Factors Affecting Cation Exchange Capacity Environmental Sciences Essay

Soils are composed of five main components (Sinha and Shrivastava, 2000): mineral particles derived from rocks by weathering; organic materials – humus from dead and decaying plant material; soil water – in which nutrient elements are dissolved; soil air – both carbon dioxide and oxygen; and living organisms including bacteria that help plant decomposition. Soils differ in their fertility levels, because they have different proportions of these components and because the mineral particles have been affected to different degrees by weathering. Age of soil minerals, prevailing temperatures, rainfall, leaching and soil physico-chemistry are the main factors which determine how much a particular soil will weather (Sinha and Shrivastava, 2000).

Soil thus, is important to everyone either directly or indirectly. It is the natural bodies on which agricultural products grow and it has fragile ecosystem (Sinha and Shrivastava, 2000). South Africa ranks among the countries with the highest rate of income inequality in the world (Aliber, 2009). Compared to other middle income countries, it has extremely high levels of absolute poverty and food insecurity threat (FAO, 2009). As part of this, a potential contributor to food security might be small-scale agricultural production. Aliber (2009) indicated that input support targeting smallholder farmers could boost production and food security. Utilisation of uncultivated arable lands and subsistence agriculture might be one option to contribute to incomes and/or savings, as well as to encourage food diversification (Altman et al., 2009). Land with high agricultural suitability is considered to have greater long-term security with regards to both agricultural production and development. From a planning perspective, high agricultural flexibility is therefore considered an appropriate measure of high quality agricultural land that is highly productive and fertile.

Only a small proportion of world’s soils have a very good level of fertility, most of which have only good to medium fertility and some have very low fertility, and are often referred to as marginal soils (Ashman and Puri, 2002). Well-known fertile soils are deep alluvial soils formed from river mud, organic matter- rich soils on loess material, nutrient rich Vertisols and volcanic soils (Brady and Weil, 2004). Under poor management, soil fertility can be seriously depleted and soils may become useless for agriculture.

2.2. SOIL PHYSICO-CHEMISTRY

Soil is a natural medium on which agricultural products grow and it is dependent on several factors such as fertility to be considered productive (Shah et al., 2011). The fertility of the soil is depended on concentration of soil nutrients, organic and inorganic materials and water. These soil physico-chemical properties are classified as being physical, chemical and biological, which greatly influence soil fertility (Ramaru et al., 2000). To manage soil fertility, knowledge and understanding of these properties is required (as discussed below).

2.2.1. Physical soil properties

(i) Soil texture

Soil texture refers to the relative proportions of the various size groups of individual particles or grains in a soil (Rowell, 1994). It is dependent on the mixture of the different particle sizes present in the soil. Based on these different sizes, soil particles are classified as sand (0.05- 2mm), silt (0.002-0,5mm) 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 and cation exchange capacity (Møberg et al., 1999).

Clay particles hold larger quantities of water and nutrients, because of their large surface areas (Brady and Weil, 1999). This property causes the swelling and shrinking of clay soils, but only those with smectitic group of clay minerals. The large surface area of clay particles gives nutrients numerous binding sites especially when the surface charge density is high, which is part of the reason that fine textured soils have such high abilities to retain nutrients (Velde, 1995). The pores between clay particles are very small and complex, so movement of both air and water is very slow (Brady and Weil, 1999). Clay particles are negatively charged because of their mineralogical composition. Soils with such 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 (Brady and Weil, 1999).

The knowledge of the proportions of different-sized particles in soils is critical to understand soil behavior and their management. Since sand particles are relatively large, so are the voids between them, which promote free drainage of water and entry of air into the soil (Brady and Weil, 2002). The implication of free drainage in sandy soil is that soil nutrients are easily washed down into the soil and become inaccessible for use by plants (Brady and Weil, 2002). Sandy soils are considered non-cohesive and because of their large size, have low specific surface areas and thus have low nutrient retention capacity (Rowell, 1994). Sand particles can hold little water due to low specific surface area and are prone to drought, therefore have a very low CEC and fertility status (Petersen et al., 1996).

The pores between silt particles are much smaller than those in sand, so silt retains more water and nutrients (Rowell, 1994). Soils dominated by silt particles therefore have a higher fertility status than sandy soils and provides favorable conditions for plant growth when other growth factors are favorable (Miller and Donahue, 1992).

(ii) Soil structure

The term soil structure refers to the arrangement of soil particles into aggregates (Six et al., 2000). Soil structure is affected by biological activities, organic matter, and cultivation practices (Rowell, 1994). It influences soil water movement and retention, erosion, nutrient recycling, sealing and crusting of the soil surface, together with aeration and soil’s structural stability, root penetration and crop yield (Lupwayi et al., 2001).

Soil structure can be platy, prismatic, granular, crumbly, columnar and blocky (RCEP, 1996). 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 (Duiker et al., 2003). Organic matter and humification processes improve structural stability, and can rebuild degraded soil structures (Brady and Weil, 1999). 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. Favorable soil structure and high aggregate stability are therefore vital to improving soil fertility, increasing agronomic productivity, enhancing porosity and decreasing erodibility.

(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 pore spaces in soils. Soil water can be described into the following stages: gravitational, capillary, and hygroscopic, 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 soils 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). Field capacity is the amount of soil moisture or water content held in soil after excess water has drained away and the rate of downward movement has materially decreased, which usually takes place within 2-3 days after a rain or irrigation in pervious soils of uniform structure and texture (Govers, 2002).

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 (RCEP, 1996). This is explained by the degree of soil compaction, where problems will arise if excessive compaction occurs which would results in increased bulk density, a decrease in porosity and aeration and poor water drainage (Gregory et al., 2006), all resulting in poor plant growth.

(iv) Electrical Conductivity (EC)

Soil electrical conductivity (EC), is the ability of soil to conduct electrical current (Doerge, 1999). EC is expressed in milliSiemens per meter (mS/m) or cm (cm/m). Traditionally, soil scientists used EC to estimate soil salinity (Doerge, 1999). EC measurements also have the potential for estimating variation in some of the soil physical properties such as soil moisture and porosity, in a field where soil salinity is not a problem (Farahani and Buchleiter, 2004). 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).

Salt tolerances are usually given in terms of the stage of plant growth over a range of electrical conductivity (EC) levels. EC greater than 4dS/m are considered saline (Munshower, 1994). Salt sensitive plants may be affected by conductivities below 4dS/m and salt tolerant species may not be impacted by concentrations of up to twice this maximum agricultural tolerance limit (Munshower, 1994). Electrical conductivity is the ability of a solution to transmit an electrical current. The conduction of electricity in soil takes place through the moisture-filled pores that occur between individual soil particles. Therefore, the EC of soil is determined by the following soil properties (Doerge, 1999):

. Porosity, where the greater soil porosity, the more easily electricity is conducted. Soil with high clay content has higher porosity than sandier soil. Compaction normally increases soil EC.

. Water content, dry soil is much lower in conductivity than moist soil.

. Salinity level, increasing concentration of electrolytes (salts) in soil water will dramatically increase soil EC.

. Cation exchange capacity (CEC), mineral soil containing high levels of organic matter (humus) and/or 2:1 clay minerals such as montmorillonite, illite, or vermiculite have a much higher ability to retain positively charged ions (such as Ca, Mg, K, Na, NH4, or H) than soil lacking these constituents. The presence of these ions in the moisture-filled soil pores will enhance soil EC in the same way that salinity does.

. Temperature, as temperature decreases toward the freezing point of water, soil EC decreases slightly. Below freezing, soil pores become increasingly insulated from each other and overall soil EC declines rapidly.

Plants are detrimentally affected, both physically and chemically, by excess salts in some soils and by high levels of exchangeable Na in others. Soils with an accumulation of exchangeable Na 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 defined as the mass of dry soil (g) per unit volume (cm3) and is routinely used as a measure of soil compaction (Gregory et al., 2006). The total volume includes particle volume, inter-particle void volume and internal pore volume (Gregory et al., 2006). Bulk density takes into account solid space as well as pore space (Greenland, 1998). Thus soils that are porous or well-aggregated (e.g. clay soil) will have lower bulk densities than soils that are not aggregated (sand) (Greenland, 1998).

Plant roots cannot penetrate compacted soil as freely as they would in non-compacted soil, which limits their access to water and nutrients present in sub-soil and inhibits their growth (Hagan et al., 2010). Compacted soil requires more frequent applications of irrigation and fertilizer to sustain plant growth, which can increase runoff and nutrient levels in runoff (Gregory et al., 2006).

The bulk density of soil depends greatly on the soil’s mineral make up and the degree of compaction. High 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). The presence of soil organic matter, which is considerably lighter than mineral soil, can help decrease bulk density and thereby enhancing soil fertility (Hagan et al., 2010).

2.2.2. Soil Chemical properties

Soil chemical properties which include the concentrations of nutrients, cations, anions, ion exchange reactions and redox properties, but for the purpose of this study focus will be based on properties that have an implication on soil fertility including:

(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). It 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 pH range of 5.5-6.5 which is near neutral pH range (Bohn, 2001). 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 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 Al) is increased by low soil pH, even to the extent of toxicity of higher plants and microorganisms (Bohn, 2001).

The pH of a soil is also reported to affect so many other soil properties (Brady and Weil, 1999), including nutrient availability, effects on soil organisms, fungi thrive in acidic soils, CEC and plant preferences of either acidic or alkaline soils. Most plants prefer alkaline soils, but there are a few which need acidic soils and will die if placed in an alkaline environment (Brady and Weil, 1999).

(ii) Cation Exchange Capacity (CEC)

Cation exchange capacity is defined as the sum of the total of the exchangeable cations that a soil can hold or adsorb (Brady and Weil, 1999). A cation is a positively charged ion and most nutrients cations are: Ca2+, Mg2+, K +, NH4+, Zn2+, Cu2+, and Mn2+. These cations are in the soil solution and are in dynamic equilibrium with the cations adsorbed on the surface of clay and organic matter (Brady and Weil, 1999).

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 (Peinemann et al., 2002). Sand particles have no capacity to exchange cations because it has no electrical charge (Brady and Weil, 1999).

CEC is used as a measure of soil 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). 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). Nutrients that are held by charges on a soil are termed ‘exchangeable’ as they become readily available to plants (Rowell, 1994).The higher the CEC of a soil, the more nutrients it is likely to hold and the higher will be its fertility level (Fullen and Catt, 2004).

Factors affecting cation exchange capacity

The factors affecting cation exchange capacity include the following (Brady and Weil 1999), soil texture, soil humus content, nature of clay and soil reaction.

Soil texture influences the CEC of soils in a way that it increases when soil’s percentage of clay increases i.e. the finer the soil texture, the higher the CEC as indicated in Table 2. CEC depends on the nature of clay minerals present, since each mineral has its own capacity to exchange and hold cations e.g. the CEC of a soil dominated by vermiculite is much higher than the CEC of another soil dominated by kaolinite, as vermiculite is high activity clay unlike kaolinte which is low activity clay. When the pH of soil increases, more H+ ions dissociate from the clay minerals especially kaolinite, thus the CEC of soil dominated by kaolinite also increases. CEC varies according to the type of soil. Humus, the end product of decomposed organic matter, has the highest CEC value because organic matter colloids have large quantities of negative charges. Humus has a CEC two to five times greater than montmorillonite clay and up to 30 times greater than kaolinite clay, so is very important in improving soil fertility.

Table 2.1: CEC values for different soil textures (Brady and Weil, 1999)

Soil texture

CEC range (meq/100g soil)

Sand

2-4

Sandy loam

2-12

Loam

7-16

Silt loam

9-26

Clay, clay loam

4-60

(iii) Organic Matter

The importance of soil organic matter in relation to soil fertility and

physical condition is widely recognized in agriculture. However, organic matter

contributes to the fertility or productivity of the soil through its positive

effects on the physical, chemical and biological properties of the soil (Rowell, 1994), as follows: physical – stabilizes soil structure, improves water holding characteristics, lowers bulk density, dark color may alter thermal properties; chemical – higher CEC, acts as a pH buffer, ties up metals, interacts with biological – supplies energy and body-building constituents for soil organisms, increases microbial populations and their activities, source and sink for nutrients, ecosystem resilience, affects soil enzymes. 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 of the soil (Brady and Weil, 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: increasing the soil’s cation exchange capacity and acting as food for soil organisms from bacteria to worms and is an important component 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). Organic matter can also hold large amounts of water, which helps nutrients move from soil to plant roots (Mikkuta, 2004).

An important characteristic of organic matter in soil fertility is C: N ratio. The C: N ratio in organic matter of arable surface horizons commonly ranges from 8:1 to 15:1, the median being near 12:1 (Brady and Weil, 1999). The C:N ratio in organic residues applied to soils is important for two reasons: intense competition among the micro-organisms for available soil nitrogen which occurs when residues having a high C:N ratio are added to soils and it also helps determine their rate of decay and the rate at which nitrogen is made available to plants (Brady and Weil, 1999).

(iv) Plant Nutrients

Plants require 13 plant nutrients (Table 2.2) (micro and macro nutrients) for their growth. Each is equally important to the plant, yet each is required in vastly different amounts (Ronen, 2007).

Essential elements are chemical elements that plants need in order to complete their normal life cycle (Scoones and Toulhim, 1998). The functions of these elements in the plant cannot be fulfilled by another, thus making each element essential for plant growth and development (Scoones and Toulhim, 1998).

Essential nutrients are divided into macro and micronutrients as illustrated in Table 3. Macronutrients are those that are required in relatively high quantities for plant growth and can be distinguish into two sub groups, primary and secondary ones, (Uchida and Silva, 2000). The primary macro-elements are most frequently required for plant growth and also needed in the greatest total quantity by plants. For most crops, secondary macro nutrients are needed in lesser amounts than the primary nutrients. The second group of plant nutrients which are micronutrients are needed only in trace amounts (Scoones and Toulhim, 1998). These micronutrients are required in very small amounts, but they are just as important to plant development and profitable crop production as the major nutrients (Ronen, 2007).

Classification

Element

Function in plant growth

Source

Deficiency symptoms and toxicities

Macro nutrients

– Primary

Nitrogen (N)

Chlorophyll and Protein formation

Air/Soil, applied fertilisers

Slow growth, stunted plants, chlorosis, low protein content

Phosphorus (P)

Photosynthesis, Stimulates early growth and root formation, hastens maturity

Soil and applied fertilisers

Slow growth, delayed crop maturity, purplish green coloration of leaves

Potassium (K)

Photosynthesis and nzyme activity, starch and sugar formation, root growth

Soil and applied fertilisers

Slow growth, Reduced disease or pest resistance, development of white and yellow spots on leaves

Macro

nutrients

– secondary

Calcium (Ca)

Cell growth and component of cell wall

Soil

Weakened stems, death of plants’ growing points, abnormal dark green appearance on foliage

Magnesium (Mg)

Enzyme activation, photosynthesis and influence Nitrogen metabolism

Soil

Interveinal chlorosis in older leaves,

curling of leaves, stunted growth,

Sulfur (S)

Amino acids, proteins and nodule formation

Soil and animal manure

Interveinal chlorosis on corn leaves, retarded growth, delayed maturity and light green to yellowish color in young leaves

Micronutrients

– essential

Iron (Fe)

Photosynthesis, chlorophyll synthesis, constituent of various enzymes and proteins

Soil

Interveinal chlorosis, yellowing of leaves between veins, twig dieback, death of entire limp or plants

Manganese (Mn)

Enzyme activation, metabolism of nitrogen and organic acids, formation of vitamins and breakdown of carbohydrates

Soil

Interveinal chlorosis of young leaves, gradation of pale green coloration with darker color next to veins

Zinc (Zn)

Enzymes and auxins component, protein synthesis, used in formation of growth hormones

Soil

Mottled leaves, dieback twigs, decrease in stem length

Copper (Cu)

Enzyme activation, catalyst for respiration

Soil

Stunted growth, poor pigmentation, wilting of leaves

Boron (B)

Reproduction

Soil

Thickened, curled, wilted and chlorotic leaves; reduced flowering

Molybdenum (Mo)

Nitrogen fixation; nitrate reduction and plant growth

Soil

Stunting and lack of vigor (induced by nitrogen deficiency), scorching, cupping or rolling of leaves

Chlorine (Cl)

Root growth, photosynthetic reactions

Soil

Wilting followed by chlorosis, excessive branching of lateral roots, bronzing of leaves

Additional nutrients

Carbon (C)

Constituent of carbohydrates and photosynthesis

Air/ Organic matter

Hydrogen (H)

Maintains osmotic balance and constituent of carbohydrates

Water/Organic matter

Oxygen (O)

Constituent of carbohydrates and necessary for respiration

Air/Water/ Organic matter

Table 2.2: Essential plant elements, their sources and role in plants (Ronen,2007)

Deficiency of any of these essential nutrients will retard plant development (Brady and Weil, 2004). Deficiencies and toxicities of nutrients in soil present unfavorable conditions for plant growth, such as: poor growth, yellowing of the leaves and possibly the death of the plant as illustrated in Table 3 (Ahmed et al., 1997). Therefore proper nutrient management is required to achieve maximum plant growth, maximum economic and growth response by the crop, and also for minimum environmental impact.

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). Achieving balance between the nutrient requirements of plants and the nutrient reserves in soils is essential for maintaining soil fertility and high yields, preventing environmental contamination and degradation, and sustaining agricultural production over the long term.

2.2.3. Soil Biological properties

(i) Soil organisms

Soil organisms include mostly microscopic living organisms such as bacteria and fungi which are the foundation of a healthy soil because they are the primary decomposer of organic matter (Brady and Weil, 1999). Soil organisms are grouped into two namely soil microorganisms and soil macro organisms (Table 2.3).

Table 2.3: Soil Macro and microorganisms and their role in plant and soil (Brady and Weil, 1999)

Classification

Organisms

Function in plant and/or soil

Source

Microorganisms

Bacteria

Decomposition of organic matter

Soil surface and humus particles

Actinomycetes

Source of protein and enhance soil fertility

Surface layers of grass lands

Fungi

Fix atmospheric nitrogen and enhance soil fertility

Soil (without organic matter)

Algae

Add organic matter to soil, improve aeration of swamp soils, and fix atmospheric nitrogen

Moist soils

Macro-organisms

Nematodes

They can be applied to crops in large quantities as a biological insecticide

Soil and plant roots

Earthworms

Enhance soil fertility and structural stability

Aerated soils

Ants and termites

Soil development

Dominant in tropical soils

Soil can contain millions of organisms that feed off decaying material such as old plant material, mulch & unprocessed compost (Ashman and Puri, 2002), Microorganisms constitute < 0.5% of the soil mass yet they have a major impact on soil properties and processes. 60-80 % of the total soil metabolism is due to the microflora (Alam, 2001). Micro-organisms, including fungi and bacteria, affect chemical exchanges between roots and soil and act as reserve of soil nutrients (Kiem and Kandeler, 1997).

Soil organic matter is the main food and energy source of soil microorganisms (Ashman and Puri, 2002). Through decomposition of organic matter, microorganisms take up their food elements. Organic matter also serves as a source of energy for both macro and micro organisms and helps in performing various beneficial functions in soil, resulting in highly productive soil (Mikutta et al., 2004).

Macro-organisms such as insects, other arthropods, earthworms and nematodes live in the soil and have an important influence on soil fertility (Amezketa, 1999). They ingest soil material and relocate plant material and form burrows. The effects of these activities are variable. Macro-organisms improve aeration, porosity, infiltration, aggregate stability, litter mixing, improved N and C stabilization, C turnover and carbonate reduction and N mineralization, nutrient availability and metal mobility (Amezketa, 1999; Winsome and McColl, 1998 and Brown et al., 2000).

The various groups of soil organisms do not live independently of each other, but form an interlocked system more or less in equilibrium with the environment (Brady and Weil, 1999). Their activity in soil depend on moisture content, temperature, soil enzymes, dissolution of soil minerals and breakdown of toxic chemicals. All have a tremendous role in the development of soil fertility (Alam, 2001). Their actions involve the formation of structural systems of the soils which help in the increase of agricultural productivity (Alam, 2001).

2.3. SOIL CLAY MINERALOGY

The clay fraction of soil is dominated by clay minerals which control important soil chemical properties including sorption characteristics of soils (Dixon and Schulze, 2002). Minerals are naturally occurring inorganic compounds, with defined chemical and physical properties (Velde, 1995). Minerals that are formed in the depths of a volcano are called primary minerals (Pal et al., 2000). Feldspar, biotite, quartz and hornblende are examples of primary minerals. These minerals and the rocks made from them are often not stable when exposed to the weathering agents at the surface of the earth (Dixon and Schulze, 2002). These rocks are broken down (weathered) continuously into small pieces by exposure to physical and chemical weathering processes (Dixon and Schulze, 2002).

Some of the elements that are released during weathering, reform and crystallise in a different structure forming secondary minerals (Melo et al., 2002). Secondary minerals tend to be much smaller in particle size than primary minerals, and are most commonly found in the clay fraction of soils (Guggenheim and Martin, 1995). Soil clay fractions often contain a wide range of secondary minerals such as kaolinite, montmorillonite and aluminum hydrous oxides, whereas the sand or silt particles of soils are dominated by relatively inert primary minerals. The clay fraction is usually dominated by secondary minerals which are more chemically active and contribute the most to soil fertility (Melo et al., 2002). Two major secondary mineral groups, clay minerals and hydrous oxides, tend to dominate. These groups can appear in various mixtures often in association with soil organic matter (Brady and Weil, 2004).

Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earth metals and other cations, (Joussein et al., 2005). They are derived from weathering of rocks and are very common in fine grained sedimentary rocks such as shale, mudstone and siltstone and in fine grained metamorphic slate and phyllite (Van der Merwe et al., 2002). There are also non-clay minerals such as quartz and calcite which are derived from weathering of igneous rocks, (Van der Merwe et al., 2002).

Clay minerals are essential phases in soil chemistry and play extremely important roles in ion exchange reactions (Brigatti et al., 1996; Barrow, 1999). Soils which are texturally and chemically similar may differ in productivity or fertility due to the presence or absence of small amounts of particular clay minerals (Van der Merwe et al., 2002). For example, smectite clays are versatile and strong cationic exchangers and their presence can greatly influence the mobility of potentially toxic elements. Vermiculite has been widely used in the study of short- to medium-term variations (seasonal and annual) in soil processes (Monterroso and Macias, 1998).

Soil clay 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 (Velde, 1995). Therefore, it is vital to know the types of minerals that make up a soil so as to predict the degree to which the soil can retain and supply nutrients to plants.

Knowledge of the clay mineralogical composition and the different clay minerals present in soil is important in understanding use, and management of the soil, and in determining the agricultural potentials of soils.

2.3.1. Occurrence of clay and clay minerals

Clays and clay minerals occur under a fairly limited range of geologic conditions (Velde et al., 2003). The environments of formation include soil horizons, continental and marine sediments, geothermal fields, volcanic deposits, and weathering rock formations (Joussein et al., 2005). Most clay minerals form where rocks are in contact with water, air, or steam (Hillier, 1995). Examples of these situations include weathering boulders on a hillside, sediments in sea or lake bottoms, deeply buried sediments containing pure water, and rocks in contact with water heated by magma (molten rock) (Hillier, 1995).

A primary requirement for the formation of clay minerals is the presence of water. Soil clay minerals’ formation occurs in many different environments, including the weathering environment, the sedimentary environment, and the digenetic-hydrothermal environment (Brady and Weil, 1999). Clay minerals composed of the more soluble compounds e.g. smectites are formed in environments where ions can accumulate (e.g. in a dry climate, in a poorly drained soil, in the ocean, or in saline lakes) (Velde 1995). Clay minerals composed of less soluble compounds (for example, kaolinite and halloysite) form in more dilute water such as that found in environments that undergo severe leaching (for example, a hilltop in the wet tropics), where only sparingly soluble elements such as aluminum and silicon can remain (Brady and Weil, 1999). Illite and chlorite are known to form abundantly in the diagenetic-hydrothermal environment by reaction from smectite (Brady and Weil, 1999).

2.3.2. Weathering of minerals

The minerals’ parent materials form in the crystallisation of molten rock material: these are known as primary minerals, and include olivine, quartz, feldspar and hornblende. Primary minerals are not stable when exposed to water, wind and extremes of temperature (Hillier, 1995). Some of the elements that are released during weathering reform and crystallise in a different structure: these are the secondary minerals, and include vermiculite, montmorillonite and kaolinite (Hillier, 1995). Secondary minerals tend to be much smaller in particle size than primary minerals, and are most commonly found in the clay fraction of soils. As minerals weather, they lose silicon (as soluble silicic acid), leading to increasing proportions of aluminates in weathered clays, such as kaolinite. Aluminium hydroxide species are amphiprotic.

The rate and nature of the weathering process very much depends on climatic conditions. Intense weathering produced in a hot and moist climate can lead to major changes in mineral structure and the conversion to hydrous oxides. There are four phases to be considered in the system that model the formation of clay minerals by the weathering of granitic rocks as the clays have a definite composition: K-feldspar, Muscovite (illite), Kaolinite and gibbsite:

3KAlSi3O8) +2H+ +12H2O «2K+ +6Si (OH)4 +KAl3Si3O10(OH)2

(K- Feldspar) (Illite) …………… [Eqn. 2.1]

2KAl3Si3O10 (OH)2 + 3H2O + 2H+ «2K+ + 3Al2Si2O5(OH)4

(Illite) (Kaolinite) …………. [Eqn. 2.2]

Al2Si2O5+ (OH)4 5H2O «€ 2Si(OH)4 + 2Al (OH)3

(Kaolinite) (Gibbsite) …………………. [Eqn. 2.3]