Factors Affecting Soil Fertility Environmental Sciences Essay

1. INTRODUCTION

In Southern Africa the most limiting factor to agricultural productivity is soil fertility (Ramaru et al., 2000). Soil fertility is defined as the condition of a soil that enables it to provide nutrients in adequate amounts and in proper balance for the growth of specified plants when other growth factors, such as light, water, temperature, and physical condition of soil, are favorable (van der Watt and van Rooyen, 1995).

This phenomenon coupled with shortages of rainfall could results in a compounded problem of food shortage and famine. For soil fertility to be sustained, extracted soil nutrients must equal replenished soil nutrients, but in large areas of Africa, more soil nutrients are extracted than replenished (Ndala and Mabuza, 2006); thus, soil fertility and its management play an important role in farm productivity. Farmers, their advisors, and any growers need to know the soil properties which have an influence on soil fertility, some of which include soil texture, structure, organic matter, anion and cation retention, cation exchange capacity, base saturation, bulk density and pH. This information will assist them in managing their soils more efficiently in terms of understanding how the soil will behave under various conditions. This explains that the more properties known for a soil, the better the knowledge on the nature of that soil, (Buol et al., 2003).

Insert a statement that highlights the fact that soils have physicochemical, biological, chemical and mineralogical properties before saying different soils have different properties

Soils vary in their fertility, physico-chemistry and mineralogy and as a result they have different levels of fertility. There are different types of soils with varying fertility status. The following are the four examples of soil types: (i) Soils that are highly weathered for example Ultisols and Oxisols behave differently due to differences in mineralogy, (Hassett and Banwart, 1992). A highly weathered soil may usually contain kaolinite and oxides, and nutrient additions may be necessary to make it suitable for agricultural use, (Zhou and Gunter, 1992). (ii) Soils developed in volcanic material such as Andisols contain amorphous minerals, such as Allophone and Imogolite, which bind strongly with organic matter and iron and aluminum oxides, (Govers, 2002). Although volcanic soils have a great water holding capacity like organic soils, they are well-aggregated, resist erosion and have good drainage like well aggregated Oxisols, and they could be a good source of plant growth when managed properly, (Gruhn et al., 2000).; (iii) Soils that primarily consist of partially decomposed organic matter called Organic soils, (Gruhn et al., 2000). Since organic matter has a tremendous water holding capacity, these soils generally hold a lot of water and perform well in agriculture, (Mathe et al., 2007); (iv) Soils developed in expanding clays called Vertisol. Vertisols are moderatelely weathered shrink-swell soils characterised by high clay content, high cation exchange capacity, and contains Montmorillonite as the dominat mineral (reference). These soils are generally fertile, but carefully managed irrigation may be required, (Parker and Rae, 1998).

Avoid numbering within paragraphs. If you must number the points, then present a numbered or bulleted list

Mention the differences in properties among the different soil types you have presented above and indicate how these differences have resulted in the soils having different fertility levels

Soil fertility is greatly influenced by clay minerals, beside other properties or conditions, that a particular soil type contains, (Douglas, 1989). The properties of the inorganic fraction (e.g clay particles) in soil holds the key to many processes related to soil fertility including: 1) the soils’ ability to hold and release nutrients in plant available form; 2) the ability of the soil to chemically and physically withstand detrimental influences caused by human or natural impact; and 3) the resilience of the soil after periods of degradation caused by erosion or nutrient depletion (Lal, 1997; Sanchez et al., 2003). Thus, fertility and crop production can be high on a soil rich in easily weatherable minerals such as montmorillonites, and illites, whereas a soil dominated by weathering-resistant, nutrient poor minerals such as quartz and kaolinite has a low natural potential for crop production (Borggaard & Elberling, 2004; Buol et al., 2003). The clay-sized particles play a dominant role in holding certain inorganic chemicals and supplying nutrients to plants. To anticipate the effect of clay on the way a soil will behave, it’s not enough to know only the amount of clay in a soil, but it is also necessary to know the kinds of clays present.

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Soil is characterized by ongoing complex interactions involving decomposition of rocks, and organic matter by animals, and microbes to form inorganic compounds in soil. Roots absorb these mineral ions if they are readily available and not ‘tied up’ by other elements or by alkaline or acidic soils, (Tucker, 1999). In general soil fertility varies with plants or the purpose for which the soil is considered for, e.g. agriculture, forestry or forage fodder. Consequently, soil fertility and its management play an important role in farm productivity, (Kayombo and Mrema, 1998).

This study aims to determine soil physico-chemistry and clay mineralogical properties of agricultural soils in selected areas of the Limpopo province andto understand how these interact to influence soil fertility in the —- for agricultural efficiency.

2. LITERATURE REVIEW

The literature of this research is reviewed below, with the latest developments in the area of this research, to identifying, evaluating and synthesizing the existing body of completed and recorded work produced by researchers, scholars and practitioners.

You cannot begin the section on literature review by presenting factors affecting soil fertility. What is soil fertility? How does it affect agricultural productivity?

2.1. FACTORS AFFECTING SOIL FERTILITY

Soil fertility is affected by several factors such as climate, rainfall, soil biological, chemical and physical properties, etc, (Ramaru e t al., 2000).

2.1.1. Climate

Temperature and rainfall are key features of climate, which affect agricultural productivity. is defined as the prevailing weather conditions over an area, (IPPC, 2001). The atmospheric temperature is the degree of heat that is produced by the heating of the earth’s surface, especially the ocean by the energy from the sun. Rainfall is the condensation of atmospheric moisture. Agriculture production depends on rainfall and atmospheric temperature, (ICIMOD/ UNEP, 2007).

Rainfall is affected by the change of atmospheric temperature or global warming. In the recent years scientific research based on reliable world climate data reveal that the climate is being affected by the green house effect and temperature and precipitation are changing globally (IPPC, 2001).

Rainfall affects horizon development factors like the translocation of dissolved ions through the soil, (Maiha, S. 2006). Areas with sufficient rainfall will have greater weathering and greater leaching of soil nutrients and organic matter, and also the enhanced decomposition of organic materials in soils. Insufficient rain and high temperature cause drought, whereas intense rain in short period reduces ground water recharge by accelerating runoff and causes floods, (Maiha, 2006). Both the situations induce negative effects in the agriculture. These factors combine to create soils lacking much organic matter in their upper horizons and therefore resulting in soils with low fertility status.

Temperature influences vegetation cover which in turn influences soil organic matter and the activity of organisms in soil. Hot, dry arid regions have sparse vegetation and hence, top soil horizon usually lacks organic matter, (Sherchan et al., 2007). The soil gets its energy for normal activity from the sun. The amount of energy entering the soil is dependent largely upon the color, the slope, and the vegetative cover of the soil under consideration, (Malla, 2003).

Climate change is a phenomenon due to emissions of greenhouse gases from fuel combustion, deforestation, urbanization and industrialization resulting in variations of temperature and precipitation, (Pathak and Kumar, 2003). Its main effect on soil fertility is through reduced rainfall and seasonal changes, (Regmi, 2007).

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2.1.2. Soil properties affecting soil fertility

Soils have different properties such as physical, chemical, biological and mineralogical properties, some of which greatly influence soil fertility. To manage soil fertility, knowledge and understanding of these properties is required. They are discussed below.

2.1.2.1. Physical characteristics:

(i) Soil texture

Soil texture refers to the relative proportions of the various size groups of individual particles or grains in a soil. It is dependent on the mixture of the different particle sizes present in the soil. Based on these different sizes, soil particles are classidied as sand (0.05- 2mm), silt (0-0,5mm), and clay (<0,002mm), (Rowell, 1994). Soil texture is arguably the single most important physical property of the soil in terms of soil fertility, because it influences several other soil properties including density, porosity, water and nutrient retention, rate of organic matter decomposition, infiltration, cation exchange capacity, etc, (Møberg et al., 1999).

Soil particles with diameter smaller than 0.002 mm (clay) can hold larger quantities of water and nutrients, because of their large surface areas of up to 90 acres/pound of soil and the presence of micro pores that prevent water from draining freely (Brady and Weil, 1999). This high surface area gives nutrients numerous binding sites, which is part of the reason that fine textured soils have such high abilities to retain nutrients.

The knowledge and understanding of the proportions of different-sized particles in a soil is critical to understand soil behavior and their management, they are explained as follows, (Brady and Weil, 1999):

Sand

Sand particles are those with diameters of between 2mm-0.05mm. As sand particles are relatively large, so are the voids between them which promote free drainage of water and entry of air into the soil. Sand particles are considered noncohesive, i.e. they do not tend to stick together in a mass and because of their large size, have low specific surface areas and thus has low nutrient retention capacity. Sand particles can hold little water due to low specific surface area and are prone to drought, therefore has a very low CEC and fertility status.

Silt

Silt particles are those with diameters of between 0.05mm – 0.002mm. The pores between silt particles are much smaller than those in sand, so silt retains more water and nutrients. Soils dominated by silt particles therefore have a high fertility status and provides favorable conditions for the plants growth when other growth factors are favorable.

Clay

Soil particles less than 0.002mm are classified as clay and have a very large specific surface area. This large adsorptive surface causes clay particles to cohere together in a hard mass after drying. The pores between clay particles are very small and complex, so movement of both air and water is very slow. Clay particles are negatively charged because of their mineralogical composition. Soils with such clay particles usually have high CEC and can retain water and plant nutrients. ; thus such soils are considered to be fertile and good for plant growth.

Since clay particles are so small, pure clay has at least 1000 times more external surface area than coarse sand, and because clays have a large surface area and negative charges, they can attract and hold positively charged ions. This characteristic is important because many positively charged ions are plant nutrients, such as calcium, magnesium and potassium, (Miller and Donahue, 1992). A maximum of 30% clay content is desirable to provide favorable conditions for plants growth when other growth factors are favorable.

(ii) Soil structure

Soil fertility is greatly influenced by soil structure, (Foth and Ellis, 1997). The term soil structure is used to describe the way soil particles (sand, silt and clay) are grouped into aggregates, (Rowell, 1994). Soil structure is affected by biological activity, organic matter, and cultivation and tillage practices. Soil structure is important in determining the infiltration rate of water in soils, sealing and crusting of the soil surface, together with aeration and structural stability. Soil aggregation is an important characteristics of soil fertility; the greater the degree of aggregation, the better the soil ’tilth’ and the more the pore space, the more available will be the water and air to plant roots, (Brady and Weil, 1999). An ideal soil structure for plant growth is often described as granular or crumb-like, because it provides good movement for air and water through a variety of different pore sizes and it also affects root penetration, (RCEP, 1996). An ideal soil structure is also stable and resistant to erosion, (Rowell, 1994). Organic matter and the humification process improve structural stability, and can rebuild degraded soil structures. Therefore it is vital to return or add organic material to the soil and to maintain its biological activity in order to enhance soil structure for plant growth.

(iii) Water retention capacity

Water holding capacity refers to the quantity of water that the soil is capable of storing for use by plants, (Brady and Weil, 1999). Soil water is held in, and flows through, the pore spaces in soils. Classification of soil water is divided into three main categories: gravitational, capillary, and hygroscopic is based upon the energy with which water is held by the soil solids, which in turn governs their behavior and availability to plants (Rowell, 1994). Water holding capacity is an important factor in the choice of plants or crops to be grown and in the design and management of irrigation systems, (Brady and Weil, 1999). The total amount of water available to plants growing in field soil is a function of the rooting depth of the plant and sum of the water held between field capacity and wilting percentage in each of the horizons explored by the roots, (Brady and Weil, 1999). The ability of the soil to provide water for plants is an important fertility characteristic, (RCEP, 1996). The capacity for water storage varies, depending on soil properties such as organic matter, soil texture, bulk density, and soil structure, that affect the retention of water and the depth of the root zone, (RCEP, 1996).

(iv) Electrical Conductivity (EC)

Soil salinity is quantified in terms of the total concentration of the soluble salts as measured by the electrical conductivity (EC) of the solution in dSm−1, (Farahani and Buchleiter , 2004).

Electrical conductivioty is an indirect measure of salinity so define and explaine the significance of EC to soil fertility

Soil salinity refers to the presence of major dissolved inorganic solutes in the soil aqueous phase, which consist of soluble and readily dissolvable salts including charged species (e.g., Na+, K+, Mg+2, Ca+2, Cl−, HCO3−, NO3−, SO4−2 and CO3−2), non-ionic solutes, and ions that combine to form ion pairs , (Smith and Doran, 1996 ). The predominant mechanism causing the accumulation of salt in irrigated agricultural soils is loss of water through evapotranspiration, leaving ever-increasing concentrations of salts in the remaining water. Effects of soil salinity are manifested in loss of stand, reduced plant growth, reduced yields, and in severe cases, crop failure, (Eghball, 2002).

By agricultural standards, soils with an EC greater than 4 dS/m are considered saline. In actuality, salt-sensitive plants may be affected by conductivities less than 4 dS/m and salt tolerant species may not be impacted by concentrations of up to twice this maximum agricultural tolerance limit, (Munshower, 1994). Plants are detrimentally affected, both physically and chemically, by excess salts in some soils and by high levels of exchangeable sodium in others. Soils with an accumulation of exchangeable sodium are often characterized by poor tilth and low permeability and therefore low soil fertility status, making them unfavorable for plant growth, (Munshower, 1994).

(v) Bulk Density (BD)

Soil bulk density is an expression of the mass to total volume ratio of a soil, (Brady and Weil, 2004). The total volume includes particle volume, inter-particle void volume and internal pore volume. Thus, bulk density takes into account solid space as well as pore space, (Greenland, 1998). Soils that are loose, porous, or well-aggregated (e.g. clay soil) will have lower bulk densities than soils that are not aggregated (sand). The bulk density of soil depends greatly on the mineral make up of soil and the degree of soil compaction.. if soil is too compact it will impede the movement of water down to the roots and also the penetration of the roots down in the soil, (Brady and Weil, 1999). Bulk density is an indirect measure of pore space and is affected primarily by soil texture and soil structure. Soils with high proportion of pore space to solids have lower bulk densities than those that are more compact and have less pore space, (Unger et al., 1994). Consequently, any factor that influences soil pore space will affect bulk density.

Increases in bulk density usually indicate a poorer environment for root growth , reduced aeration and undesirable changes in hydrologic function, such as reduced infiltration, (Brady and Weil, 1999). Bulk density data are used to compute shrink-swell potential, available water capacity, total pore space, and other soil properties. The moist bulk density of a soil indicates the pore space available for water and roots. A bulk density of more than 1.6g/cm³ can restrict water storage and root penetration, (Brady and Weil, 1999).

2.1.2.2. Chemical properties:

Soil chemical properties include the concentrations of nutrients, cations, anion concentrations, ion exchange reactions, redox properties and etc, but for the purpose of this study focus will be on properties that implicate soil fertility.

(i) Soil pH

Soil pH is an important soil property that affects several soil reactions and processes and is defined as a measure of the acidity or alkalinity of the soil, (Bohn, 2001). This property has considerable effect on soil processes including ion exchange reactions and nutrient availability, (Rowell, 1994). Soil pH is measured on a scale of 0 to 14, where a pH of 7.0 is considered neutral, readings higher than 7.0 are alkaline, and readings lower than 7.0 are considered acidic, (McGuiness, 1993). Most plants are tolerant of a fairly wide range of pH of 5.5-6.5 which represents the middle of the range, (Blake et al., 2002). As the amount of hydrogen ions in the soil increases, the soil pH decreases thus becoming more acidic, (Fullen and Catt, 2004). Soil pH is one of the most important characteristics of soil fertility, because it has a direct impact on nutrient availability and plant growth. Most minerals and nutrients are more soluble in acid soils than in neutral or slightly alkaline soils, (Bohn, 2001). In strongly acidic soils the availability of macronutrients (Ca, Mg, K, P, N and S) as well as molybdenum and boron is reduced. In contrast, availability of micronutrient cations (Fe, Mn, Zn, Cu and Co) is increased by low soil pH, even to the extent of toxicity of higher plants and microorganisms, (Bohn, 2001).

(ii) Cation Exchange Capacity (CEC)

Cation exchange capacity is defined as the sum of the total of the exchangeable cations that a soil can adsorb, (Brady and Weil, 1999).

Cation exchange capacity is used as a measure of soil fertility, nutrient retention capacity, and the capacity to protect groundwater from cation contamination, (Brady and Weil, 1999). It buffers fluctuations in nutrient availability and soil pH, (Bergaya and Vayer, 1997). Clay and organic matter are the main sources of CEC, (Peinemann et al., 2002). The more clay and organic matter (humus) a soil contains, the higher its CEC and the greater the potential fertility of that soil. CEC varies according to the type of clay. It is highest in montmorillonite clay, lowest in heavily weathered kaolinite clay and slightly higher in the less weathered illite clay. Sand particles have no capacity to exchange cations because it has no electrical charge. This means sandy soils such as podzolic topsoils have very low CEC.

Soils that are high in clay generally have higher CEC values, although the type of clay can substantially affect CEC, (Kahr and Madseni, 1995). These soils hold few nutrients and lose them easily as water moves through the soil. The soil’s capacity to hold nutrients comes from a coating of clay and organic matter on the sand particles, (Meier and Kahr, 1999). The higher the cation exchange capacity of a soil, the more nutrients it is likely hold, and the higher will be its fertility level (Fullen and Catt, 2004). Nutrients that are held by charges on soil are termed ‘exchangeable’ as they become readily available to plants. Plants obtain many of their nutrients from soil by an electrochemical process called cation exchange. This process is the key to understanding soil fertility, (Rowell, 1994).

(iii) Plant Nutrients

Plants require water, air, light, suitable temperature, and 18 essential nutrients to grow, (Brady and Weil, 1999). Each is equally important to the plant, yet each is required in vastly different amounts. These differences have led to the grouping of these essential elements into two categories; macro nutrients (primary and secondary) and micronutrients, (Brady and Weil, 1999):

Macronutrients

Macronutrients are those that are demanded in relatively high levels for plant nutrition and can be distinguish into two sub groups, primary and secondary ones. The nutrients nitrogen (N), phosphorus (P) and potassium (K) are referred as the primary macro-elements; they are the most frequently required in a crop fertilization program. Also, they are needed in the greatest total quantity by plants as fertilizer. Calcium (Ca), magnesium (Mg), and sulfur (S) are the secondary nutrients. For most crops, these three are needed in lesser amounts than the primary nutrients. They are growing in importance in crop fertilization programs due to more stringent clean air standards and efforts to improve the environment.

The following are the role of macro nutrients and in plant growth: Nitrogen for Chlorophyll, Proteins formation; Phosphorus for Photosynthesis ; Potassium for enzyme activity, starch formation, sugar formation; Calcium for cell growth, component of cell wall; Magnesium for Enzyme activation and Sulfur for amino acids and proteins formation.

Micronutrinents

The second group of plant nutrients is micronutrients, which are needed only in trace amounts, and include iron (Fe), manganese (Mn), boron (B), zinc (Zn), copper (Cu), molybdenum (Mo), chloride (Cl), sodium (Na), nickel (Ni), silicon (Si), cobalt (Co) and selenium (Se). These elements are used in very small amounts, but they are just as important to plant development and profitable crop production as the major nutrients.The importance of micro-elements in plant nutrition is high and they should not be neglected although they are needed in minor quantities. The following are essential micro nutrients and their role in plant growth: Boron for reproduction; Chlorine for root growth; Copper for enzyme activation; Iron for photosynthesis; Manganese for enzyme activation; Sodium for water movement; Zinc for enzymes and auxins component; Molybdenum for Nitrogen fixation; Nickel for Nitrogen liberation; Cobalt for Nitrogen fixation; Silicon for Cell wall toughening.

In addition to the nutrients listed above, plants require carbon, hydrogen, and oxygen, which are extracted from air and water to make up the bulk of plant weight, (Brady and Weil, 1999).

Deficiency of any of these essential nutrient, will hold back plant development, (Brady and Weil, 2004). If the deficient element is supplied, growth will be increased up to the point where the supply of that element is no longer the limiting factor. Increasing the supply beyond this point will not be helpful, as some other elements would then be in minimum supply and become the limiting factor, (Scoones and Toulmin, 1998). Deficiencies and toxicities of nutrients in soil cause infertile soil, poor growth, yellowing of the leaves and possibly the death of the plant, (Ahmed et al., 1997). Therefore proper nutrient management is required to achieve maximum uptake efficiency of plant nutrients and maximum economic and growth response by the crop, and also for minimum environmental impact.

Exchangeable cations (bases)

Closely related to CEC is the base saturation, which is the fraction of exchangeable cations that are base cations (Ca, Mg, K and Na), (Miller and Donahue, 1992). The higher the amount of exchangeable base cations, the more acidity can be neutralised in the short time perspective. Thus, a site with high CEC takes longer time to acidify (as well as to recover from an acidified status) than a site with a low CEC (assuming similar base saturations). According to van Reeuwijk (2002) the amount of exchangeable bases are an important property of soils and sediments. They relate information on a soil’s ability to sustain plant growth, retain nutrients, buffer acid deposition or sequester toxic heavy metals.

2.1.2.3. Biological properties

(i) Organic Matter

Soil organic matter consists of a wide range of organic substances, including living organisms, carboneous remains of organisms which once occupied the soil, and organic compounds produced by current and past metabolism in the soil, (Brady and Weil, 1999).

For simplicity, organic matter can be divided into two major categories: stabilized organic matter which is highly decomposed and stable, and the active fraction which is being actively used and transformed by living plants, animals, and microbes, (FAO, 1999). Soil organic matter plays a critical role in soil processes and is a key element of integrated soil fertility management (ISFM), (Brady and Weil, 2004). Organic matter is widely considered to be the single most important indicator of soil fertility and productivity, (Rowell, 1994). It consists primarily of decayed or decaying plant and animal residues and is a very important soil component. Benefits of Organic matter in soil according to Ashman and Puri, (2002) include: (1) hold and supply available plant nutrients (N,P and S), and (2)augment the soil’s cation exchange capacity, (3) is food for soil organisms from bacteria to worms and is an important element in the nutrient and carbon cycles.

Organic matter, like clay, has a high surface area and is negatively charged with a high CEC, making it an excellent supplier of nutrients to plants. In addition, as organic matter decomposes, it releases nutrients such as N, P and S that are bound in the organic matter’s structure, essentially imitating a slow release fertilizer, (Myers, 1995). The CEC of organic matter can be as high as 215 meq/100 g, a much higher value than for clay. Organic matter can also hold large amounts of water, which helps nutrients move from soil to plant roots, (Mikkuta, 2004).

Carbon to Nitrogen ratio (C: N) of organic materials and soils

The C: N ratio in the organic matter of arable surface horizons commonly ranges from 8:1 to 15:1, the median being near 12:1, (Brady and Weil, 2009). The ratio is generally lower for subsoils than for surface layers in a soil profile. The carbon content of typical plant dry matter is about 42%; that of soil organic matter is much lower and varies widely (from<1 to >6%). The C:N ratio in organic residues applied to soils is important for two reasons: (1) intense competition among the micro-organisms for available soil nitrogen occurs when residues having a high C:N ratio are added to soils, and (2) the C:N ratio in residues helps determine their rate of decay and the rate at which nitrogen is made available to plants, (Brady and Weil, 2009).

(ii) Microorganisms

Soil microorganisms, take in food and excrete by products, which are either products of respiration or components in the food supply, (Brady and Weil, 2004). Microorganisms usually excrete the nitrogen of originally combined nitrogen, which is surplus to their requirements as ammonium ions under aerobic conditions, and they excrete urea, uric acid acids such as: citric, tartaric, formic, lactic, oxalic, dibasic acids, succinic acids. But, if the aeration is reduced, or anaerobic conditions set in, complex and usually foul smelling amines will also be produced such as the alipathic amines cadaverine and putrescine and the aromatic indols. All these products are harmful to the plant growth.

Microorganisms can only use insoluble substances, such as cellulose and other polysaccharides and insoluble proteins as source of nutrients, (Uchida and Silva, 2000). But, due to production of enzymes, these substances convert into simpler compounds such as simple or amino acids and they are utilized by the microorganisms. On this concept, a fertile soil is one, which contains either an adequate supply of plant food in an available form, or a microbial population, which is releasing nutrients fast enough to maintain rapid plant growth; an infertile soil is one in which this does not happen, as for example, if the microorganisms are removing and locking up available plant nutrients from the soil.

Earthworms are one of the organisms composing the soil population, (Brady and Weil, 2004). Earthworms are very important in the development of soil fertility. They vary in size from large Lumbricus terrestris, which may have a length exceeding 25 cm fled weighing between 2 and 7 g to small pieces with lengths about 2.5 cm and weighing about 50 mg. The principal food of earthworms is dead or decaying plant remains including both leaf litter and dead roots.

Earthworms can only thrive in soils under certain specific conditions, (Alam, 2001). They are intolerant to drought and frost and hence the dry sandy soils and thin soils overlying rock are not usually favourable environments for them. They need reasonably aerated soils, hence heavy clays or undrained soils are also unfavourable as are pastures whose surface is pudded by over grazing in wet weather. They are numerous in loams and less in sands, gravels and days. Many can survive up to a year in water if it is reasonably aerated. Most earthworms, including all the larger species, need a continuous supply of calcium and if they are feeding on a calcium-rich material will excrete calcium surplus to their requirement as calcite from special plants in their digestive tract, (Alam, 2001). They overall play important role in improving the fertility of the soils.

2.1.2.4. Soil mineralogy

Soil mineralogy plays a vital role in soil fertility since mineral surfaces serve as potential sites for nutrient storage, (Tucker, 1999). However, different types of soil minerals hold and retain differing amounts of nutrients. Therefore, it is vital to know the types of minerals that make up a soil so as to predict the degree to which t