose sand may be turned into a sand stone or a sandstone into quartzite. Some of the rocks and the minerals commonly dominant in them are given in Table 1.3. 15 Soil Table 1.3: Some of the important sedimentary and metamorphic rocks and the minerals commonly dominant in them.

Conglomerate: coarse - Sedimentary rocks Dominant minerals Metamorphic rocks grained rock with rounded fragments that Limestone Calcite [CaCO3] Marble are greater than 2 mm in size. Dolomite Dolomite CaMg(CO3)2 Marble

Sandstone Quartz [SiO2] Quartzite Shale Clays Slate Conglomerate a Varies b Gneiss

a Small stones of various mineralogical make up are cemented into conglomerate.

b The minerals, present are determined by the original rock, which has been changed by metamorphism. Primary minerals present in the igneous rocks commonly dominant are also present.

To conclude we can say that all life on Earth is locked in the minerals and through weathering (subsection 1.4.2) nutrients essential to life are made available. Even life in the sea gets nutrients that are released by weathering on the land, then carried to the sea by rivers. For these reasons, we need to understand what weathering is and how does it take place. You will study this in the next subsection. Before that try to answer the following SAQ.

SAQ 4 a) Name the primary minerals present in an igneous rock. ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………….

b) Name the secondary minerals present in a . ………………………………………………………………………………………… ………………………………………………………………………………………… …………………………………………………………………………………………

1.4.2 Weathering and Soil Formation Weathering refers to the combined activity of destruction and synthesis of rocks. Natural weathering processes occur around us everyday, continually rearranging and building landforms on Earth's surface. In weathering, the rocks are first broken down physically into smaller sizes , may be even to individual minerals of which they are composed, or chemically into new minerals. These subsequently decompose and decay in a variety of ways. The chemical and physical processes of weathering aid each other. A rock mechanically broken into smaller pieces exposes more surface area of the original rock which increases its chemical weathering potential, while chemical weathering of rock makes it softer that eases its physical disintegration.

Weathering and soil development proceed almost simultaneously in case of soft rocks, whereas in case of hard rocks weathering precedes soil development. Two basic processes, mechanical and chemical, are involved in the changes indicated in Fig.1.4. The former is often designated as disintegration , the latter as decomposition . All these processes are discussed briefly as follows:

16 Nature and Formation of Soil Rock

Chemical Processes Physical Processes •Hydration and Hydrolysis •Temperature variations •Dissolution •Action of water, ice and wind •Oxidation •Action of living organisms

Decomposed Rock Disintegrated Rock

Continued Physical Chemical Continued Chemical Processes Processes Processes Physical Processes

Weathered Rock

Fig. 1.4: Pathways of weathering.

Mechanical or Physical Weathering Processes A rock broken into smaller pieces is said to be physically or mechanically weathered. It has a large exposed surface area which further increase its weathering potential. Rocks are broken and degraded by the mechanical action of water, ice, wind, plants and animals in a number of ways as described below. Temperature has an indirect effect in the weathering process. The variation in temperature actually results in mechanical way of weathering. Let us try to understand how does it happen?

Temperature variations: Different minerals present in the rock behave differently to the changing temperatures. The rocks expand on getting heated at day time due to sun’s heat and shrink at night on cooling. The alternating heating and cooling of the rocks generates a kind of stress which eventually causes cracks in the soil. In glacial regions of the world or during forest fires the rocks are exposed to extreme temperatures. Since the conduction of heat is very slow, the temperatures at the surface and the interior of the rock are quite different thereby a kind of lateral stress is generated which may cause the peeling off of the external layers of the rock. This phenomenon is called as exfoliation and is accelerated by freezing of water trapped inside the rock crevices and unloading i.e., the removal of overlying rocks or sediments causing the release of pressure.

Abrasive action of water, ice and wind: Water is an important mechanical weathering The force developed by agent both in liquid as well as in the frozen state. Rain water carrying sediments, ice the freezing of water is glaciers moving under the influence of gravity and wind loaded with dust particles equivalent to about 1465 (dust storms) have enough cutting power to cause abrasion of the mineral particles metric tonnes (mega they come across on the surface of the weathering rock. Further, water can enter into grams per square metre, Mg/m 2) or 150 tons/ft.2 small crevices of rocks where it can freeze in winter season. The frozen water on expansion puts a force which is sufficient to split minerals and rocks. Repeated cycles of freezing and melting of water in rock called as freeze -thaw activity or frost action widen the cracks and may even split the rock apart. Even alternate wetting and drying 17 Soil of rocks may also bring about disruption of layers and finally swelling of layers on wetting.

In dry climates a yet another mechanism operates much like frost action, in this process the ground water is drawn to the surface, which on evaporation leaves behind salts such as halite, calcite, and gypsum. This salt-crystal growth in pore spaces cracks the mineral particles apart. In igneous rocks, such as granite, water can enter into in the fractures between the grains and break up the igneous rock into its separate grains. This is called granular disintegration .

2+ 2+ 2+ − 23−− Solution: Various ions such as Ca , Mg , Fe , C1 , SO44,PO etc. are invariably present in rocks. The water soluble salts are removed by continuous action of flowing water that also percolate through rocks. This process cracks the rocks, so that it is no longer a solid mass and eventually breaks dow n. Lichens: Dual organisms consisting of fungi and algae in mutualistic Plants and animals: The activity of living organisms like lichens, fungi, bacteria, association blue green algae, bryophytes is also responsible for weathering. It is referred to as biological weathering. The lichens are able to extract nutrients from the rocks . They Bryophytes: Lower are capable of retaining water for a long time during which chemical processes take plants growing in shady and moist places place, e.g. alumino-silicates which are important components of rocks are converted to simple clay molecules by the process of hydrolysis and carbonation. Algae also plays an important role because it brings about increase in organic matter due to Humus: Organic matter photosynthesis. The organic matter and CO2 form H2CO3 which dissolves various that has undergone minerals and results in the breaking of rocks. Growing plant roots are capable of extensive decomposition splitting many rocks. Constant digging by animals also adds slowly to physical and is quite resistant to further alteration disintegration. In addition, humus accelerates the slow process of physical weathering by ploughing and cultivation.

Chemical Weathering Processes The chemical process of decomposition starts with the physical disintegration of rocks and minerals. In chemical weathering processes, the minerals in rocks are chemically altered partially or wholly to give secondary minerals. Increasing precipitation (rain) speeds up the chemical weathering of minerals in rocks, as seen on tombs and monuments made of limestone and marble. In fact, water is an essential factor of chemical weathering. Chemical weathering is accelerated by the presence of oxygen, and organic and inorganic acids resulting from the microbial breakdown of plant and animal residues. Increasing temperature also accelerates the chemical reaction that causes minerals to degrade. This is why humid, tropical climates have highly weathered soils, and buildings.

Water: As mentioned above, water is an essential factor of chemical weathering and when it contains dissolved salts and acids it is all the more effective. It enhances the degradation, alteration, and re-synthesis of minerals by causing a wide variety of chemical reactions like hydrolysis, hydration, and dissolution etc. For example, the mineral anhydrite (CaSO4) on hydration, chemically changes to gypsum (CaSO4.2H2O). Gypsum is used in the construction industry. Another example showing the action of water on microcline (KAlSi3O8), a potassium-containing feldspar, is given below:

hydrolysis + − KAlSi 83 + 2OHO   → HAlSi 83 ++ OHKO (solid) (liquid) (solid) (solution) hydrolysis 2HAlSi O83 +11 2 OH   → Al O32 + 6 H SiO 44 (solid) (liquid) (solid) (solution) hydration Al O +3 232 OH   →Al O .3 232 OH (hydrated solid)

18 The above reactions illustrate how different types of reactions (hydrolysis, dissolution Nature and Formation and hydration) involving water convert microcline into hydrated aluminum oxide of Soil (alumina), release potassium - an important plant nutrient and silicic acid (H4SiO4) into the soil.

Acid solution weathering: Weathering is accelerated by the presence of the hydrogen ions in water. Rain water dissolves CO2 from air to form carbonic acid, H2CO3. When this acid comes in contact with rocks that contain lime, soda and potash, the minerals containing calcium, magnesium and potassium in these rocks chemically change into carbonates and dissolve in rain water. For example calcite in limestone reacts as given below:

H2CO3 Ca(OH)2 CaCO3 + H2O

2CaCO3 + H CO32 → 2Ca(HCO )23 calcite (solution) (solution) (solid)

As bicarbonates are highly soluble, Ca(HCO3)2 goes into solution. Further, stronger It may be mentioned here acids, like nitric and sulphuric acid present in soils through acid rains and some that the whiteness of Taj organic acids generated in the soil also cause decomposition e.g., marble stone Mahal is also decreasing decomposes as given below. because of the conversion of marble into CaSO4. + 2+ 2CaCO3 + 2H → Ca(HCO )23 + Ca

2 2−+ Ca + SO4 → CaSO4

Oxidation: Weathering by oxidation is common in rocks which are rich in the elements that can be easily oxidised e.g. iron. Iron as Fe2+ present in a mineral is oxidised to Fe3+ state. This change in the oxidation state causes an adjustment leading to a less stable mineral which is subject to both disintegration and decomposition. In other words, when ions such as Fe2+ are removed or are oxidised within the minerals, the rigidity of the mineral structure is weakened and the mechanical breakdown is made easier. This provides a favourable environment for further chemical reactions. Increased temperatures and the presence of precipitation will accelerate the oxidation process.

After weathering, the soil undergoes further changes. These changes are in fact numerous and complex. The fac tors which bring about these changes are dealt in the next subsection. You can study those after solving the following SAQs.

SAQ 5 Name the important reactions which take place in chemical process of weathering? ………………………………………………………………………………………… ………………………………………………………………………………………… …………………………………………………………………………………………

SAQ 6 Name the orgranisms which take part in biological process of weathering? ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… 19 Soil 1.4.3 Factors Affecting Soil Formation Parent materials formed due to weathering of rocks are transported from the place of The parent materials their origin and redeposited before they are subjected to further modification. We must transported are named clearly understand that weathering and soil formation or developments are not two according to the main force distinctly separate processes; these may be consecutive and may even be overlapping. responsible for the transport The point where rock weathering ends and soil formation begins is not always clear; and redeposition. For however, soil body is the final product of these processes. As mentioned before, soil example, soil transported by water and deposited along development or soil genesis as it is called, is the result of a number of factors such as with stream banks is named parent material, topography, time, climate and biosphere and collectively known as alluvium. If it is deposited soil formers. by gravity at the base of a strong slope, it is called colluvium etc. According to Jenny, soil formation is a function of all these factors and can be expressed as:

S = f (cl, o, r, p, t. . . . )

where S, soil formation is the function of climate (cl), organisms(o), topography or relief (r),parent material (p) and time or age of the land (t). Jenny emphasised that a soil property is determined by the relative influence of these factors. He specified temperature and rainfall as climate; flora and fauna as biosphere organisms; elevation, slope and depth of water table, as relief. Parent material is the unconsolidated and more or less chemically weathered mineral or organic matter from which the solum of soils is developed by pedogenic processes. These factors are generally divided into two categories: i) Passive factors or passive soil formers and ii) Active factors or active soil formers which are discussed below .

Passive Factors or Passive Soil Formers Passive factors are by and large slow acting factors and include parent material, topography and time. Let us briefly discuss each of these in the following paragraphs.

Parent material: Parent material refers to the primary material from which the soil is Parent material: the formed. The parent material may be either a solid rock, a decomposed rock, organic state of soil system at time zero of soil material or a deposit from water, wind, glaciers, volcanoes, or material moving down a formation - Jenny slope. In general, the influence of the climate agents in soil formation is so pronounced that the influence of the parent material is suppressed i.e., the parent material plays a passive role in soil development. Different rocks or parent materials placed under similar climatic conditions give rise to soils which are very similar to one another. On the other hand, similar rock formations or parent materials, placed under different or dissimilar climatic conditions, give rise to diff erent types of soils. Though the nature of changes taking place is more or less the same under different climatic conditions, the extent, rate and direction of changes and their extent of completion vary with the nature of climatic elements. Hence, the same original rock or parent material gives rise to different soils in different regions.

There are instances where the effect of climate is subdued or masked by the parent material. Soils formed under such conditions where the influence of the parent material is supreme are known as endodynamorphic soils as against the ectodynamorphic soils where the influence of climate has been predominant. Young soils and soils formed on mountain slopes are examples of endodynamorphic soils.

Soil erosion: Wearing Topography: The location of a soil on a landscape i.e., topography or relief of the away of the land surface by running water, wind, land is an important passive factor in soil formation as it can affect the impact of ice or other geological climatic processes. It influences soil formation through retention of water, drainage agents run off, and microclimate i.e., exposure of land surface to the sun and wind. If the rock is very steep its retention capacity is poor. If ditches and holes are Drainage : outflow of there, the capacity is more. Soils at the bottom of a hill will get more water than soils water from soil on the slopes, and soils on the slopes that directly face the sun will be drier than soils on slopes that do not. Further, mineral accumulations, plant nutrients, type of 20 vegetation, vegetation growth, erosion, and water drainage are also dependent on Nature and Formation topographic relief. of Soil

Time: Time plays a very important role in the soil formation as it provides for the soil forming processes to act and cause significant changes in the parent material. Hard The period from the inception or zero point rocks require more time for weathering while soft rocks weather quite fast. Time also of soil development to controls the degree of maturity of a soil body. When the soil body has been acted the present stage is upon by the soil formers for a comparatively longer period of time, and when the soil called as soil age. formation processes are more or less complete, it is known as mature soil. In mature soil, the horizons are usually well developed. In a young or immature soil which is still undergoing pedogenic processes, horizon differentiation is not well marked. In course of time young soils become mature and mature soils sometimes get degraded.

Active Factors or Active Soil Formers It has been recognised for many years that climate is one of the major state factors controlling soil formation. Soil climate regimes which manifest in the combined factors of and soil temperature, are important in a wide range of applications including improvement, precision farming, integrated crop management, and ecosystem management. Besides the climate, the biosphere comprising of flora and fauna is also an important active soil forming factor. Let us briefly study their effect in soil development in the following paragraphs.

Rainfall: Of the various climatic elements that take part in active soil formation, rainfall or precipitation is the most important factor. It supplies water whose movement through the parent material controls the nature of the soil body to a very large extent. As it percolates and moves from one part of the parent material to another, it carries with it substances in solution as well as in suspension. The substances so carried are either redeposited in another part of the parent material when the movement of water stops or leached out and completely removed from the soil body. The movement may take place downward, upward or even lateral. As a result of this, one part of soil body is deprived of some constituents and other enriched.

The movement of water is determined by the nature and amount of rainfall. Where the rainfall is excessive or intense, the constituents carried in solution are completely removed from parent material through percolation, leaching and runoff. On the other hand, if the rainfall is not so intense or is less in amount, the movement of water especially the downward movement, is restricted and the constituents are still retained in soil body. They are leached from upper layers to lower depth where they are re- deposited. When the soil moisture at the surface evaporates causing an upward movement of water, the soluble substances move with it and are translocated to upper layers or even brought to the surface. Rainfall thus brings about a redistribution of substances both soluble and those in suspension in the soil body.

The ratio of rainfall and evaporation is called as P/E ratio, where P stands for precipitation and E for evaporation. If P/E ratio is large the weathering will be quick. Water table is the upper If P/E ratio is less weathering will be slow. In combination with other factors rainfall surface of ground water also determines the depth of water table and in long run would affect the capillary or the level below which movement of water and aer ation. Rainfall influences soil formation indirectly through an unconfined aquifer (in which water table defines the activities of plant and animal life. The nature, type and amount of vegetation the upper water limit) is which it supports in turn modify the effects of rainfall. permanently saturated with water. Temperature: Temperature is another climatic agent which influences the process of soil formation. It affects the P/E ratio. If the temperature is high P \E ratio is poor. If temperature is low P/E ratio is more. Temperature also governs the process of various chemical reactions and also the growth of various .

21 Soil High temperatures hinder the process of leaching and cause an upward movement of soluble salts. Wherever, moisture conditions are favourable, high temperatures favour It is computed by Jenny the growth of luxurious vegetation and at the same time bring about a rapid (1941) that in tropical decomposition of organic matter due to increased activities of microorganisms. Low regions the rate of temperatures, on the other hand, induce leaching by reducing evaporation. They weathering proceeds favour the accumulation of organic matter by slowing down the process of three times faster than in temperate regions and decomposition. Temperature thus controls the rate of chemical and biological nine times faster than in reactions taking place in the parent material and the rate of decomposition of organic arctic. matter.

The effect of climatic agents on the changes taking place in the parent materia l is determined by the ratio of rainfall to temperature. A high rainfall coupled with low temperature, giving a wide ratio, brings about more dissolution and more leaching. By keeping the atmosphere more humid, it checks evaporation. It brings about an accumulation of humus. A low rainfall coupled with high temperature, giving a narrow ratio, on the other hand brings about more evaporation and keeps the atmosphere dry. Leaching and eluviation are restricted and salts are precipitated from their solutions. They either accumulate where they are deposited into lower layers or are brought to or near the surface by the upward movement of water.

Biosphere: Besides the above two types of factors, biosphere (the living environment) is another important active soil forming agent. The activity of living plants and animals and the decomposition of their organic wastes and residues have marked influence on soil development. Differences in soils that have resulted primarily from differences in vegetation are especially noticeable in the transition where trees and grasses meet.

SAQ 7 How do time and temperature work in soil formation? ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………...

1.5 SOIL CLASSIFICATION The purpose of any classification is to organise the knowledge and ideas of a given field in the best possible manner so that the relationships between different properties are understood and it provides means for future developments. Soils had been class ified ever since human beings started using it as a media for plant growth. In those terms it was called good or productive with respect to certain crops or bad or nonproductive with respect to the others; or these were differentiated into heavy soils and light soils etc. In terms of geological parent material it was categorised as sandy or clayey soils etc. The concept of soils as natural bodies was first developed by Russian soil scientist V.V Dokuchaev and his associates when they noted the relationship amongst climate, vegetation and soil characteristics in 1883. After a long gap of many years, soil survey staff of the U.S. Department of Agriculture in cooperation with soil scientists in other countries developed a new comprehensive system of soil classification based on soil properties. This soil classification system used by many countries is called Soil Taxonomy originally published in 1975 by the soil survey staff. In India also we have adopted the same classification system.

The system of Soil Taxonomy is based on soil properties which in turn depend on soil genesis i.e., the way soil is formed. A number of properties like, colour, texture, structure, pH, organic matter, moisture, cation exchange capacity, exchangeable ions (Na, K, Ca, Mg), extractable acidity, and temperature etc. are considered for this 22 purpose. On this easily verifiable basis the soils are arranged into six groups or Nature and Formation categories discussed below in their hierarchical order. of Soil

Order: At this highest level of generalis ation, the soils are distinguished on the basis of the degree of horizon development and the kinds of horizons present. There are Hierarchy of soil taxonomy: eleven orders whose names end in sol (Latin, solum, soil). Order Suborder Suborder: The orders are subdivided into suborders on the basis of physical, chemical Greatgroup and morphological properties that reflect the presence or absence of waterlogging and Subgroup vegetation. Each order has a number of suborders and as many as 47 suborders are Family Series recognised.

Greatgroup: This category contains soils that have same kinds of horizons in the same sequence having similar temperature, moisture regimes and also have similar base status. More than 230 greatgroups are recognised.

Subgroup: Three kinds of subgroups have been recognised; these are typic groups, intergrades and extragrades. The typic groups show the central properties of the greatgroup; the intergrade subgroup shows properties of more than one greatgroup while the extragrade subgroup has properties which do not belong to any greatgroup. More than 1200 subgroups are recognised.

Family: In this category the soils have been grouped together largely on the basis of physical and mineralogical properties that are important for plant growth. On the basis of properties like , temperature regime, mineralogy and thickness of the soil permeable to the roots, as many as 6600 families of soils have been recognised.

Series: The is the most specific unit of classification which consists of soils that are similar in all major profile characteristics. Conc eptually it represents a contiguous polypedon, however in the field it may include aggregates of polypedons and the associated inclusions. In field soil surveys the soil series are sometimes further subdivided into soil phases on the basis of surface soil texture or other characteristics.

1.5.1 Soil Types of India

Soils were classified as urvara (fertile) and anuvara or usara (barren) in the ancient The details of Indian soil period. The soils were then given local names by cultivators like, Matasi, Reh, Regur, types is given for the Chopan etc. In the modern time these are classified from geological point of view by sake of information for the geologists. The major soil groups found in India have been discussed briefly in the those who would be following paragraphs. These are given here to impress upon the variety existing. interested to know little more about these.

Alluvial soils: Alluvial soils, covering the largest area in India (approx. 700,000 km2) cover large parts of Rajasthan, Punjab, U.P., Bihar and West Bengal and extend even into west Assam and north Gujarat. These soils are most important from agriculture point of view . By definition, alluvial soils are formed on parent material transported by different means viz., water, ice, gravity and wind. The soils developed in the western parts of the Indo-Gangetic plains are, however, markedly different from those in the eastern region. The semi-arid climate of the Punjab and adjoining Uttar Pradesh has led to the development of saline and alkaline soils wherever the water is high, in contrast to the absence of such soils in the more humid climate of West Bengal. There is also a greater accumulation of lime carbonates in the western region.

Alluvial soils are characterised by their extreme depth, often several hundred feet, and a grey or greyish brown colour. Their texture varies from sandy to clay loam. Their structure is also variable, being loose, open and free draining in the case of sandy soils and compact and impervious in many clayey soils. Being immature there is no distinct horizon differentiation. At some places, however, the deposition of

23 Soil alternate layers of sandy and clayey materials tends to form different layers though not true horizons.

These soils are considered to be the most fertile of Indian soils, but they are deficient in nitrogen, phosphoric acid and humus and are well supplied with lime. Some of these soils also show deficiency of potash. The pH varies from 7.0 to 8.0. The prominent clay minerals are illite and chlorite.

Black soils: These soils have been formed mainly under semi-arid conditions and are derived from a number of rock formations. Basaltic trap is the most common rock from which they seem to have originated. As their name implies these soils are black or dark brown in colour. The colour however, varies considerably from light brown in the case of murmad soils to blackish brown as in the case of deep alluviums of the Narmada and Tapti. Their depth also varies considerably, from a few inches in highly eroded soils to several feet, some times more than 20 feet in thickness particularly of those formed on banks of rivers. They include soils locally known as Regur or black cotton soil. Their texture ranges from sandy loam to heavy clay.

These soils are on the whole low in fertility constituents being deficient in nitrogen, phosphoric acid and humus. They are rich in lime and most of them contain sufficient potash. They are widely distributed and extend over later parts of Maharashtra, Saurashtra and Madhya Pradesh. They are also present in parts of Rajasthan, Uttar Pradesh, Andhra Pradesh, Tamil Nadu and Karnataka.

Desert soils: These soils occur in the hot desertic region, extending over approximately 290,000 km in the north-western part of India. They form a major part of Rajasthan, southern part of Haryana and Punjab, and northern part of Gujarat. The soils consist mostly of sand believed to have been derived from old sea coast. They contain large amounts of soluble salts and varying proportions of lime. They have a high pH, and are very poor in fertility constituents.

Forest and hill soils: The hill and fores t soils of the Himalayan region seem to be akin to soils developed in humid temperate regions, and have been found to include podsolised soils, brown Earths and meadow soils.

Laterite and lateritic soils: These soils are derived from the subaerial weathering of several types of rocks, both basic and acid under climatic conditions of alternate wet and dry seasons. They are found mostly in areas of high rainfall. They are light in texture and have an open free-draining structure. They are found mostly in areas of high rainfall and do not retain moisture. There is practically no horizon differentiation in the soil profile. They are deficient in lime and are slightly to moderately acid in reaction. The pH varies from 5.0 to 6.0.

Lateritic soils formed at high levels have a pale red colour, have high gravel content and are poor in all fertility constituents. Those formed at low levels have a darker colour probably due to greater accumulation of humus, a slightly finer texture and are quite well drained. These soils are found all along the west coast of Maharashtra, Karnataka and Kerala, on tops of hills in the Deccan, Madhya Pradesh, and in Orissa along the Eastern Ghats.

Peaty and marshy soils: Peaty soils occur in humid regions and have an accumulation of high organic matter. Such peaty soils containing considerable amount of soluble salts occur in parts of Ker ela and are locally called ‘Kari’ soils. During the monsoon, the soil gets submerged in water. After the monsoon, the water recedes and rice is cultivated. The soils are black, clayey, and highly acidic (pH as low as 3.5), and contain 10 to 40 percent organic matter. The acidity is due to formation of sulphuric acid and decomposition of organic matter under anaerobic condition. Free aluminium and ferrous sulphates are present. 24 Marshy soils of this type occur in the coastal tracts of Orissa, in the Sunderban area of Nature and Formation West Bengal, in the central portion of North Bengal and south east coast of Tamil of Soil Nadu.

Red soils: Red soils are characterised by a rusty red colour due in most cases to the presence of various oxides of iron. They are either formed in situ or from the products of decomposition of rocks washed to a lower level. They include soils locally known as red loam, red sandy soil and red alluvium. Their main features are a light texture, porous structure, absence of lime, and low soluble salts. They are generally poor in fertility constituents such as nitrogen, phosphorus, potash and lime, and are highly deficient in organic matter. They are neutral in reaction, pH ranging from 7.0 to 7.5. The dominant clay mineral in these soils is kaolinite.

Red soils cover very large parts of the country and are found in almost every state. Practically the whole of Tamil Nadu, Karnataka, parts of Andhra Pradesh, Madhya Pradesh, Orissa and Chota Nagpur contain red soils. In the north they extend into the Birbhum district of West Bengal, Santhal parganas of Bihar, the Mirzapur, Jhansi and Hamirpur districts of Uttar Pradesh and Eastern half of Rajasthan.

Saline and alkaline soils: The saline and alkaline soils are known under various names such as Reh, Kallar, in the North, Khar or Khajan in the West and Karl, in the South. Sodium, calcium and magnesium salts are by and large commonly present in these soils. They have originated mainly as a result of the capillary rise of sub and the consequent transfer of soluble salts from lower layers to the surface. Most of these are saline soils but quite a few are saline-alkali and alkali soils. They are mainly found in the black soil region in the south and west, in the Indo-Gangetic alluvium in the North, and in the deltaic and coastal regions all along the East coasts. These soils are characterised by a high salt content in the case of saline soils, and high sodium saturation and high pH in cases of alkali soils. Most of these soils contain fair quantities of lime.

Tarai soils: The word ‘tarai’ in Hindi means moist, indicating thereby that the tarai soils have a wet regime and high water table conditions for most part of the year. The tarai soils are foothill soils and extent in strips of varying widths at the foot of the Himalayas in Jammu and Kashmir, Uttar Pradesh, Bihar and West Bengal. Soils under natural conditions are thickly vegetated. Several types of these soils are highly productive.

The texture of the soils is sandy loam to silty loam. The soils are fertile and with proper drainage, become productive. The relatively dry soils of tarai are recognised as Bhabar soils.

In West Bengal, these soils are brought down by hilly rivers from high mountains and the deposits are mainly sandy, raw humus type and deep black to grey black in colour. The soils are acidic (pH 4.7 to 5.8), poor in bases and available plant nutrients.

SAQ 8

Why are peaty and marshy soils highly acidic? ………………………………………………………………………………………… ………………………………………………………………………………………… …………………………………………………………………………………………

25 Soil 1.6 SUMMARY Let us briefly state the significant points about the nature and formation of soil as discussed in this unit.

Soil has different meanings to different people and it is difficult to give a unique definition for soil. Different definitions including the one given by the Soil Survey Staff of USDA have been discussed. Soil is an important natural resource which needs to be studied so as to be used in the best possible manner. There are two approaches of studying soil; Pedologists study soil as a natural body while Edaphologists consider the various properties of soils in relation to plant production.

Soil morphology is the description of the soil body, its appearance, features, and general characteristics. Study of soil profile from a vertical section of the soil reveals the presence of distinct layers called horizons. These horizons possess relatively homogeneous physical, chemical, and biological properties which provide information of the processes that led to their formation. These horizons are located on the basis of colour differentiation. Munsell notation - a soil colour system based on the three variables of colour viz., hue, value and chroma is assigned to it.

Different horizons and layers in a profile are identified by certain designations. Capital letters, lower case letters, and Arabic numerals are used as symbols for this purpose. The master horizons and layers of soils are represented by the capital letters O, A, E, B, C and R while subordinate distinctions within a master horizon are identified by using lower case letters that designate specific characteristics.

The formation of soil is a consequence of interplay of a number of factors over a long period of time. The study of origin and formation of soil i.e. soil genesis leads to an understanding of the processes of soil development and the reasons for the differences in their properties and productivity. Soil genesis embodies two distinct phases; in the first phase, physical and chemical weathering processes cause the disintegration and decomposition of rocks and minerals while in the second phase, a number of factors collectively known as soil formers lead to the development of soil. The climate and living organisms act on the parent material, while the topography exerts a modifying influence and time allows the soil forming processes to take place.

As a consequence the possible var iability in the soil forms is enormous and it is quite difficult to study all of them. Soil taxonomy attempts to classify soils on the basis of their properties. All possible soil types are put into six groups or categories with the following levels of hierarchy: order, suborder, great group, subgroup, family and series.

Soil types of India have been divided into nine major groups; alluvial soils, black soils, desert soils, forest and hill soils, laterite and lateritic soils, peaty and marshy soils, red soils, saline and alkaline soils and tarai soils

1.7 TERMINAL QUESTIONS 1. What are the main factors responsible for the formation of metamorphic rocks?

2. List the main weathering agents involved in mechanical or physical process of weathering.

3. Describe soil horizons and explain their importance in determining the properties of soil.

4. Distinguish between primary and secondary minerals and give examples of each. 26 5. Explain how water and temperature interact in the physical weathering of rocks Nature and Formation and minerals. of Soil

6. Describe the terms pedon and polypedon.

1.8 ANSWERS

Self-Assessment Questions 1. Pedology is concerned with the study of the composition, distribution and formation of soils as it occurs in nature. On the other hand edaphology is the study of propertie s of soil from the plant productivity point of view.

2. a) Soil colour is significant in soil classification, i.e. the characterisation of soil, interpreting soil properties and understanding soil formation. b) Chroma

3. i) Organic horizon (O) with organic material that has undergone decomposition (a) and has been disturbed or ploughed (p) ii) Mineral horizon (B) that has been stable long enough to allow an accumulation of clay (t) and calcium carbonate (k) to take place.

4. a) Quartz (SiO2), feldspars, amphiboles, pyroxenes and micas are the most common primary minerals in igneous rocks. b) Calcite (CaCO3), dolomite [CaMg(CO3)2], quartz and clays are the secondary minerals in sedimentary rocks.

5. Hydration, hydrolysis, oxidation, reduction and carbonation.

6. Bacteria, fungi, algae, bryophytes and lichens.

7. The time taken for the soil formation may be more in some cases or less in others. Hard rocks require more time as compared to soft rocks and accordingly the formation of mature soil which has clear horizon development depending upon the nature of rock. Temperature is one of the climatic factors which works by affecting the P/E ratio. High temperature causes poor ratio and low temperature causes good ratio. It works by affecting chemical reactions and influencing growth of various . Rainfall is another climatic factor which coupled with temperature, determines the process of soil development.

8. The acidity is due to decomposit ion of organic matter under anaerobic conditions and formation of sulphuric acid.

Terminal Questions 1. Water, heat or pressure or combined action of any two of these or all three.

2. Temperature, water pressure and living organisms.

3. The horizon are O, A, E, B and C. They are indicative of the soil forming process i.e. the genesis of soil. The horizons are specific for a particular type of soil. The horizons define various properties like if there is more of organic matter accumulation, more of minerals or a mixture of the two.

4. Primary minerals are formed by the solidification of molten lava during the formation of Earth e.g. quartz, feldspar. Secondary minerals are recrystallised products of the chemical breakdown of primary minerals e.g. silicates, iron oxides. 27 Soil 5. During day time rocks become heated and at night they cool resulting in differential stresses with every temperature change and eventually causing cracking of rocks. The rocks are warmer on the outer surface than the inner and the water getting into crevices freezes and exerts a high pressure resulting in cracks in rocks.

6. Pedon is a three dimensional body which represents the nature and arrangement of horizons and variability in the properties of soil while a polypedon is a group of similar pedons surrounded on all sides by ‘non-soils’ or by pedons of unlike character.

28 UNIT 2 SOIL QUALITY PARAMETERS

Structure

2.1 Introduction Objectives 2.2 Mechanical Parameters Soil Texture and Methods of Analysis Soil Textural Classes S oil Aggregation and Soil Aeration Soil Water 2.3 Biological Parameters Soil Flora Soil Fauna Beneficial Role of Soil Organisms 2.4 Physico-Chemical Parameters Crystal Structure of Clays Ion Exchange Property of Soils Soil pH – Acidity and Alkalinity 2.5 Summary 2.6 Terminal Questions 2.7 Answers

2.1 INTRODUCTION In the previous unit, you learnt about the nature of the soil in terms of its constituents, appearance and the formation of soil as a consequence of interplay of a number of factors over a long period of time. The formation of soil did not take place in isolation but soil minerals, organisms and other constituents, each contributed into its development in their own way. In the process of its formation, the soil acquires certain properties, determined by these constituents, which in turn determine the quality of the soil. In this unit, we will consider the mechanical, biological and physico-chemical parameters of soil quality, their significance and determination. An understanding of these parameters will help in assessing the productive value of soil and, further it may help in determining the reasons for and ways of retarding the soil erosion a destructive phenomenon as far as the soil itself and its nutrients are concerned. In the next unit, we shall take up the issues of and productivity.

Objectives After studying this unit, you should be able to: • describe the procedure to determine soil texture and soil structure, • explain the role of pore spaces in soils and various ways by which water is held by soil, • explain the role of various macro- and microorganisms in ascertaining th e soil quality and the beneficial roles of soil organisms, • describe the general structure of layer silicate minerals, • describe ion exchange properties of soil, and • explain the various causes and the significance of soil pH.

2.2 MECHANICAL PARAMETERS You have read in Unit 1 that soil is a heterogeneous mixture of organic matter, salts, metal oxides, water and air with silicates as dominant constituents of varying size and composition. The physical properties of soil constituting the mechanical parameters depend to a good extent on all these components. These properties are extremely important in determining how soils can and should be used. Thus the suitability of soil 29

Soil for many uses is also determined. Soil texture is the most important and permanent characteristic physical property of soil which is related to several soil properties such as soil structure, aeration, water holding capacity, nutrient storage, water movement and bearing strength. Soil texture and its analysis are discussed in the following subsection.

2.2.1 Soil Texture and Methods of Analysis Soil texture is the basic property of a soil related to the size of individual mineral particles, which cannot be easily altered. It specifically refers to the relative size of soil particles. The rate and extent of many important physical and chemical reactions in soils are governed by texture because it determines the amount of surface on which the reaction can occur. Different size groups of soil particles, termed as soil separates were arbitrarily chosen by International Society of (ISSS) as clay, fine sand, coarse sand, and gravel. The United States Department of Agriculture (USDA) has further subdivided the sand fraction. The two systems of textural classification are described in Table 2.1

Table 2.1: USDA and ISSS soil textural classification.

USDA Classification ISSS Classification Size(mm) Name Size (mm) Name More than 2.0 Gravel More than 2.0 Gravel 1.0 – 2.0 Very coarse sand 0.5 – 1.0 Coarse sand 0.2 – 2.0 Coarse sand 0.25 – 0.50 Medium sand 0.10 – 0.25 Find sand 0.02 – 0.2 Find sand 0.05 – 0.10 Very fine sand 0.002 – 0.05 Silt 0.002 – 0.02 Silt Less than 0.002 Clay Less than 0.002 Clay

The texture is generally determined by the relative proportions of sand, silt and clay. If various size fractions of the particles could be isolated and their relative proportion determined the soil texture can be defined. Following methods are generally used for the purpose of determining the relative proportion of the particles in a soil sample.

1) Analysis by “Feel” Method Soil texture was recognised from very early times by experienced farmers from the “feel” of moist soil placed between the thumb and the forefinger. The soil may be smooth or fine or it may be coarse or gritty. In the former case the soil is composed dominantly of small sized particles whereas in the later case the particles are of bigger size. Thus the feel of gritty, sticky or smooth characterises sand, clay and silt, respectively. When this soil is extruded between the thumb and the forefinger to make a “ribbon”, the longer the ribbon the more the clay in the sample. The accuracy of the determination by this method depends on the experience of the field worker.

2) Mechanical Analysis Mechanical analysis is actually the particle size analysis which is done to determine the amount of various separates or the groups of particles present in the soil. The following methods are used for this purpose:

i) Sieve Analysis Method As the name suggests sieve analysis is done using sieves and may give a true distribution of soil particles. This method involves passing a soil sample through 30 sieves with successively smaller holes and collecting different fractions. This method Soil Quality Parameters cannot determine individual particle sizes, it only divides the particles into size categories bracketed by the sieve opening sizes. By dividing the mass retained on each sieve by the total mass of the soil taken, the percent of the particles in each size range can be determined. ii) Hydrometer Method Bouyoucos devised a simple and rapid technique using a specially designed hydrometer to determine sand, silt and clay content of soil without separating them. In this method a known amount of the air dry soil (say 50 g of fine soil) is dispersed in a known amount of water containing calgon (sodium hexametaphosphate) solution. After a certain period of time, in this case 40 seconds, when the largest particle (the sand) will have settled on the bottom of the cylinder, the density of the liquid is determined with the help of a hydrometer. This gives a measure of the silt and clay in the soil. So, the 40 sec reading is the reading “grams of silt and clay in suspension”. Similarly, a density measurement is performed at two hours when the silt would also have settled. This two hour reading reads “grams of clay”. The amount of sand is determined by subtracting the 40 second reading from the total amount. iii) Pipette Method The sieve analysis, though is a true particle size analysis, is unable to separate −6 particles smaller than 20 microns. Such particles are separated by the pipette method 1 micron = 10 m −4 or the technique. This method is based on the principle that when soil = 10 cm = 10 −3 mm particles are suspended in water, they tend to sink. Since there is little variation in the density of most soil particles, then according to Stokes law the velocity (V) of settling (also called sedimentation velocity) is proportional to the square of radius (r) of each particle i.e., V = kr2 where k is a constant. Considering the velocity as distance of fall (h) / time (t), we can calculate the travel time for a given diameter of soil particle for a fixed distance (say 10 cm). At the stipulated time a small aliquot of the suspension is pipetted out from the designated distance, oven dried (100 - 105°C) and weighed to find out the amount of a particular fraction.

By examining the result of mechanical analysis we come to know of the percentage of clay, sand and silt in soil, which can be used to assign a textural clas s to a soil. Let us study in the next subsection what are textural classes and how do we assign the textural class for a soil. Before going to that try to answer the following SAQ. Particles of different sizes move at different rates SAQ 1 What is the significance of texture and texture analysis in deciding the soil quality? ………...……………………………………………………………………………… ………………………………………………………………………………………… …………………………………………………………………………………………

2.2.2 Soil Textural Classes On the basis of the proportion of different soil separates present, the soils are classified into different groups called as soil classes. The soil classes are named according to the separate(s) which contribute the most to the characteristics of the soil. Therefore, the soil textural class names convey the overall textural make up of soils and give an indication of their physical properties. Three broad groups of these classes are recognised, these are , and clays. There are specific textural class names within each of these classes (Table 2.2).

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Soil Table 2.2: Basic soil textural class names according to USDA.

Common name Texture Basic soil textural class names Sandy soils Coarse Sand Loamy sand Moderately coarse Sandy loam Fine sandy loam Medium Very fine sandy loam Loamy soils Loam Silt loam Silt Moderately fine Sandy clay loam Silt y clay loam Clay loam Clayey soils Fine Sandy clay Silty clay Clay

Thus we see that these textural class names form a graduated sequence from soils that are coarse in texture and easy to handle to the clay soils which are very fine and Tillage: mechanical difficult to manage. These classes markedly affect physical properties like, soil manipulation of soil for aeration, and ease of tillage. There are two methods used to determine the soil class. many purposes e.g. These are: modifying soil condition for crop production. Field Method: This method has been discussed in subsection 2.2.1 under “Feel” method of analysis. As mentioned earlier, this method involves rubbing of a moist soil sample between thumb and forefinger.

Laboratory Method: The mechanical analysis methods discussed in subsection 2.2.1 have been used by USDA to develop a method for naming the textural class. This is depicted by a triangle shown in Fig. 2.1. For a given composition of soil, the textural class can be named using this triangle.

32 Fig. 2.1: Percentages of sand, silt and clay in the major textural classes.

The sides of the triangle represent mass percentage of clay, silt and sand. The sum of Soil Quality Parameters the percentages of sand, silt and clay at any point in the triangle is 100. To find the textural class on the basis of the mechanical composition of soil, the percentage of one of the separates (say clay) is first located in the diagram and projected towards the centre of the triangle running parallel to the adjacent side. Similarly it is done for any of the other two separates, silt or sand. The point at which the two projections cross will identify the class name. Let us take an example to understand this. Suppose a soil has the following composition of the three separates, 35% clay, 25% silt and 40% sand, then we may use any two of the three values and use the above method to arrive at the soil textural class as indicated by pairs of green lines in the figure. This soil is an example of the class, ‘clay loam’. You can verify this from Fig. 2.1.

The grouping of soil particle distribution into different groups has be en done, though arbitrarily, each group represents a range. The limit does not correspond to a marked and discontinuous change in properties. Thus a particle having 0.019 mm diameter does not show any abrupt difference in property from that of 0.021 mm diameter. This may be expressed by saying that mechanical composition is a continuous function of particle size. Mechanical composition of a few Indian soils are given in Table 2.3.

Table 2.3: Mechanical composition of some Indian soils.

Soil Mechanical Composition (%) Sand Silt Clay Black (Padegano, Maharashtra) 10.2 13.0 76.8 Alluvial (Burden, W. Bengal) 18.5 28.0 53.5 Laterite (Kutaparma, Kerala) 62.5 3.0 34.5 Red (Raipur, MP) 34.9 33.7 31.4 Forest (Dehradun, Uttaranchal) 52.2 16.5 31.3 Red (Coimbatore, TN) 75.9 3.5 20.6 Black (Chittorgarh, Rajasthan) 38.4 44.2 17.4 Red (Yemmiganur, AP) 78.6 7.1 14.3 Alluvial (Ludhiana, Punjab) 71.4 15.0 13.6 Desert (Bikaner, Rajasthan) 85.5 8.4 6.1

As we have read, the term texture is used in reference to the size of soil particles. However, when the arrangement of the particles is being considered the term structure is used. Let us study this in the next subsection. Before going further try to answer the following SAQ.

SAQ 2 Suggest the textural class for the compositions of the following Indian soils using the data given in Table 2.3 and Fig. 2.1.

Indian soil Textural class Black (Padegano, Maharashtra) ……………………….... Alluvial (Burden, W. Bengal) ………………………… Laterite (Kutaparma, Kerala) ……………………….... Red (Raipur, MP) ……………………….... Forest (Dehradun, UP) ……………………….... Red (Coimbatore, TN) ……………………….... Black (Chittorgarh, Rajasthan) ……………………….... Red (Yemmiganur, AP) ……………………….... Alluvial (Ludhiana, Punjab) ……………………….... Desert (Bikaner, Rajasthan) 33

Soil 2.2.3 Soil Aggregation and Soil Structure In the context of soils the term ‘structure’ refers to the shape that the soil takes. It may exist as individual particle or as a combination of primary soil particles into secondary particles called as aggregates. The individual unit of soil structure is called a ped whether as an aggregate or otherwise. An aggregated structure modifies the influence of texture with regard to moisture and air relationships, availability of plant nutrients, action of microorganisms, and root growth. In general there are three broad categories of soil structure. These are: 1) Single grained, where each particle in a as an individual and is not attached to other particles. The examples are sand and silt. 2) Massive, has a dense structure and forms large clods on drying. It is commonly referred to as puddled and generally constitutes C-horizon. 3) Aggregated, is formed by aggregation of primary soil particles viz., sand, silt and clay. The structure type is determined by their shape. These types are described briefly in Table 2.4. The five type of structures based on the shape are spheroidal, platy, prismatic, columnar and blocky.

Table 2.4: Various soil structures and their description.

S.No. Structure Diagrammatic Description Common type aggregate horizon location 1 Spheroidal Rounded aggregates, A horizon 1-10 mm diameter, may be granular or

porous

2 Platy Horizontally layered, A2 horizon thin and flat aggregates occurring in recently deposited clay soils.

3 Prismatic 1-10 mm diameter B horizon (in cell layers)

Vertically oriented 4 Columnar pillars, often six sided, B horizon upto 15 cm in diameter

5 Blocky These are similar to the prisms, however their tops are rounded B horizon Six faced irregular blocks normally with all sides more or less equal in sizes upto 10 cm.

2.2.4 Soil Aeration Whatever may be the structure of aggregate, it introduces pores in the soils. These pores are filled by air and/or water and have a role in the plant growth. Since plants 34 breathe through their roots and if pores are not there, oxygen may not be available to them to breath and they may die. Therefore, the process of soil aeration is one of the Soil Quality Parameters important determinants of productivity. The presence of pores in soil changes its density i.e. the bulk density and the particle density. Bulk density, Db of soils is defined as the mass per unit volume of soil consisting of soil particles and pore air. Particle density, Dp of a soil is the mass per unit volume occupied by soil particles The size of the particles of alone and is expressed as grams per cubic centimeter or as megagrams per meter cube. a given material and the Since the particle density depends upon the chemical composition and crystal arrangement of the soil structure of the mineral particles and is not affected by pore space, it is generally solids have no relationship to the particle density. taken to be 2.65 g cm−3 or 2.65 Mg m −3.

The percentage of solid particles and pore space in a soil may be calculated from the bulk density and particle density if both are expressed in the same units of measurement. The percentage of solid particles in the soil is given as,

 bulk density      100 =× %solids  particle density 

This percentage, subtracted from the total (100 percent) will give the percentage of pore space, hence the formula,

% pore space = 100 − % solids

 bulk density    % pore space = 100 −  ×100  particle density 

particle density − bulk density % pore space = × 100 particle density

Effects of Soil Aeration

Some soil pores are full of O2 and/or CO2 gas. These gases take part in the respiration of plants. We know that respiration involves the oxidation of organic compounds as follows:

C6H12O 6 + 6O2 6CO2 + 6H2O

Another important consequence of soil aeration is oxidation-reduction reactions that take place in soil and this may decide the oxidation state of various ions in a soil. If 3+ 4+ −−2 soil is well aerated, oxidised forms of Fe, Mn, N and S as Fe , Mn , NO34,SO 2++22+− dominate. If soil is poorly aerated the reduced forms i.e. Fe,Mn,NH4 ,S are found.

The colour of the soil gets influenced by the oxidation states of metal ions. Colours such as red, yellow and reddish brown are found under well oxidising conditions. More subdued shades such as grey and blue predominate if insufficient oxygen is present. Imperfectly drained soils are characterised by alternate streaks of oxidised and reduced material. The mottled conditions indicate a zone of alternate good and poor aeration, a condition not conducive to proper plant growth.

Oxidised forms of elements are more desirable for most common crops grown in acid soils and humid regions while the reduced forms of Mn and Fe are toxic to plants in acid soil. In drier soils , opposite is true when Fe2+ and Mn2+ are preferred. In neutral and alkaline dry soil Fe3+, Mn4+ and other micronutrient metal ions get precipitated resulting in their deficiency.

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Soil SAQ 3 Calculate the percentage pore space in soil with bulk density 1.2 Mg/m 3 and a particle density of 2.6 Mg/m3. ………………………………………………………………………………………… ………………………………………………………………………………………… …………………………………………………………………………………………

2.2.5 Soil Water Water, as you are well aware, is the most common substance on earth which is necessary for life. Plants need water continually to satisfy their evapo-transpiration (loss of water through different parts of plants like, leaves etc.) requirem ent, as solvent and for various reactions. The water requirement varies depending upon the atmospheric conditions and nature of plants. of soil can be expressed on mass basis or on volume basis. The common practice is to express water content You would recall that surface tension is defined on the basis of the loss in mass of soil on drying in oven. This mass of water relative as the force per unit length to mass of dry soil is called soil water content (?). perpendicular to a liquid surface. It is also defined Water has an important property called surface tension that markedly influences its as the energy required to behaviour in soils and may be measured by level of water rise in a capillary. Water increase the surface area by one unit by moving the moves up a capillary tube because of the surface tension while the downward pull of molecules from the interior gravity holds it back. of the liquid to the surface. Capillary forces are observed in all moist soils, however, the rate of movement and the rise in height may sometimes be less than one would expect on the basis of soil pore size. The rise may be less due to the following reasons: • Soil capillaries are not continuous and straight. • Capillaries have radius that varies. • Capillaries have air blockage.

A fine textured soil will hold a different amount of water at a given pressure than a coarse textured one. Electrical energy, potential energy and kinetic energy together are referred to as free energy or the energy status of water. The difference between the free energy of soil water and that of pure water in a standard reference state is termed soil water potential. It determines the state and movement of water in soils. It arises due to the position or internal conditions of soil water and is expressed relative to the energy conditions of free water. The potential of free water is arbitrarily taken to be zero. ψ m is always negative. Soil water which is subject to three major force fields involves expense of energy. ψ o can be negative or positive. The force fields affecting the soil water are due to attraction between solid matrix and

ψ g is always positive. water, gravitational field and due to presence of solutes in soils. The corresponding potentials are matric potential ψm, gravitational potential ψg, and osmotic potential ψo respectively.

Moisture Content of Soil As mentioned above the degree of moisture pertains to relative concentration (rather than the absolute amount) of water, independent of the size of sample. In order to find out the moisture content of soil, a sample of moist soil, usually taken from core of field is weighed. It is then dried in oven at a temperature of 100 - 110ºC, and weighed again. The water lost by the soil represents the soil moisture content in the moist sample. The most common means of expressing soil moisture content is the mass or volume of water associated with a given mass or volume of soil solids.

As a result of the rain or , there is continued, relatively rapid, downward 36 movement of some of the water in response to the hydraulic (water flow through soil due to pressure of liquid and that exerted by gravity) gradient, mostly gravity. After Soil Quality Parameters two to three days, this rapid downward movement becomes negligible. The soil then is said to be at its field capacity. As plants absorb water from a soil, they lose most of it through evaporation at the leaf surface. Some water is also lost by evaporation directly from the soil surface. As the soil dries, plants begin to wilt to conserve moisture during the day time. Initially, the plants regain their vigour at night, but ultimately they will remain wilted night and day. Although not dead, the plants are in a permanent wilted condition and will die if water is not provided. The moisture content of the soil at this stage is called wilting coefficient or permanent wilting percentage. This water in soil is found in the smallest of the micropores and around individual soil particles and is not available to higher plants.

Plant-Available Soil Moisture Plant available soil moisture or total available water is the difference between the amount of water held in the plants root zone when soil is at yield capacity and permanent wilting point. Matric potential, ψm influences the amount of soil moisture plants can take up because it affects the amount of water at the field capacity and at the wilting coefficient. These two characteristics, which determine the soil water available to plants are influenced by the texture, structure and organic matter content of the soil. The general influence of texture is shown in Fig. 2.2.

Texture of the soil

Fig 2.2: General representative relationship between soil moisture characteristics and soil texture.

It can be seen from Fig. 2.2 that as fineness of texture increases, there is a general increase in available moisture storage from sands to loams and silt loams. However, clay soils frequently provide less available water than do well granulated silt loams. The comparative available water holding capacities are also shown by this graph.

Most of the benefits of organic matter are attributable to its favourable influence on soil structure and in turn on the volume of soil pores. Although humus has high moisture content at field capacity, its wilting coefficient is also proportionately high. Thus, the contribution of humus towards available moisture is primarily indirect through its effect on soil structure. The presence of salts in soil can reduce the range of available moisture.

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Soil SAQ 4 A clay soil at a field capacity of 35 kg water/100 kg soil may provide less water to plants than loam soil of equal depth at a field capacity of 25 kg water/ 100 kg soil. What is the likely explanation for this difference? ………………………………………………………………………………………… ………………………………………………………………………………………… …………………………………………………………………………………………

2.3 BIOLOGICAL PARAMETERS Biological parameters which include living organisms constitute an important component of soil. Even though organisms form only a fraction of the total soil mass, they play an important role in the fertility of soil. The soil organisms comprise of macroorganisms which are visible to naked eye and microorganisms or microbes which can be seen only with the help of a microscope. The important role of microorganisms was recognised as early as 1838, when J.B. Boussingeutt a French agricultural chemist observed that legumes could utilise the atmospheric nitrogen for their nutrition. It was further confirmed when, M.W. Beigerinch, a Dutch scientist, isolated bacteria from root nodules of legumes. Investigations have shown that soil is not an inert body but a medium pulsating with a large number of microbes that are always viable and active and thus full of life. These organisms transform soil structure as part of their normal activities and add fertilizers and organic matter to the soil.

The soil organisms are classified into two broad groups viz. soil flora (related to plants) and soil fauna (related to animals) which are again subdivided, depending on their size into macroflora, microflora, macrofauna and microfauna. The number of organisms that commonly occur in surface soil (considered to be 15 cm deep) are given in Table 2.5.

Table 2.5: Relative number of some organisms found in surface soils.

Number Organisms Per sq. meter Per gram Bacteria 1013 – 10 14 108 – 109 Actinomycetes 1012 - 10 13 107 – 108 Fungi 1010 – 10 11 105 – 106 Algae 109 –1010 104 – 105 Protozoa 109 –1010 104 – 105 Nematoda 106 – 107 10 – 10 2 Other Fauna 103 – 105 – Earthworm 30 – 300 –

2.3.1 Soil Flora Higher plants are, by and large, the most important group of primary producer organisms in the whole soil. The continuous decay of plants and plant roots contributes to the formation of soil microorganisms and thereby changes the soil properties, viz. soil aggregation, cation exchange capacity, water, air and nutrient retention capacity. The proliferating roots exert tremendous pressure on surrounding soil particles, thereby compacting and aggregating them. When the roots decay, the vacant space makes room for water and air to move in. The microbes in the flora class are bacteria, actinomycetes, fungi and algae. This is also their order of abundance.

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Bacteria, the most abundant soil organisms are primitive, very minute, unicellular Soil Quality Parameters organisms devoid of chlorophyll. Bacteria are most abundant in soil with an average mass of about 1.5 x 10-12 g. Some 300 to 3000 kg of live bacteria are present in each hectare of surface soil. Most of the bacterial cells remain adsorbed on the clay particles and humus part of soil. Their number in a soil depends upon the type of soil and the climatic conditions. Bacteria, in general, prefer near neutral to slightly alkaline medium, between pH 6.5 to 8.00 for their growth. Soil bacteria may be heterotrophic, obtaining their energy and carbon from complex organic substances or autotrophic, obt aining their energy from the oxidation of the organic compounds, their carbon from the carbon dioxide, their nitrogen and other minerals from inorganic compounds.

Actinomycetes, have characteristics which are transitional between bacteria and fungi and are sometimes called fungi-like bacteria. They are heterotrophic aerobic organisms and need plenty of organic material, optimum moisture and aeration for Actinomycetes are also growth. The activity of these organisms is low when pH is below 5.0. Actinomycetes called as ray fungi or thread can degrade all sorts of organic substances, but at a rate slower than that for bacteria bacteria. and fungi due to their slow growth rate. For this reason, organic residues added to soil are first attacked by bacteria and fungi and later on by actinomycetes. Actually, they start func tioning when the readily decomposable organic compounds have been decomposed by fungi and bacteria.

Fungi, are heterotrophic plants, larger than the bacteria. Those that live on the dead tissues of organic substances are called saprophytic. They play a significant role in Fungi may be regarded as soils and plant nutrition. The number of fungi may vary from 105 to 106 per gram of the scavengers which decompose almost anything soil (Table 2.5) and the number is less than that of bacteria but the total mass is of organic nature present in probably of the order of that of bacteria. The environmental factors which influence soil including lignin that the occurrence, distribution and activity of soil fungi are more or less the same as bacteria cannot tackle and those of soil bacteria, except that fungi dominate in acid soils. As these soil many of them serve as food for the bacteria. Humus is organisms are aerobic and heterotrophic, they need an abundant supply of oxygen and primarily formed by these organic matter in soil. Therefore, due to the aeration factor, the number of fungi in organisms. coarse textured soil is more than that in fine textured clay soil. Optimum soil reaction lies between pH 4.5 and 6.5. However, some soil fungi can tolerate a pH as high as 9.0.

Soil algae, are microscopic, chlorophyll containing organisms, being the simplest The thick growth of algal chlorophyllous plants. These are phototrophic aerobic organisms who obtain energy population on rock surfaces from sunlight through chlorophyll and fix atmospheric carbon dioxide and synthesise acts as one of the agents in the weathering of rocks. their own food. Soil algae occur as unicellular organisms or as filaments or as colonies as seen on the surface soil, which have adequate moisture level and receive sunlight. A few algae may be found as heterotrophs below surface soil, some of the algae are symbiotic while others are nonsymbiotic. Moisture and adequate sunlight Symbiotic: An association are the most significant environmental conditions influencing the algal population. where both the partners derive The optimum activity pH range varies from strain to strain. Algae also help in mutual benefit. building up of soil structure, but do not contribute significantly to the biochemical transformation in soil. They contribute a lot to the organic matter level of soil; the blue green algae, in particular, adding nitrogen to the soil. Both these properties are of significance under the tropical conditions in India.

2.3.2 Soil Fauna The soil fauna inhabiting the soil are active partners of the soil flora in the decomposition of plant tissues. When dead leaves fall on moist soil, they are immediately attacked by mites which perforate and fragment the dead plant bodies, thus facilitating microbial entry and offering a larger surface area for microbial action. Soil fauna ingest bacteria with food, which remain active in their digestive tract. The excreta of animals are similarly attacked by flora and fauna. The more common and abundant macro and microfauna are briefly described below.

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Soil Earthworms: There are several species of earthworms, having slender cylindrical bodies with varying diameters. These organisms ingest the dead organic tissues in soil along with the soil material. The mineral soil is digested, and excreted as granular soil aggregates richer in bacteria and mineral nutrients. These organisms only transform the food so as to be more beneficial for the higher plants and are more abundant in soils which are moist, have high organic matter and undecomposed plant residues. They are more common in fine textured neutral soils with high reserves of lime than in the coarse sandy soils.

Moles: Molehills, frequently observed in some fields are composed of subsoil deposited by moles. They need a good reserve of calcium in soil.

Ants : Ants are the most widespread species of insects in soil and consume plant residues, but are more active in humifying insects than plants.

Soil protozoa: These are unicellular but larger than bacteria, the size varying from a few microns to a few centimeters. The protozoa exist in all aerable soils, but numerically form only a small part of the soil population. A fertile surface soil may contain 10,000 to more than a million of protozoa per gram of soil. Soil protozoa tolerate adverse environmental conditions such as moisture, acidity, and climate to a greater extent than other soil microbes. The role of protozoa in soil is not definitely known. As they feed on the bacteria and actinomycetes, they probably help to maintain a favourable balance of the microflora in soil.

Nematodes: These are next to protozoa and are the abundant soil microfauna in soil. They are also called eelworms, thread worms or roundworms. Nematodes that feed on decaying organic matter are called saprophytic; those which feed on earthworms, other nematodes, etc, are predatory, and those feeding on roots of higher plants are parasitic. The saprophytic group is abundant in soils, but those belonging to the other two groups are agriculturally important in the sense that they cause loss of vigour of the root system and make plants growing in nematode infested soil liable to diseases. The damage to crops may be controlled by crop rotation, ploughing, use of organic manures, and oil cakes etc. The most successful method in controlling nematodes is soil fumigation with nematicides.

Viruses : Viruses are the ultramicroscopic parasites which always require a living host for their multiplication. Since they are much smaller than bacteria they can be observed only through an electron microscope. Chemically, the viruses are nucleic acids contained within a shell of protein. The viruses in the soil are known as parasitised bacteria and are named bacteriophages. When they destroy the bacteria of agricultural importance like rhizobium, they attain economic importance. Clay and organic matter in the soil adsorb bacteriophages and thus cause their retention and spread in the soil.

2.3.3 Beneficial Role of Soil Organisms The soil organisms play a significant role in the life cycles of plant and animals through decomposition, synthesis and transformation, in which a group of organisms rather than a single individual is involved. The most important reactions which the microorganisms carry out and have significant bearing on soil properties and plant The conversion of nutrients growth are decomposition of organic matter and synthesis of humic substances in soil, in organic matter into the biological fixation of nitrogen, microbial transformation of nutrients into the mineral mineral inorganic form is inorganic form and granulation of soil. These are discussed in the following termed mineralisation. paragraphs.

Biofertilizers, are the cultures of microorganisms used for inoculating seed or soil or both under ideal conditions to increase the availability of plant nutrients. Their purpose is to supplement chemical fertilizers and not to replace them. Some of the microorganisms fix nitrogen to be supplied to crops, convert insoluble phosphates to 40

soluble forms to make them available to crops, synthesise biomass for manuring Soil Quality Parameters crops, particularly rice, and hasten the process of decomposition of cellulose in composts and farmyard manures. If these specific organisms are not present in the soil The process of introducing or in the decomposing substrate, they have to be in oculated into the medium to initiate pure or mixed cultures of and accelerate biological activity. microorganisms in natural or artificial culture media is Soil Aggregation : Some organisms may play a beneficial role indirectly by creating called inoculation. Change of atmospheric better soil physical conditions e.g., by improving soil aggregation. You have read compounds in the soil by about the role of earthworms in soil granulation in subsection 2.3.2. Plant roots also microorganism is known promote soil aggregation with the help of their decay product. Gum or polysaccharides produced by certain microorganisms also cause soil aggregation. Azotobacter, Baijerinckia and Rhizobium are examples of gum producing bacteria.

Cultural Practices: Continuous cultivation of a single crop over years on the same site causes accumulation of a particular group of microbes, which dominate over the other. Crop rotation with a legume disturbs the unfavourable population balance. Irrigation, of acid soils (treating with lime to decrease acid), gypsum application to soil significantly increase bacterial activity. Fertilizers and manures increase crop production and microbial population in soil.

Before proceeding to the next section, try to answer the following SAQ.

SAQ 5

Fill in the blanks with appropriate answers:

i) The soil organisms which can synthesise their own food are ______. ii) Fungi are abundant in ______textured soil due to their ______nature. iii) The most abundant soil organisms are ______. iv) Two important functions carried out by soil microorganisms are, ______and ______.

2.4 PHYSICO-CHEMICAL PARAMETERS You have read that rocks and minerals of the earth’s crust are important contributors in the soil development. The possible combination of various elements found in the earth’s crust gives rise to a large number of minerals. Out of all the elements present, oxygen and silicon are the most abundant and compose 75% of it. That’s also the reason of great abundance of silicate minerals in the crust. The solid phase of soil is broadly composed of inorganic and organic constituents. The inorganic constituents consist of silicates, both of primary and secondary origin, having a definite chemical Colloids: particles with a composition and well defined crystalline structure. They are classified as primary and diameter ranging from secondary minerals. The fraction of secondary minerals is often mixed with primary 1-100 nanometer (nm) minerals and oxides of iron, aluminium and silicon and has a size of less than having a large surface 0.002 mm. This is the clay fraction (inorganic soil colloids) called clay minerals. The area per unit mass organic constituents (organic colloids) are due to mainly the plants and partly the animals. They form only a small fraction and get transformed into new products which get thoroughly mixed up with the soils leading to the formation of soil humus. The surfaces of colloidal fractions of clay minerals and humus are responsible for the cation and anion exchanges between soil particles and growing plant roots. Most of the physical, chemical and biological reactions also take place on these colloidal particles. The inorganic fraction i.e. clay is most reactive being small sized, both physically and physic o-chemically. Let us discuss the structure of only the fundamental units present in the clay minerals.

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Soil 2.4.1 Crystal Structure of Clays Most clay minerals have two types of basic structural units – tetrahedral and 4− octahedral. The tetrahedral unit is SiO 4 in which silicon is equidistant from the four oxygen atoms. The octahedral unit consists of an aluminium, iron or magnesium ion, surrounded by oxygen or hydroxyl ions in the form of an octahedron. The tetrahedral and octahedral units giv e rise to their respective sheets by joining of the units. These structures are depicted schematically in Fig. 2.3. An aluminium-dominated sheet is known as a dioctahedral sheet and the one dominated by magnesium is called trioctahedral sheet.

The tetrahedral and octahedral sheets are the fundamental structural units of silicate clays. They bound together within the crystals by shared oxygen atoms into different layers (Fig. 2.3). This layer structure is responsible for the different physical and chemical properties of clays.

Fig. 2.3: Structure of silicate clays.

4− This common architectural unit (SiO4 ) of silicates is of remarkable stability. The different silicates may be classified primarily according to the manner in which these 4− SiO4 units become associated with one another. This results into continuous chains, sheets or complete three dimensional structures of silicates depending upon the manner in which the extension of linked tetrahedrons takes place in space. The stability of a given structure is maintained by the complex interplay of geometrical and electrical factors involving all the atoms in the final compound. The minerals in which one tetrahedral sheet and one octahedral sheet form the crystal unit, are known as 1:1 type minerals e.g., kaolin, while the crystal unit in which one octahedral sheet is sandwiched between two tetrahedral sheets is called 2:1 type minerals e.g. montmorillonite. When two tetrahedral sheets are present with one shared and one unshared octahedral sheet, a 2:1:1 type of mineral is obtained e.g. , structures with alternate mica layer followed by brucite layer. 42

The silicate clays develop charges because of an imbalance which takes place due to Soil Quality Parameters an exchange of one cation by the other in the crystal structure. The charge which develops may be positive or negative. For example, a positive charge would develop when Al3+ replaces a Mg2+ ion and a negative charge would be developed when it is the other way round. There are some pH dependent charges also which develop due to either an excess or low concentration of OH− and H+ ions. Due to the presence of cations in silicate clays, they are capable of exchanging their ions with the ones in soil solution. The exchange is easy in case the particle size of exchanger is very small and this is true with clays. We will discuss the ion exchange property of clays in the next subsection.

SAQ 6 Differentiate between inorganic and organic soil colloids. ………………………………………………………………………………………… ………………………………………………………………………………………… …………………………………………………………………………………………

2.4.2 Ion Exchange Property of Soils Ion exchange is a reversible process. Soil has capacity to exchange its ions due to its colloidal property. The water existing in the immediate vicinity of clay and humus particles which are both colloidal in nature is considered to be water associated with exchangeable cations. The ion exchange reactions may be cation exchange or anion exchange reactions. The cations adsorbed on the surface are subject to exchange with cations held in the soil solution. For example, a calcium ion held on the colloidal surface is subject to exchange with two H+ ions in the soil solution.

+ Soil — Ca2+ + 2H+ Soil H + Ca2+ H+ The cation exchange The capacity to exchange positively charged ions is called cation exchange capacity capacity is an (CEC), e.g, Ca2+, Mg2+, Na+, K+, NH+ , etc. The cation exchange capacity of a given expression of the 4 number of cation soil is determined by the relative amounts of different colloids in that soil and adsorption sites per unit expressed in centimoles of positive charge per kilogram of oven dry soil. The mass of soil. monovalent cations are replaced more easily than divalent or trivalent cations. The order of preference for some tri, di and monovalent ions, in cation exchange reactions is gener ally as given under.

A13+ > Ca2+ > Hg2+ > K+ > Na+ > Li+

The sandy soils have lower CEC than clay soils because the coarse textured soils are commonly lower in both clay and humus content.

The CEC of most soils increases with pH, below pH 6.0 the charge for clay mineral is relatively constant, while above pH 6.0 the charge on the mineral colloid increases slightly because of ionisation of hydrogen from exposed hydroxyl groups at crystal edges. At very low pH values, however, the CEC is generally low.

The cation exchange property is very significant because it affects the physical and chemical properties of soil. The most important ions being the hydrogen, sodium and calcium. For example, exchange of hydrogen ions affects the soil pH. If there is predominance of exchangeable hydrogen, the soil is acidic as a result Al3+ ions become exchangeable and may be toxic to plants. On the other hand with a higher exchange of sodium ions, soil becomes alkaline. Calcium is capable of formulating stable aggregates of finer particles.

43

Soil The ion exchange data is helpful • in , • indicating nutrient storage capacity of soil, • prediction of salinity hazards, • determining liming requirement, etc.

2.4.3 Soil pH – Acidity and Alkalinity The pH scale has been devised for conveniently expressing the extremely small concentration of H+ found in water. The pH of a soil-water system is an approximate measure of the active fraction of the hydrogen ions present in the soil phase, where they remain undissociated. Soil acidity is caused by ionisable hydrogen ions or protons. The exchangeable H+ ions along with other cations are one of the sources for this as mentioned in the previous subsection. Let us see what are the other sources.

Formation of Acid Soils The soil acidity is mainly caused by hydrogen and aluminium ions, depending upon the degree of existing soil acidity. At very low pH (<5.0) aluminium is soluble and hydrolyses to Al(OH)2+, the H+ ions released are responsible for acidity. At a pH range of about 5.0 and 6.5, aluminium, in presence of OH− ions gets converted to + + Al(OH)2 ions which on further hydrolysis form aluminium hydroxide and H ions, again adding to the soil acidity. The stepwise reactions can be written as follows: 3+ 2+ + Al + H2O → Al(OH) + H

2+ + + Al(OH) + H2O → Al(OH)2 + H + + Al(OH)2 + H2O → Al(OH) 3 + H

The various other ways for the formation of acid soils are as follows: 1. Flowing water or rain water leads to formation of acid soils as per the following reaction.

Na+ + Soil − Water H+− Soil + NaOH

where alkali formed is removed by flowing water or it percolates downward leaving behind acid soil.

2. Rain water dissolves CO2 from atmosphere to form carbonic acid which converts soil to acid soil.

+ + Na − Soil + H2CO3 H − Soil + NaHCO3

In a polluted atmosphere SO2, SO3, NO2 gases are present in air, which dissolve in rain water to form H2SO3, H2SO4 and HNO3 which lead to formation of acid soils. As these acids are very strong acids, more acid soils are formed in presence of these than in presence of carbonic acid.

3. Humus in soil hydrolyses to produce its acid form which undergoes ion exchange with soil to produce acid soils.

2+ + Ca − Humus + H2O H − Humus + Ca(OH)2 H+ − Humus + Na+ − Soil Na+ − Humus + H+ − Soil

4. Nitrogen in fertilizers is ingested by the plants in its nitrate form only. The nitrogen in the fertilizers is converted to nitrates by microbes as follows:

+ + − NH 4 + 2O 2 2H + NO 3 + H2O [ 44

These hydrogen ions undergo ion exchange to make the soil acidic. Thus Soil Quality Parameters inorganic fertilizers make soil acidic. 5. Certain oxidation reactions taking place in soil are associated with the production of H+ ions as follows:

2+ −+ Mn + 2 2OH → MnO2 4H ++ 2e 2+ −+ + 3Fe2 2OH → Fe O32 6H ++ 2e −+ Fe2 + 243 OHO → Fe3 O32 2H ++ 2e

As these hydrogen ions are removed, the equilibrium shifts to right hand side.

Advantages of Acid Soils The acid soils, formed by any of the above processes, behave as very strong acids so much so that pH is quite low even in partial conversion to acid soils. Acid soils have certain advantages for plantation. These are given below: (i) All metal oxides and hydroxides undergo easy dissolution and make metal ions available as nutrients for plant growth. (ii) Carbonates in soil decompose to produce more carbon dioxide, which is essential for the growth of plants. (iii) Acid soils make availability of phosphate fertilizers easy for the plants. (iv) Acid soil decom poses illite and mica present in the soil so that more of potassium ion is also available for plants to grow.

Acidity of soil may have harmful effects on plants due to the effect on enzymatic changes, reduced availability of nutrients like copper, zinc, retarded microbial activity, leaching of some nutrients, etc.

Liming – Decreasing Soil Acidity Acid soils can be managed in two ways, viz. either by growing crops suitable for particular soil pH or by treating the soils with additives which will counteract soil acidity. The former is rather risky, intensive and continuous cropping of aerable crops in humid region will aggravate soil acidity and use of acid producing fertilizers for 20 years on an acid soil (pH 5.5 to 5.6) will render the soil unproductive due to development of more acidity (pH 4.8). On the other hand, limed plots treated with the same fertilizers maintain reasonable good level of productivity. The importance of liming acid soils is to counteract the harmful effects of soil acidity and thus help in managing acid soils.

Acid soils are made more suitable for agricultural use by liming which raises the soil pH. Soil acidity is commonly decreased by adding carbonates, oxides or hydroxides of Ca and Mg referred to as agricultural tones.

Liming, • affects the solubility and availability of most of the plant nutrients and raises the level of exchangeable base status of calcium and magnesium, • influences the nutrient uptake of plants by producing a certain antagonistic effect, • reduces toxic concentration of aluminium and manganese by neutralising the effect, • improves soil structure and promotes root distribution.

The common practice of liming is to apply ground limestone. The reactions that take place by lime application may be expressed as follows:

2+ − CaCO3 + H2O + CO2 Ca(HCO3)2 Ca + 2HCO 3 + 2+ 2+ + H – soil + Ca Ca – soil + 2H 45

Soil − + HCO 3 + H H2CO3 H2O + CO 2 The higher the soil moisture, the more rapid is the rate of reaction. The quantity of liming material to be applied to raise the soil pH to a desired level needs to be assessed. It is neither necessary nor economical to completely neutralise an acid soil production.

The lime requirement of a soil is the amount of a liming material that must be added to raise the soil pH to some prescribed value for a crop usually in the range of 6.0 to 7.0. A number of laboratory methods are available for this purpose. The buffer method of Shoemaker and coworkers for determining lime requirement is being widely used in India. In this method 5g of soil is added to 5ml of distilled water and 10ml of the extractant buffer (1.8g nitrophenol, 2.5ml triethanolamine, 3.0g potassium chromate, 2.0g calcium acetate, 53.2g calcium chloride dihydrate are dissolved in a litre of distilled water and the pH adjusted to 7.5 with dilute NaOH solution) and stirred continuously for ten minutes or intermittently for 20 minutes. The pH of the suspension is then determined. Lime requirement (in terms of pure calcium carbonate) is read from Table 2.6.

Table 2.6: Lime requirement of some of the soil samples of given pH.

Lime required to bring the soil to given pH pH of soil suspension (in tones/acre* of pure CaCO3) pH 6.0 pH 6.4 pH 6.8 6.7 1.0 1.2 1.4 6.6 1.4 1.7 1.9 6.5 1.8 2.2 2.5 6.4 2.3 2.7 3.1 6.3 2.7 3.2 3.7 6.2 3.1 3.7 4.2 6.1 3.5 4.2 4.8 6.0 3.9 4.7 5.4 5.9 4.4 5.2 6.0 5.8 4.8 5.7 6.5 5.7 5.2 6.2 7.1 5.6 5.6 6.7 7.7 5.5 6.0 7.2 8.3 5.4 6.5 7.7 8.9 5.3 6.9 8.2 9.4 5.2 7.4 8.6 10.0 5.1 7.8 9.1 10.6 5.0 8.2 9.6 11.2 4.9 8.6 10.1 11.8 4.8 9.1 10.6 12.4

* Figures to be multiplied by 2.51 for conversion into metric units, i.e., tones/hectare.

The direct and indirect effects of lime applicatio n to acid soils generally are increased crop growth, although their magnitude varies widely under different soil conditions and cropping systems. Legumes are more responsive to lime application in acid soils. In some tracts, rice responds to liming. Application of lime in combination with NPK fertilizers increases the yields of wheat, maize, jowar and jute. The main reason for differential response to lime is that soil reaction references of different crops for normal growth are not identical.

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Formation of Alkaline Soils Soil Quality Parameters Soil alkalinity may be due to high base saturation particularly with sodium and the presence of free carbonates of calcium and sodium. Alkalinity of the soils can be by two ways, 1. The soluble carbonates in the soil hydrolyse as per the following reaction,

+ − Na2CO3 + H2O → 2Na + 2OH + H2CO3

and make the soil alkaline.

2. The soils containing excess of Ca and /or Mg carbonates undergo hydrolysis as,

2+ − CaCO3 + 2H2O → Ca + 2OH + H2CO3

to make the soil alkaline, since H2CO3 is a weak acid.

Disadvantages of Alkaline Soils As most of the plants can assimilate nitrogen in its nitrate form, and oxidation of ammonium fertilizer can take place only in acidic medium, nitrogen will not be available to plants in alkaline medium. All metal ions get precipitated as hydroxides and are not available as nutrients to the plants. Phosphates also get precipitated and are not available as nutrients. In other words growth of plants is retarded in alkaline medium.

SAQ 7 Describe the role of aluminium in enhancing soil acidity. Identify the ionic species involved. ………………………………………………………………………………………… ………………………………………………………………………………………… …………………………………………………………………………………………

2.5 SUMMARY Let us summarise the various aspects of assessing the quality of a given soil discussed in the unit. There are three types of soil quality parameters viz., mechanical, biological and physico-chemical. Mechanical parameters are primarily dependent upon composition of the soil. The relative proportion of the particles of different sizes viz., sand, silt and clay determine the texture of the soil and can be used to assign a suitable soil textural class to it. Soil structure on the other hand is a manifestation of aggregation of different particles into clusters of varying shapes. These structures generate pores in the soil which hold air, water and organisms, each one of which is essential for the plant growth. Soil aeration i.e. presence of air in the soil pores governs the oxidation state of Fe, Mn, N, S, C in soil which in turn affect the colour of soil. The pores hold water which is available through different mechanisms for the growth and development of the plants and also provides a medium to make essential nutrients available for the plants.

The other important soil quality parameter is the presence of flora and fauna in a soil. The flora and fauna, act in a variety of ways and make the nutrients available to plants for growth. Organisms play significant role in the life cycle of plants through a number of processes such as decomposition, synthesis and transformation of various compounds present in the soil. Continuous cultivation of a single crop over years on the same site causes accumulation of particular group of microbes which dominate over the other. Crop rotation disturbs their unfavourable population.

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Soil The third important parameter of a soil quality is related to the physico–chemical properties of soils. The most active part of a soil for any type of the physico–chemical reactions is soil colloid which is composed of clay and humus. Structure of most of the clay minerals and isomorphous substitution in them makes clay and thus the soils to show the ion exchange property. Structure of lateral surfaces which carry positive and negative charges is the cause of anion exchange. Soil humus accounts for high cation exchange capacity. Although acid soil is essential for plant growth yet breakdown of soil clay, due to the excess acidity, may be retarded by liming the soil properly. If soils are not managed properly, the sand content may increase in a short span of time and the 15cm thick layer of good soil which has formed over some million years may get eliminated in a short span of say fifty years.

2.6 TERMINAL QUESTIONS 1. How are soil textural classes categorised and what is their significance?

2. Differentiate between soil water content and plant available soil moisture.

3. Describe three means of characterising soil aeration.

4. Explain why earthworms are said to be the most important soil animals.

5. Why are nematodes agriculturally very important?

6. Which class of microflora would you expect to be most active under following conditions. i) In rice paddies requiring nitrogen ii) in acid soils iii) in the decomposition of the most resistant organic compounds iv ) in fixing atmospheric nitrogen v) where photosynthesis occurs

7. How do you explain the fact that clay soils which commonly have higher pore space than sandy soils are often more poorly aerated than sandy soils.

8. When an acid region soil was first cleared for cropping showed a pH of about 8.0. After a few years of irrigation, crop production declined, the soil aggregation tended to break down and pH went upto nearly 10. What is the likely explanation for this?

2.7 ANSWERS

Self-Assessment Questions 1. Soil texture is one physical property of the soil which does not change over a period of time and its analysis decides the suitability for various users. The analysis also helps in determining the soil structure, aeration, water holding capacity etc.

2. Textural Class Clay Silty clay Sandy clay Silty clay Clay loam Sandy 48 Silt loam

Sandy loam Soil Quality Parameters Loam Loamy sand

(2.6− 1.2) 3. Percentage pore spaces = ×=10053.85 or 2.6 1.2 100%−×=10053.8% 2.6

4. See Fig. 2.2, as clay soil matric potential is high, it holds more water very strongly, as a result quite large amount of it is not available to plant. On the other hand, clay content of loam soil decreases and thus more water is available to plants.

5. i) algae ii) coarse, aerobic iii) bacteria iv) decomposition of organic matter and synthesis of humic substances

6. Inorganic soil colloids consist mainly of secondary minerals, mixed with primary minerals and oxides of Fe, Al and Si. Organic soil colloid is mainly the soil humus developed by plant and animal sources.

7. In acid soils exchangeable aluminium slowly hydrolyses as follow s.

3+ 2+ + Al + H2O → Al(OH) + H 2+ + + Al(OH) + H2O → Al (OH)2 + H + + Al(OH )2 + H2O → Al(OH)3 + H

In the process it provides hydrogen ions to ke ep soil acidic good enough for plant growth.

Terminal Questions 1. The soil textural classes are based on the percentage of the separate(s) which contributes maximum to the characteristics of soil. The textural class indicates the physical properties e.g. soil aeration, tillage etc. and textural make up of a particular type of soil.

2. The mass of water lost relative to mass of dry soil is called soil water content whereas plant available water is the water readily absorbed by plant roots.

3. The three means are (i) redox potential of soil (ii) colour of soil (iii) growth of microorganisms in soil.

4. Earthworms ingest organic matter as well as soil where they mix, granulate and excrete to increase stability of soil aggregates and improve soil aeration. The holes left in soil further increase aeration and drainage.

5. Nematodes are the abundant soil microfauna. Being predatory and saprophytic in nature they cause a loss of vigour of the root system. For this reason soils rich in nematodes become liable to diseases also. The damage to crops done by them may be controlled by crop rotation, summer follow and ploughing, use of organic manures and oil cakes.

6. (i) blue-green algae (ii) soil fungi (iii) soil fungi and soil actinomycetes 49

Soil (iv) symbiotic and nonsymbiotic nitrogen fixing bacteria (v) soil algae

7. Clay soils have higher pore space but clay holds water strongly so that these pore are blocked. On the other hand sandy soil water percolates through sand leaving pores open for aeration.

8. Initially at pH 8 there was lime in the soil. With time calcium was depleted and replaced by sodium ions or sodium carbonate in the soil. This led to disappearance of aggregation and sodium carbonate hydrolysed as

Na2 CO3 + 2H2O → 2NaOH + H2CO3

As H2CO3 is a very weak acid, the soil became strongly alkaline.

50 UNIT 3 SOIL FERTILITY AND PRODUCTIVITY

Structure

3.1 Introduction Objectives 3.2 Plant Nutrients Macronutrients Micronutrients 3.3 Availability of Nutrients in Soils Chemical Methods of Estimating Available Nutrients Soil pH and Nutrient Availability 3.4 Soil Fertility Evaluation Concepts in Soil Fertility Maximum Crop Yields 3.5 Management of Soil Productivity Fertilizers and Fertilizer Management Factors Affecting Fertilizer Requirements Manures Cultural Practices 3.6 Summary 3.7 Terminal Questions 3.8 Answers

3.1 INTRODUCTION In the first unit of this course you learnt about the nature and components of soil and the processes involved in its formation, along with the factors affecting it. The second unit dealt with the physical, biological and physico-chemical parameters of soil as the indicators of soil quality and their determination. In this unit we will take up the fertility and productivity aspects of the soil.

A fertile soil is considered to be the one that produces ab undant crops under suitable environmental conditions. However, a clear distinction between soil productivity and soil fertility should be made. Soil fertility is concerned with the inherent quality of a soil that enables it to provide essential nutrients in quantities and proportions for the growth of specified plants when environmental factors such as light, temperature, moisture and soil physical conditions are favourable. It is also understood that the soil should be free from any toxic substances. Thus the saline soil could otherwise be fertile but the excess of sodium salts would be toxic to plants and disturb balance between Na+, Ca2+, K+ and other nutrient ions. Soil productivity is basically an economic concept and signifies the capacity of soil to produce specified plant or sequence of plants under a specified system of management. Productivity emphasises the capacity of soil to produce crops and should be expressed in yields. Thus soil fertility, good management practices, availability of water supply and a suitable climate contribute towards soil productivity. In other words a soil can be highly fertile i.e., it has ready supply of nutrients yet it may be unproductive, say due to insufficient water supply. Soil fertility denotes the status of plant nutrients in the soil while soil productivity connotes the resultant of various factors influencing crop production, both within and beyond the soil. In this unit soil fertility and soil productivity concepts and their inter-relationship have been explained assuming that soil is of good quality, and availability of water, air and light are good enough for the plant growth.

Objectives

After studying this unit you should be able to : • describe the significance of macro and micronutrients present in soil and their functions, describe the laws of soil fertility evaluation, • 51

Soil • explain the concept of nutrient availability, and • explain the importance of soil management in management of soil productivity.

3.2 PLANT NUTRIENTS The primary source of energy for photosynthesis in plants is the sun. A complete analysis of plants shows the presence of a large number of elements, but only those which provide nourishment to the plant and take part in plant metabolism are considered essential. An element is said to be essential if ,

(i) the deficiency of an element in plant, makes it impossible to complete the vegetative or reproductive stage of its life, (ii) the element is directly involved in the nutrition of the plant, (iii) the deficiency that develops in plants in its absence can be remedied only by that element.

In practice, it becomes difficult to meet all the conditions so as to establish essentiality. This is particularly so for the elements that are required in very small The ionic forms in amounts. To overcome this difficulty an element required for normal growth is called which some of the essential element. These elements have been divided into macronutrients and elements that are micronutrients. An element necessary in large amounts (usually > 50 mg/kg in the absorbed by plants are: * Nitrogen as: plant) for the growth of plant is called a macronutrient and includes nitrogen, carbon, −+ hydrogen, oxygen, phosphorus, potassium, calcium, magnesium and sulphur. An NO,NH 34 element necessary in only extremely small amounts (< 50 mg/kg in the plant) for the * Phosphorous as: plant growth is called a micronutrient. These elements are boron, iron, manganese, − 2− H PO42 , HPO4 zinc, copper, molybdenum and chlorine. Sodium, cobalt, vanadium, silicon, selenium, * Potassium as: K + gallium, aluminium and iodine also belong to this list. Let us briefly discuss about * Calcium as: Ca 2+. both types in the following subsection.

3.2.1 Macronutrients Each element is specific in its function in plant metabolism, however, the exact Macronutrients (~50 mg/kg): functions for a number of them are still not known. These essential elements exist as N,C,H,O,P,K,Ca,Mg and S structural components of a cell, maintain cellular organisations, function in energy transformations and in enzyme reactions.

Carbon, hydrogen and oxygen form about 94% of the dry mass of plants. These elements are obtained from carbon dioxide and water which are converted to carbohydrates photochemically and ultimately transformed to protein and protoplasm. Besides their structural role, they provide energy required for the growth and development of plants by oxidative breakdown during cellular respiration. These elements, however, are not considered to be mineral nutrients. The rest of these are described below.

Nitrogen

Nitrogen is an important plant nutrient which is assimilated by most of the plants as % organic matter = % N x 20 nitrate and ammonium ions. Organic matter is the store house of all nitrogen in the soil. Regardless of the form of nitrogen absorbed by plants, it is reduced to the –N=, NH– , –NH2 forms and then forms more complex compounds and ultimately leads to formation of proteins. In addition, nitrogen is present in many biomolecules such as chlorophyll, nucleotides, phosphatides (phospholipids) and alkaloids, as well as many enzymes, hormones and vitamins. These biomolecules are of great physiological importance in metabolism.

The supply of nitrogen is related to carbohydrate utilisation. If nitrogen supply is insufficient, carbohydrates will be deposited in vegetative cells which will cause them to thicken. Excessive nitrogen fertilization reduces the sugar content of sugar beets. However, if nitrogen is available and conditions are favourable for growth, the 52 carbohydrates are converted into proteins and then protoplasm. A plant deficient in nitrogen, becomes stunted and yellow in appearance. This yellowing, or chlorosis Soil Fertility and usually appears first on the lower leaves, the upper leaves remain green. In case of Productivity severe nitrogen shortage the leaves turn brown and die. Plant growth is thus hindered with marked effect on crop yield. Chlorosis is the condition of failure of chlorophyll Phosphorus development. Colour of the leaves in such a condition Phosphorus, with nitrogen and potassium is classed as a major nutrient element. It is a ranges from light-green constituent of nucleic acids, phytin, phospholipids, cell membrane, chloroplast etc. An through yellow to almost white. adequate early supply of phosphorus in the life of the plant is important in laying down primordia for its reproductive parts. Plants absorb most of the phosphorus as − 2− H PO42 ions and small quantity as HPO 4 ions generally in the ratio of 10:1. The − relative amount of ions absorbed is pH dependent. At lower pH values the H PO 42 ion is absorbed more whereas the higher pH values increase the absorption of the 2− HPO 4 form. The large uptake of phosphorus is coupled with the formation of lateral and fibrous roots which increase the absorbing surfaces for other nutrients. An adequate supply of phosphorus is associated with greater strength of cereal straw. This improves the quality of certain fruits, vegetable and grain crops also.

Phosphorus has also been associated with early maturity of crops particularly the cereals. Its shortage is accompanied by a marked reduction in plant growth. Fodder crops grown on phosphorus deficient soils contain reduc ed amount of this element and thus are of inferior value when fed to livestock. Leguminous crops grown under phosphate deficiency conditions may suffer from nitrogen deficiency as well, since the nodule bacteria function normally only when plants are supplied with adequate amount of phosphorus.

Potassium Potassium is the third major element required for plant growth, generally absorbed as K+ ion. In soil 95 to 99 percent of potassium exists in the lattice of minerals like feldspar, muscovite, biotite and hydrous mica. The Earth's crust has an average potassium content of 2.6 percent. This would easily mean that it contains 40,000 to 50,000 kilograms of potassium per hectare. This amount of potassium is sufficient to last forever. However, this potassium is not readily available, hence money spent for potassium fertilizers is constantly on the increase.

Unlike nitrogen, sulphur, phosphorus, and several other elements, potassium apparently does not form organic compounds in the plant and thus is not an integral part of plant components as protoplasm, fats and cellulose. Its function appears to be catalytic in nature. Despite this, it is essential for the following physiological functions:

• It is vital for photosynthesis. When K+ ion is deficient, photosynthesis declines and respiration increases which lowers the plant’s carbohydrate supply. • It is essential for protein synthesis. • It is important in the breakdown of carbohydrates by a process which provides energy for plant growth. • It helps to control ionic balance. • It is important in the translocation of heavy metals such as iron. • It helps the plant to overcome the effects of diseases. • It is important in fruit formation. • It is involved in the activation of more than 60 enzyme systems which regulate the rates of major plant metabolic reactions. • The process of opening and closing of plant leaf pores (stomata) is regulated by potassium, and thus regulates the use of water efficiently.

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Soil The gross impact of these effects on crop production is manifested in several ways. Perhaps the first visible indication of a potassium deficiency is the development of leaf characteristics. Potassium is a mobile element which is translocated to the Meristematic tissues: younger meristematic tissues if a shortage occurs. As a result, the defic iency tissues consisting of symptoms usually appear first on the lower leaves of plants, progressing towards the dividing cells. top as the severity of deficiency increases. In addition potassium shortage is frequently accompanied by a weakening of the straw grain crop particularly rice and wheat which results in lodging of small grains. Potassium deficiency greatly reduces crop yields, and decreases resistance to plant diseases. The quality of some crops, particularly fruits and vegetables, is decreased with low supplies of potassium.

Calcium Calcium is absorbed as Ca2+ ion and is a constituent of cell wall, an activator of different plant enzymes and is essential for the stability of cell membranes. The specific physiological functions performed by calcium in plants are not clearly defined. It is considered to be essential to the formation of the middle lamellae of cells because of its alleged role in the synthesis of calcium pectate. It has also been suggested that calcium favours the formation of protein and increases the protein content of mitochondria. Since it is a constituent of the cell wall, it therefore helps in increasing stiffness of straw. It acts to neutralise organic acids and thus acts as a detoxifying agent. It also encourages seed production.

Calcium in soils is usually abundant except in acid soils, which occur in humid areas due to excessive leaching. Deficiency of calcium leads to increased nucleotide formation but the synthesis of nucleic acid decreases. Also calcium deficiency in soils leads to accumulation of Al3+, Mn2+ ions in plants in harmful concentrations. When compared with most potassium minerals, calcium bearing minerals are usually much more soluble. As a consequence, there is nearly always more exchangeable calcium on the clay/humus colloid than potassium.

Increasing Ca:K ratio in soil solution depresses the uptake of potassium. Thus one of the harmful effects of over liming may result in decreased uptake of potassium. However, when potassium is in excess, calcium prevents luxury consumption of potassium by the plant and thus avoids wastage.

Magnesium Magnesium helps in translocation of carbohydrates and regulates the uptake of other nutrients, presumably by helping in the formation of phosphorylated compounds. It appears to be related to phosphorus metabolism increasing the efficiency of phosphorus absorption and is considered to be specific in the activation of a number of plant enzymes. Appreciable quantities of it are frequently found in seeds.

Since, each chlorophyll molecule contains magnesium ion, there could be no green plants without magnesium. As magnesium is present in the leaves, stem and roots which fall over the soil and may be used again and again in the growth process so that total supply of magnesium is not in excess but sufficient. However, its deficiency may cause chlorosis.

Sulphur Sulphur is found in small amounts in soil perhaps 0.15% only. A large part of sulphur used by plants comes from decomposing organic matter or from fertilizers. It is required for the synthesis of sulphur containing amino acids, cystine, methionine and cysteine and proteins. Many reactions take place due to the presence of sulphydryl group (- SH) of enzymes. Sulphur is also present in coenzymes such as coenzyme A, thiamine pyrophosphate and biotin. These coenzymes are involved in the metabolism of carbohydrates, fats and proteins. Sulphur is known to stimulate root growth, seed formation and nodule formation. Although not a constituent of chlorophyll, sulphur 54 also helps in its formation. Sulphur also increases the oil content of crops such as flax and soyabeans. Disulphide linkages (-S-S-) have recently been associated with the Soil Fertility and structure of protoplasm, and quantity of sulphydryl group in plants has in some cases Productivity been related to increased cold resistance. Deficiency of sulphur causes a pale green or yellow colour of leaves.

3.2.2 Micronutrients

All the micronutrients except molybdenum, boron and chlorine are available more in Micronutrients acid soils. These elements may limit plant growth either because of insufficient (< 50 mg/kg): amount in the soil or as is more often the case, under some conditions their B, Fe, Mn, Zn, Cu, Mo, availability in soil is reduced. The others are described below: Na, Co, V, Si, Se, Ga, Al, Cl and I. Boron Boron is required in extremely small amounts in the range of 0.01 to 1.0 ppm. The ppm: parts per million, actual amount may vary from crop to crop but seldom exceeds 33-56 kg of borax per it equals the number of milligrams per kilogram. hectare when broadcast (spread in the form of a shower from above) or 4.5 kg when applied in the rows. 1 kg of borax ~ 114 g of boron. • It influences cell development by affecting the polysaccharide formation and regulates translocation of sugars across membranes. • It affects the intake of calcium and its efficient utilisation by the plant. • It acts as a regulator of the K+/Ca2+ ratio. • It is intimately related to the absorption of nitrogen and is necessary in cell division. • It appears to be fixed in plant tissues so that it cannot move to parts where it is most needed. As a result continuous supply is needed to meet plant requirement. • If plant contains a large supply of potassium there is a boron deficiency. Xylem: conducting tissues in plants meant Inadequate availability of boron tends to increase the loss of phosphorus from plant to conduct water and roots and also affects the synthesis of proteins. Symptoms of boron deficiency are mineral salts. dying of growing tips, inhibition of blossoming, disorder in xylem, poor development Phloem: conducting of root system and break down and discoloration of cell walls especially in phloem. tissues in plants meant to conduct prepared It is toxic to virtually all plants when present in small quantities but there is great food material. variability in the susceptibility of plant species to boron toxicity.

Iron Iron is a constituent of cytochromes, haem and non-haem enzymes. It is also essential for chlorophyll formation though is not a part of it. It is involved in several redox reactions in plants and is therefore essential for synthesis of proteins and several metabolic reactions.

Man ganese Manganese performs some functions in photosynthesis and acts to regulate the intake of certain elements and their oxidation states. Several crops have been found to be lower in reducing sugar and sucrose when the Mn2+ supply is limited. Also sugar content from sugarcane is higher where there is ample manganese supply.

An excess of manganese induces chlorosis due to inactivity of iron because of its oxidation by manganese. The deficiency results in a limited development of chlorophyll and hence chlorosis. Excess iron brings about symptoms of manganese deficiency. The optimum ratio of Fe/Mn is 2.0 for plant growth.

Zinc It is concerned with the functioning of sulphydryl compounds, such as cysteine, in regulation of redox potential within the cell. Zinc is sometimes related to the growth promoting substance, auxin. It is generally believed that zinc has something to do with chlorophyll formation but the cause is not known. 55

Soil Copper Copper is known to act as an electron carrier in enzymes which bring about oxidation - reduction and regulates respiratory activity in plants. It is helpful in the reactions brought about by many enzymes. Its deficiency leads to death of young growing tips followed by development of auxillary buds below the dead tips which result in bushy growth.

Molybdenum Molybdenum is intimately related to nitrogen metabolism of plants. It is part of the Symbiotic: An association in which both nitrate reductase system which helps to utilise nitrate for nitrogen requirements prior partners derive mutual to amino acid synthesis. It is also essential for symbiotic and nonsymbiotic nitrogen benefit. fixing organisms.

Chlorine The application of chlorides results in leaves of greater spread, a lighter green colour and with a smoother surface. However, the combustibility of the dried leaves is poor proba bly due to low content of potassium in combination with organic acid. An excess chlorine interferes with carbohydrate metabolism to an extent sufficient to check growth.

SAQ 1 Fill in the blanks with appropriate answers: a) If soil is deficient in phosphorus (i) Crop maturity will be ______. (ii) Leguminous crop will suffer deficiency of ______. b) The nutrient element not an integral component of plants but has catalytic effect in many physiological functions is ______. c) Supply of ______increases the stiffness of straw. d) ______is associated with an early maturity of crops. e) Calcium acts as detoxifying agent to ______. f) The element ______is associated with the greenness of plants. g) Calcium deficiency leads to accumulation of toxic ______and ______ions. h) The element that regulates K/Ca concentration is ______. i) Excess as well as deficiency of ______ion leads to chlorosis. j) Purity of sugar from sugarcane is higher if soil contains sufficient ______.

3.3 AVAILABILITY OF NUTRIENTS IN SOILS The productivity of a crop depends largely on the nutrients discussed in the previous section. It is important to find out methods which will help in estimating the availability of these nutrients in soil. The fraction of nutrient actually available in the soil for plant utilisation is called as available soil nutrient. An evaluation of these You will be studying and available nutrients in the soil is called as soil fertility evaluation. The solid phase, actually doing the made up of soil and other source material for nutrients, is a heterogeneous mixture. determination of a few Each nutrient source contributes to solution phase according to its interaction with elements available to the plant in your lab work for environment. It is difficult to understand this interaction because of the lack of this course given in information about exact chemical nature of the nutrient. In addition, a variety of Block-7. chemical and biological reactions takes place leading to the formation of the solution phase. If we analyse the solution phase by separating it from the solid phase, we get information about the nutrient content of the solution phase at that time. The solid phase may be allowed to equilibrate with fresh water and the solution phase analysed again. This may be repeated for many times till no more nutrient comes into water. The sum of the amounts of nutrients present in successive solution phases may be assumed to represent the quantity of available nutrients. This however is not a practical procedure. Apart from being time consuming, there is uncertainty of the number of times the equilibrium has to be carried out. The amount of nutrients taken 56 up by a plant during its entire span or upto the desired stage of its growth gives the Soil Fertility and correct measure of nutrient availability but this also is time consuming. Productivity

Therefore, to get a quick measure of nutrients available we have to depend upon empirical methods. These are mostly indirect and are not very accurate. Some of these empirical methods are discussed below.

3.3.1 Chemical Methods of Estim ating Available Nutrients Water soluble forms of nutrients are available to plants but forms other than water soluble may also become available to plants. In estimating these nutrients, the choice of the particular extractant is decided finally by establishing a correlation between the amount of extracted nutrient and crop growth yield. Estimation of some of the nutrients is discussed in brief below. A few of these estimations are dealt in detail in Block 7 of this course. In fact you will be carrying out some of these estimations as laboratory work.

Nitrogen - Most of nitrogen is available as organic nitrogen which has to be mineralised to inorganic form to become available to plants. Estimation of organic Experiment 3 in Block 7 carbon is widely used as a measure of available nitrogen in soil. It is calculated as: deals with the estimation of organic carbon as a Percentage of N= Percentage of carbon × 0.05 measure of available nitrogen in soil. Phosphorus/phosphate - Phosphorous is estimated as phosphate. Some methods for extracting phosphorous are as follows:

Dyer method : 1% Citric acid used for extracting available soil phosphate.

Bray and Kurtz method : Phosphate is extracted with a solution of 0.03M NH4F in 0.025M HCl and 0.5M NaHCO3 and solution adjusted to pH 8.5.

You will learn the Olsen's method : Phosphate is extracted with 0.5M NaHCO3 adjusted to pH estimation of N, S, P, and 8.5 with 10% caustic soda. Mn & Fe in Experiments 4, 5, 6 and 7 respectively The extracted phosphate develops colour when treated with ammonium molybdate in Block 7 of this course. and stannous chloride. The colour intensity is measured with the help of a colorimeter.

Potassium - Water soluble and exchangeable potassium is readily available to plants. This is extracted with Morgan reagent (10% sodium acetate in 3% acetic acid solution) or 1M ammonium acetate (neutral). From the extractant, potassium is estimated by flame photometer. In the absence of flame photometer, turbidity method can be used. In this method the turbidity is developed by adding sodium cobaltinitrite and is measured with the help of a photoelectric colorimeter.

Calcium and Magnesium - Exchangeable forms of these ions are considered to be available nutrients. The concentration from extractant is found by titrating against ethylene diamminetetraacetate, EDTA.

Sulphur and Boron - Boron is extracted with hot water. Sulphur is extracted with 500 ppm solution of P solution (prepared from potassium dihydrogen orthophosphate or monocalcium phosphate) and analysed colorimetrically or turbidimetrically.

Soil testing as mentioned above is done to assess the nutrient supplying capacity of the soil. The data obtained as a result of testing is useful for recommending the type and amount of fertilizer and other additives for improving crop production. This tells the fertility levels of assessed area. On the basis of values of available nutrients, N P K, the soils are grouped into classes as low, medium and high. Table 3.1 shows the rating used in the soil testing labs. 57

Soil Table 3.1: Classes of soils on the basis of available nutrient.

Nutrient availability Class of soil

N% P2O5 (kg/ha) K2O (kg/ha) Below 0.5 Below 10 Below 100 Low 0.5 to 0.75 20 to 40 150-250 Medium Above 0.75 Above 40 Above 250 High

There is a greater probability of obtaining a profitable response from the use of fertilizers on soils testing low in an element than on soils testing high in that element.

The amount of a particular nutrient determined analytically does not necessarily represent what is available but is merely taken to be a rough measure of it. For a plant the quantity of available nutrient present does not matter so much. But the rate at which the required quantity is supplied to the root system is very important. The intensity i.e., rate and capacity (amount analysed) determines the actual availability of nutrients to crops and therefore the knowledge of both is necessary. The availability will further depend on the nature and characteristics of crop grown, growing conditions and soils characteristics. No extractant can take care of all the variables and in that sense the methods are empirical.

3.3.2 Soil pH and Nutrient Availability Soil pH is the most important factor which governs the availability of nutrients in soil. All the nutrients are absorbed by plants in their ionic form, since the solubility of these ions is pH dependent, the availability of nutrients in soil is also pH dependent. Fig.3.1 gives an idea of the pH range for availability of different nutrients in soils. The pH range of 6.0 to 7.5 appears to be optimum for the availability of most of the nutrient elements.

Nutrient availability

Optimum range

Fig. 3.1: Effect of pH on availability of nutrients and other elements in soils.

The ammonifier and nitrifier organisms are reactive in pH range of 5.5 to 6.5, below which nitrogen availability decreases. Most of the phosphate gets precipitated in soil in acidic region but nature of species available in soil solution is pH dependent. At low pH, H PO − ion usually exists, at pH 7 HPO 2− dominates while at pH 12 PO 3− 58 42 4 4 will dominate. At intermediate pH any two of the ions coexist depending on the pH. Soil Fertility and Liming of acid soils increases phosphate availability. The simple mechanism may Productivity − involve exchange of H PO42 from insoluble phosphates for the hydroxyl ions. Probably the pH rise towards neutral range causes increased microbial activity resulting in increased phosphate availability. On the other hand, organic matter affect phosphorus availability either as a result of displacement of phosphate from iron and aluminium phosphate by humate or fulvate ions or by lowering redox potential and thus pH. Humic acid forms chelate complexes with iron and aluminium which are water soluble and thereby reduce phosphate fixation. Phosphate availability is maximum in the pH range of 6.5 to 7.5.

Potassium, calcium and magnesium are available in cationic form and their availability is governed by the factors which control exchange equilibria in soil. At pH less than 6.0, the soil is partly base saturated and their availability decreases. At this 2− − 2+ 2+ pH CO 3 gets conver ted to HCO 3 making more of Ca and Mg available from carbonate and phosphate.

The solubility of micronutrient cations is at a maximum at low pH values. As the pH increases the ions of these elements first change into hydroxy l ions and then to insoluble hydroxides or oxides. For example, iron undergoes the following reaction due to pH change.

− − − 3+ OH 2+ OH + OH Fe → Fe(OH) → Fe(OH)2 → Fe(OH)3

Iron, manganese, zinc, aluminium and copper are available at low pH range while the availability of molybdenum is low in acidic range but increases above a pH of 6.5.

SAQ 2 Why micronutrients are available in acid soils and not in alkaline soils? ………………………………………………………………………………………… ………………………………………………………………………………………… …………………………………………………………………………………………

3.4 SOIL FERTILITY EVALUATION

In order to maintain soil fertility, nutrients removed from the soil by crops must be restored by the application of manures and fertilizers. The selection of the right kind of nutrient to be replenished and the proper quantity to be applied is based on the nutrient requirement of the crop and the nutrient supplying capacity of the soil. The assessment of nutrient supplying capacity of a soil is called soil fertility evaluation . Let us first understand a few basic concepts involved in soil fertility.

3.4.1 Concepts in Soil Fertility The soil fertility evaluation can be assessed by having an idea of the amount of nutrient which will be required for the growth of plant. On the basis of numerous experiments two quantitative formulations have been propounded to explain this. These are i) Law of minimum and ii) Law of diminishing return. These are briefly explained below.

Law of Minimum This is one of the earliest hypotheses put forward by von Liebig on the relationship between the amount of plant nutrient in the soil and the growth of the plant. The growth of the plant is directly related to the yield. This law is based on the fact that if a soil contains optimum/ adequate amounts of all but one nutrient element, crop 59

Soil growth is regulated by that single nutrient. This means that the crop growth can be varied by varying the amount of that single nutrient. This law is valid upto the stage at which growth starts decreasing (Fig 3.2).

Law of Diminishing Return A German scientist Mitscherlich calculated the effect of different quantities of various growth factors on the yield of plants by means of a mathematical equation. This equation assumes that a plant, under optimum conditions should give maximum yield. A “theoretical highest possible yield” is assumed for each set of growth conditions when an optimum supply of a given growth factor e.g. a plant nutrient is present. This yield, designated as A, will vary with the supply of other nutrients, climatic conditions, system of cultivation, etc. When the said factor is not at its optimum concentration, then a corresponding decrease in yield takes place and the obtained yield (actual yield) is given as ‘y’.

According to the ‘Law of diminishing return’ the increase in the yield by a unit increase of the deficient factor is proportional to the decrease from maximum i.e., (A−y). The mathematical equation expressing this relationship of growth-to-growth factor is as follows:

log (A−y) = log A – C x ….. (3.1)

Where, C = proportionality constant (an effectiveness factor) and x = quantity of a nutrient applied; when x = zero, y = zero.

On increasing the amount of the nutrient the yield increases and the decrease from the maximum i.e., (A−y) becomes less. Subsequent addition of the nutrient would cause an increase proportional to this decrease from maximum and therefore there would be a lesser increase.

It can also be shown mathematically that by increasing the amount of nutrient (x), the rate of increase in yield becomes smaller and smaller. This is the basic premise of the ‘law of diminishing return’.

Curves showing the relation of available N, P2O5, and K2O to yield in percent of A according to Eq. 3.1 are shown in Fig. 3.2.

95 P2O5; c = 0.6 80 K2O; c = 0.4

60 Standard curve; c = 0.301 N; c = 0.122 40

20

Yield percent of A 0 0.5 0.75 2.20 2.47 3.30 4 5 6 7 8 9 10 10.8

Nutrient in quintals per hectare

Fig. 3.2: A correlation betwee n available nutrient in quintals per hectare and yield in percent of A.

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The Baule unit - A mathematician from Gottingen by the name of B. Baule assisted Soil Fertility and Mitscherlich with his calculations. The quantity of any growth factor (nutrient) which Productivity will produce one half of the theoretical highest yield, A (Fig 3.2) is termed a Baule represented as bu. These quantities for N, P 2O5, K2O in quintals per hectare are 2.47, 0.5 and 0.75 respectively. It should be pointed out that these values include the amounts of the available nutrients in the soil in addition to quantities that are supplied through fertilizers.

3.4.2 Maximum Crop Yields Mitscherlich conducted a large number of experiments with different plants, making use of pots 20 cm in diameter and 20 cm in depth. By adding what he considered a sufficiency of all growth factors but one, and then increasing the amounts of this one factor or nutrient by given increments, he determined the quantity of this nutrient that would give maximum yield. By this procedure it was determined that 3.50 grams of N, 0.70 grams of P2O5 and 1.30 grams of K2O gave maximum yields in these pots. Converting these quantities in quintal per hectare gives 11.15 quintals of N, 2.23 quintals of P 2O5 and 4.14 quintals of K2O as the quantit ies required for maximum crop yields. As a matter of convenience it is considered that the quantity of any growth factor, which will give maximum yield, is 10 Baules. Accordingly, 1 Baule of N is 1/10 of 11.15 quintals per hectare or 1.115 quintals. Likew ise, 0.223 quintals of P 2O5 constitutes 1 Baule of this nutrient and 0.414 quintals a Baule of K2O. Spurway, by similar calculation, determined that 0.0672 quintals constitute 1 Baule of Mg and 8.967 x 10 −3 quintals constitute 1 Baule of S.

In determining maximum crop yield it was thought that a given plant let us say corn, is only inherently capable of producing a given amount of plant tissue regardless of how favourable all exterior factors such as nutrient supply, soil conditions, water supply and climate may be. The maximum yields of several crops are shown in Table 3.2.

Table 3.2: Maximum yields (in bushels) per acre of some crops when all growth factors are at the optimum.

1 bushel = 63 po unds (lbs) Crop Yield Crop Yield 1 bushel (UK) = 36.368 litres 1 bushel (US) = 35.239 litres Corn 225 Potatoes 1550.0

Wheat 171.2 Rice 252.5 Bales: so named because cotton was baled or tied * Oats 395.0 Sugarbeet 54 into big bundles.

Barley 308 Sugarcane 192 *

# 1 bale of cotton in US = 500 lbs Rye 198.0 Cotton 4.6 Average bale = 470 lbs

* yields are in ‘tons’ and # yields are in ‘bales’

Calculation of Maximum Crop Yield Percent It was pointed out previously that 1 Baule of any growth factor is equal in effect on growth to the effect of 1 Baule of any other factor, and further that 10 Baule of any factor gives maximum growth so far as that factor is concerned. Accordingly the effect of any quantity, in Baule units, of any factor on growth can be calculated in terms of percent of maximum crop yield by use of the yield formulae of Mitscherlich, which is

y = 100 – [0.1 x 2(10-x)] ….. (3.2) w here y = percent of maximum crop yield and x = plant nutrient in Baule units per acre. By this procedure Spurway derived the values given in Table 3.3. 61

Soil Table 3.3: Potency of a single plant nutrient in terms of Baule units and percent of maximum crop yield.

Baule Percent of Baule Percent of units maximum units maximu m crop yield crop yield 0.1 4.5 1.8 70.6 0.2 10.9 1.9 72.6 0.3 16.9 2.0 74.4 0.4 22.4 2.2 77.7 0.5 27.6 2.4 80.6 0.6 32.5 2.6 83.1 0.7 37.0 2.8 85.3 0.8 41.2 3.0 87.2 0.9 45.2 3.5 90.9 1.0 48.2 4.0 93.6 1.1 52.3 4.5 95.5 1.2 55.5 5.0 96.8 1.3 58.4 6.0 98.4 1.4 61.2 7.0 99.2 1.5 63.8 8.0 99.6 1.6 66.2 9.0 99.8 1.7 68.5 10.0 100.0

The potential or expected yield of any crop may be calculated by use of the factors given in Table 3.3, if the growth factors in Baule units are known. For example, assume that N supply is 0.7 Baule, P2O5 supply is 1.0 Baule, K2O supply is 2.0 Baule, and climatic and soil factors are 4.5 Baule. The crop to be grown is wheat, the maximum theoretical yield of which is 171.2 bushels (Table 3.2). Using Table 3.3 one can calculate the maximum crop yield as follows:

Factor for N (0.7 Baule) = 37/100 = 0.37 Factor for P2O5 (1.0 Baule) = 48.2/100 = 0.482 Factor for K2O (2.0 Baule) = 74.4/100 = 0.744 Soil factors (4.5 Baule) = 95.5/100 = 0.955

Factor for maximum crop yield = 0.37 x 0.482 x 0.744 x 0.955 = 0.1267

Maximum crop yield = 0.1267 x 171.2 = 21.69 bushels.

It is seen that with this formula one may readily calculate the increase in yield that can be expected from the addition of a given amount of the nutrient which is in shortest supply. Likewise, when the deficiency for one nutrient is supplied, the results to be obtained from additions of other nutrients, as needed, may be determined. Furthermore, the quantities of various nutrients which can be profitably applied may be calculated by giving consideration to the costs of the nutrients and the value of the crop increase obtained. It should also be noted that the addition of a nutrient which is present in considerable quantity, K2O in the above example, will give some increase in yield even if no N is added.

SAQ 3

Assume that N supply in a given soil is 0.9 Baule, P2O5 is 1.9 Baule, K2O supply is 4. 0 Baule and soil factor is 5.0 Baule. The crops to be grown are corn, wheat and sugarcane, calculate the maximum crop yield in each case. Which nutrient should be increased to increase the yield substantially. Hint: Make use of Table 3.2 and 3.3. ………………………………………………………………………………………… …………………………………………………………………………………………

62 …………………………………………………………………………………………

Soil Fertility and 3.5 MANAGEMENT OF SOIL PRODUCTIVITY Productivity The continuous prosperity and well being of the people of any nation is dependent upon several factors, one of the most important being the level of soil fertility. The soil must always be kept in a fertile condition, so as to be produce high yields. If any essential nutrient is lacking, it should be supplied because any deficient nutrient limits the crop yields. The soil fertility should not only be maintained but also constantly improved to reap rich harvest. Fertilizers and manures are necessary for maintaining the soil in a high state of fertility and productivity. It is important to know the relative level of nutrients and nutrient deficiency, if any, in the soil, before any corrective fertilizer recommendation can be made.

3.5.1 Fertilizers and Fertilizer Management Chemical fertilizers do Fertilizers, in a broad sense, include all materials that are added to soils to supply not contain plant certain elements essential to the growth of plants. However, fertilizer usually refers nutrients in elemental to chemical fertilizers and unlike in manures, the nutrient elements in fertilizers are form as nitrogen, present in higher concentrations and in forms which can be readily assimilated by phosphorus or potassium plants. Sulphur, nitrogen, phosphorus and potassium are the macronutrients applied in but the nutrients exist in compounds which commercial fertilizers. Nitrogen, phosphorus and potassium are referred to as the provide the ionic forms fertilizer elements. Being concentrated in nutrient elements the fertilizers have the of nutrients that plants advantage of smaller bulk, economy and ease to transport, storage and handling. can absorb. Moreover, their dose can be adjusted to suit the requirement as determined by soil fertility evaluation. Some of the commonly used fertilizers in India and their available nutrient contents are given in Table 3.4.

Table 3.4: Available nutrient contents of some common fertilizers.

Fertilizer % N % P2O5 % K2O

Straight Fertilizers

Nitrogenous fertilizers 46.6 Urea 26.0 Calcium ammonium nitrate (CAN) 21.0 Ammonium Sulphate 25.0 Ammonium Chloride

Phosphatic fertilizers 16.0 Superphosphate (single) 46.0 Superphosphate (triple) 34.0 Dicalcium phosphate

Potassic fertilizers 48-60 Muriate of potash; KCI 48-50 Sulphate of potash; KSO 2 4 25-30 Schoenite, K SO . MgSO . 6H O 2 4 4 2

Complex Fertilizers Complex fertilizers are those which have two or Diammonium phosphate (DAP) 18.0 46.0 more nutrients in one compound or mixture. Ammonium phosphate sulphate 20.0 20.0

Urea ammonium phosphate 28.0 28.0

Nitrophosphate with potash 15.0 15.0 15.0

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Soil Different grades of complex and mixed fertilizers are being marketed to suit the soil and crops of a region. The recommendations for amount of fertilizers currently being given to the farmers in Delhi region of India are presented in Table 3.5. The recommendations are made on the basis of soil testing. The soil test information is compiled soil-wise or area-wise in the form of soil test summaries. This summary indicates the number of samples falling in the category of high, medium or low N, P or K content.

Table 3.5: Soil test values and recommended fertilizer dose.

Organic Dose Available Dose Available Dose carbon (%) (kg N/ha) phosphorus (kg P2O5/ha) potassium (kg K2O/ha) (kg P/ha) (kg K/ha)

LOW Below 0.20 160* Below 10 80 Below 100 60 0.21-0.40 140 11-20 60 101-150 50

MEDIUM 0.41-0.60 120 21-30 45 151-200 40 0.61-0.80 90 31-40 30 201-250 30

HIGH 0.81-1.0 75 41-55 15 251-300 20 Above 1.0 60 Above 55 10 Above 300 0

* It should be supplemented by Farm Yard Manure (FYM).

The ratings seem rather arbitrary and empirical but are meant to get a reasonable yield. Outputs for different doses of fertilizers are obtained from the response. On the basis of costs of input and outputs, the following parameters may be derived, (i) maximum yield (ii) maximum profit per hectare (iii) maximum rate of return per rupee of investment (iv) minimum profitable application, and used by a farmer to make his choice. Recommendations of fertilizers are valid to the extent that the soil sample collected for testing represents the soil of the area of operation. In case of doubt or wide soil heterogeneity, a large number of soil samples should be tested.

The amount of micronutrients in fertilizers must be much more carefully controlled than the macronutrients. The difference between the deficiency and toxicity levels of a given micronutrient is extremely small. Consequently, micronutrients should be added only when their need is certain and when the amount required is known. When a trace element deficiency is to be corrected, a salt of the lacking nutrient is usually added separately to the soil. Table 3.6 gives a list of some of the common micronutrient carriers.

Table 3.6: Micronutrient carriers commonly used in fertilizers.

Compound Formulae Nutrient composition (%)

Sodium borate (borax) Na2B4O7 .10H2O 11% B Copper sulphate CuSO4 .5H2O 25% Cu Ferrous sulphate FeSO 4 .7H2O 20% Fe Manganous sulphate MnSO4 .4H2O 23-25% Mn (depending on extent of hydration) Ammonium molybdate (NH4)6Mo7O24 .2H2O 54% Mo Sodium molybdate Na2MoO 4 .2H 2O 40% Mo Zinc sulphate ZnSO4.7H2O 35% Zn

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3.5.2 Factors Affecting Fertilizer Requirements Soil Fertility and Productivity The rapid increase in the use of fertilizers is based on the fact that large profits are obtained from the money invested in them. The factors affecting the use of fertilizers are related to soil, climate, crop and agronomic practices. These are discussed briefly below:

Soil factors: Physical condition of the soil, soil fertility and soil reaction are three important soil factors. A poor physical condition does not allow efficient use of fertilizer nutrients due to impeded drainage, restricted aeration and unfavourable soil temperature. The coarse textured soils are usually poorer in available nutrients than fine textured soils and therefore it is necessary to apply nitrogenous, phosphatic and potassic fertilizers more frequently in such cases. The higher the fertility of soil, lower is the response of crops to fertilizers. The soil reaction is very important in selection of the right type of phosphatic fertilizers.

Climatic factors: These include temperature, rainfall and its distribution, evaporation, length of day and growing season. A cool climate and regions of high rainfall require Leaching: Removal of more amounts of ammoniacal nitrogenous fertilizers, as high rainfall causes leaching soil materials in of soil and loss of fertilizer nutrients. Similarly, increased light intensity and length solution. of day increase fertilizer requirement of crops.

Crop factors: Crop responses to fertilizers vary with the nature as well as the variety of the crop which in turn is related to the cation exchange capacity of the roots. With a legume in the crop sequence, nitrogen requirement may be less but the other nutrients must be there for optimum utilisation of nitrogen built up by the legume.

Agronomic factors: These factors are key to efficient use of fertilizers for crop production and include fertilizer responsiveness of crops, spacing, proper dose, time and method of fertilizer application. Fertilizers that supply or produce ammonia when added to the soil tend to increase soil acidity due to the following reaction.

+ + − NH4 + 2O2 2H + NO3 + H2O

In addition to ammonium compounds, materials such as urea which, upon hydrolysis yield ammonium ion, are again a potential source of acidity. The phosphorus and potassium fertilizers commonly used have little effect on soil pH unless they also contain nitrogen. The appropriate acid forming capacity of some fertilizer materials expressed in kg of CaCO3 needed to neutralise the acidity produced by 100 kg nitrogen supplied are as given below:

Fertilizer CaCO3 (kg) Fertilizer CaCO3(kg)

(NH4)2SO4 535 NH4NO3 180

(NH4)2HPO4 500 Cotton seed meal 145

Urea 180 Castrol bomace 90

Continuous and careless use of acid forming fertilizers, may convert a fertile soil to arid soils in short span of two to three years.

3.5.3 Manures The way fertilizers affect soil productivity, org anic matters in soil are also associated with soil fertility. This is because a soil rich in organic matter is often productive and also that the organic manures were the only ways of improving soil from much earlier than the fertilizers came into use. Manures are the organic materials derived from animal, human and plant residues which contain nutrients in complex organic forms. 65

Soil These are evaluated mainly on the basis of their nitrogen content and the amount of organic matter present in them. The two important manures in the category of bulky Farm yard manure: organic manures are farm yard manure (FYM) and compost. The basic ingredients prepared from cattle dung of these are either animal or human excreta containing microorganisms which and urine, waist feeds and fodder and litter (prepared decompose carbonaceous and nitrogeneous materials. There are other categories like in ~ 2 months time). concentrated organic manures, edible cakes and meals from animal wastes belonging to manures.

Compost: obtained as a Manures contain 50 to 80 percent water. A figure of 10 - 2.2 - 8.3 or (10-5-10) NPK in result of microbiological decomposition of organic a ton of manure is often used as a guide. Thus an application of 10 tons per acre residues like farm wastes, would supply 2 to 5 tons of organic matter, which would help to maintain the soil in weeds, cattle dung, litter, better tilth, improve water intake and increase release of carbon dioxide. tree leaves, night soil, slaughter house waste (takes ~ 4 months time). Dry manure though contains 80 percent carbon, yet it is considered primarily as a nitrogen fertilizer and, to a lesser extent, one of potassium. Losses of nutrients from manure are serious. For example, if fermented manure is left to dry on the soil surface after being spread and before ploughing under, as much as 25 percent of the nitrogen may be lost by volatilis ation in one day and as much as 50 percent in four days. For most efficient use manure should be ploughed under the same day it is spread.

The large quantity of carbon dioxide evolved during organic matter decomposition is thought to be important to the release of certain nutrients, particularly inorganic phosphorus. The carbon dioxide dissolves in water and forms carbonic acid and decreases soil pH. This effect would be of greater importance on neutral and alkaline soils. Under such conditions the temporary reduction in pH would increase the rate of release of other elements such as boron, zinc, manganese and iron as well as phosphorus.

Some of the other important functions of organic matter are as follows:

• It acts as a store house of nutrients: nitrogen, phosphorus, sulfur etc. • It increases exchange capacity of soil. • It provides energy for microorganisms’ activity. • It releases carbon dioxide and thus clears soil air pores. • It stabilises soil structure and improves tilth.

It is interesting to note that all these functions except last depend on decomposition. Hence the production of large quantities of residues and their subsequent decay is necessary for a good crop and soil fertility.

SAQ 4 Why does a soil with poor physical conditions and high rainfall require more fertilizer nutrients? ………………………………………………………………………………………… ………………………………………………………………………………………… …………………………………………………………………………………………

SAQ 5 Name the nutrient present in a fertilizer which causes acidity in soils. Also explain how it does so. ………………………………………………………………………………………… ………………………………………………………………………………………… …………………………………………………………………………………………

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3.5.4 Cultural Practices Soil Fertility and Productivity Continuous cultivation of a single crop (monoculture) over years at the same site causes accumulation of a particular group of microbes which dominate over the others. Thus irrigation of soil, particularly in arid regions, brings about a significant proliferation of soil microbes. We know that the microflora in the soil play an important role in the decomposition of dead plants and animal and the recycling of nutrient. Liming of acid soil increases the activity of bacteria and actinomycetes and lower fungal population. Crop rotation i.e., changing the crops disturbs such unfavourable population balances. However, crop rotation and monoculture both have advantages and disadvantages associated with them. These are given below:

Crop Rotation 1. Deep rooted legumes may be grown periodically over all fields. 2. There is more continuous vegetative cover and probably less erosion. 3. Tilth of the soil may become superior. 4. Crops varying in feeding range of roots and nutrient requirements may be grown. 5. Weed and insect control are favoured, although chemicals are becoming increasingly effective. 6. Disease control is favoured; changing the crop residues fosters competition among soil organisms and may reduce the pathogens.

Continuous Cropping or Monoculture 1. The climate may favour one crop or a soil may be especially adapted to one crop. 2. Machinery and building costs are probably lower thereby the profits may be greater. 3. The grower may prefer a single crop and become specialist. Few people can become well enough informed to do an expert job of growing a large number of crops and also produce livestock. Monoculture demands greater skills including pest, soil erosion and soil fertility control. 4. The grower may not wish to be fully occupied with farming the year around.

3.6 SUMMARY Let us summarise the various aspects of soil fertility and productivity discussed in this unit. Soil fertility and productivity are inherent qualities of a soil that need to be improved upon by proper management so as to get better yields of the crop. A large number of elements called as essential elements provide nourishment to the plant and take part in plant metabolism. These elements have been divided into macronutrients and micronutrients depending on whether they are required in large amounts or in only extremely small amounts respectively. These nutrients perform a wide range of functions in the plants and their deficiency affects the growth which is manifested in terms of deficiency symptoms which are specific for a particular nutrient.

The productivity of a crop does not depend only on the presence of the nutrients in the soil but also on the fraction of nutrients actually available, called available soil nutrient, for plant utilization. This nutrient availability depends on a number of factors, soil pH being the most important. A number of chemical methods can be used to estimate available nutrients in a soil and such an evaluation of the available soil nutrients in the soil is called as soil fertility evaluation. Two quantitative formulations called, law of minimum, and law of diminishing return have been put forth to undertake soil fertility evaluation and thereby estimate fertilizer requirements for a certain crop production in a given soil. Even maximum crop production under ideal conditions can be calculated.

The prosperity of any nation depends a great deal on the level of soil fertility and its management. The soil fertility should not only be maintained but constantly improved. The deficiency, if any, of the essential nutrient should be removed because any 67

Soil deficient nutrient limits the crop yields. Fertilizers and manures are necessary for maintaining the soil in a high state of fertility and productivity. Mere addition of fertilizers is not sufficient, one needs to evaluate the fertility level of the soil and then use suitable fertilizers in proper proportion. Addition of nitrogen based fertilizers if not associated with lime may produce so much acid that within a few years a fertile soil may become nonfertile. Further, a number of factors related to soil, climate, crop and agronomic practices are to be considered to manage the productivity of the soil.

In addition the practice of crop rotation should be followed to check the unfavourable population growth of certain soil organisms and to keep the soil stay in good condition. This rotation may be planned in a number of ways but rotation with legumes that supply nitrogen and organic matter to soil is commonly employed. Even farm manure should be used to increase the productivity of soil.

3.7 TERMINAL QUESTIONS

1. Under what conditions an element is called an essential element?

2. Leguminous crop grown under phosphate deficiency conditions may suffer from N deficiency as well. Explain.

3. Why is there more exchangeable calcium than potassium on clay/humus colloid?

4. Explain the law of diminishing return.

5. A soil in Delhi has 0.67% carbon, 50 kg P 2O5/ha available phosphorus and 125 kg K2O/ha of available potassium. What dose of fertilizer will you recommend? Also calculate the weight of some fertilizer that should be applied on this field. (Hints: use Tables 3.4 and 3.5)

6. Why chemical fertilizers should be supplemented by organic manure?

7. In what way is crop rotation good for soil management?

3.8 ANSWERS

Self-Assessment Questions 1. a) (i) delayed (ii) nitrogen b) potassium c) calcium d) phosphorus e) organic acids f) magnesium g) Al3+and Mn2+ h) boron i) Mn2+ j) manganese

2. Because iron, manganese, zinc and copper get precipitated in neutral to alkaline region.

3. Factor for N (0.9 Baule) = 45.2/100 = 0.452 Factor for P2O5 (1.9 Baule) = 72.6/100 = 0.726 Factor for K2O (4.0 Baule) = 93.6/100 = 0.936 Soil factors (5.0 Baule) = 96.8/100 = 0.968

Factor for maximum crop yield = 0.452 x 0.726 x 0.936 x 0.968= 0.297

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Maximum crop yield Soil Fertility and Productivity corn = 0.297 x 225 bushels = 66.825 bushels. wheat = 0.297 x 171.2 bushels = 50.84 bushels sugarcane = 0.297 x 192 tons = 57.02 tons

Since soil and K2O are sufficient for a good crop, nitrogen and phosphorus should be increased above 3 Baule. Say N = 3.5 and P = 4.0 Baule.

The new factor for maximum crop yield = 0.909 x 0.936 x 0.936 x 0.968= 0.771

Maximum crop yield will be

corn = 0.771 x 225 bushels = 173.5 bushels. wheat = 0.771 x 171.2 bushels = 132.0 bushels sugarcane = 0.771 x 192 tons = 148.0 tons

i.e., the change will give maximum yield of 173.5 bu, 132 bu and 148 tons of corn, wheat and sugarcane respectively.

4. A high rain causes leaching of soil and loss of fertilizer nutrients.

5. Fertilizers producing ammonia increase soil acidity. The ammonium ions formed by ammonia, in presence of soil air produce H+ ions responsible for the acidity in soils.

Terminal Questions

1. An element is called as an essential element, if:

a. The plant can not complete its life cycle without it. b. The element is specific in its phys iological function in plants. c. The deficiency that develops in plants in its absence can be remedied only by that element.

2. Rhizobium bacteria infect the roots of leguminous plants and lead to the formation of root nodules. These organisms derive carbohydrates from root tissues of host plant and synthesise nitrogenous compounds to fix nitrogen. Rhizobium bacteria do not grow in soils deficient of phosphorus, which means that in case of phosphorous deficiency, there will be less infection in plant roots and thus lesser nodules and thereby lesser excretion of nitrogen compounds. This will result into a nitrogen deficient soil.

3. Calcium is usually abundant except in acid soils which occur in humid area due to excessive leaching. Compared with K-minerals like mica, Ca-bearing mineral like lime are usually much more soluble. Therefore, there is more exchangeable Ca than K on clay/humus.

4. The increase in the yield by a unit increase of the deficient factor is proportional to the decrease from maximum (pl. see subs ec . 3.4.1)

5. Using Table 3.5 recommended dose corresponding to these values are N= 90 kg N/ha P = 15 kg P2O5/ha K= 50 kg K2O/ha

If fertilizers selected are urea, superphosphate (single) and potassium sulphate their mass can be calculated 100/46 x 90 ≈ 196 kg of urea (since urea contains ~ 46 % of N) 100/48 x 50 ≈ 104 kg of K2O (since K2SO4 contains ~ 48% of K2O) 69

Soil 100/16 x 15 ≈ 94 kg of P2O5 (since superphosphate single contains ~ 16% of P 2O5)

These quantities of fertilizers can be mixed and applied per hectare of field.

6. Exclusive use of chemical fertilizer will eliminate organic matter and thus + organisms from the soil. Important reaction like conversion of NH4 and organic nitrogen to nitrate and numerous other reactions may not be feasible. As a consequence the soil productivity may decrease and soil granulation structure is lost and it may become arid with time.

7. Continuous cultivation of a single crop over the years on the same site causes accumulation of a particular group of microbes which dominates over the other. This decreases soil productivity, rotation of crops disturbs this unfavourable population and help in maintaining the soil productivity.

FURTHER READINGS

1. Chemistry of Water, Earth and Environment, P.S. Sindhu, New Age International (P) Ltd, New Delhi 1998.

2. Test Book of Soil Science, T.D. Biswas and S.K Mukherjee, Tata Mc Graw Hill, New Delhi 1992.

3. Soils, Their Chemistry and Fertility in Tropical Asia R.V. Tamhane, D.P. Motiramani, Y.P. Bali and Roy L. Donature Prentice Hall of India (Pvt.) Ltd. New Delhi 1970.

4. Soil Fertility C.E. Millar John Wiley and sons, New York, 1955.

5. The nature and properties of soils, Nyle C. Brady, Maxwell Macmillan International Editions, New York, 1990.

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