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.
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 the soil can retain and supply nutrients to plants. There are numerous types of minerals found in the soil, which vary greatly in size and chemical composition, (Pal et al., 2000).
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 processes at the surface of the earth. These rocks are broken down (weathered) continuously into small pieces by exposure to physical and chemical weathering processes, (Dixon and Schulze, 2002). Secondary minerals e.g. Kaolinite, Montmorillonite and aluminum hydrous oxides, are new minerals that are synthesized from the weathering products of primary minerals or they are produced from simple alterations to existing primary mineral (Melo et al., 2002).
Secondary minerals are extremely important in soil as they provide a mechanism for retaining essential nutrients through ion exchange therefore enhancing soil fertility, (Dixon and Schulze, 2002). Secondary minerals also buffer the soil against rapid changes in acidity or alkalinity and help create soil structure. During the weathering process secondary minerals are formed. Soils often contain a wide range of secondary minerals. 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 insert a reference here.
In summation of all factors affecting soil fertility, fertile soil has the following properties: It is rich in the nutrients which are necessary for plant nutrition, including nitrogen, phosphorus and potassium (N-P-K); It contains trace elements which are also critical for plant nutrition, but are needed in much smaller quantities than N-P-K, these include boron, cobalt, copper, iron and several others; It contains good levels of organic matter, which improves soil structure and its ability to hold water; The soil pH (an indicator of acidity or alkalinity) is balanced; It has good soil structure, so it drains well; It has a good CEC which is the sum of all the exchangeable cations (Ca, Mg, K and Na) in available form for plants; It contains a range of macro-organisms (earthworms, termites, etc.) and micro-organisms (fungi, bacteria, etc.) and that help support plant growth and health.
2.2. SOIL CLAY MINERALOGY
the clay fraction of soils is dominated by clay minerals which control important soil chemical properties including cation exchange capacity and surface area, (Dixon & Weed, 1989).
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). Clay minerals are a type of secondary mineral. For example, the clay mineral Kaolinite Al2(Si2O55)(OH)4 is formed by the weathering of the primary mineral Feldspar (KAlSi3O8).
The inorganic minerals in the soil are the original source of most of the chemical elements essential for plant growth, (Brady and Weil, 1999). Although the bulk of these nutrients are held rigidly as components of the basic crystalline structure of the minerals, a small but important portion is in the form of charged ions on the surface of clays and organic matter, (Brady and Weil, 1999). Primary minerals are inert and mainly secondary minerals are needed in soils for agricultural purpose.
2.2.1. OCCURANCE OF CLAY AND CLAY MINERALS
Clays and clay minerals occur under a fairly limited range of geologic conditions. 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. Examples of these situations include weathering boulders on a hillside, sediments in sea or lake bottoms, deeply buried sediments containing pore water, and rocks in contact with water heated by magma (molten rock), (Velde , 1995).
2.2.2. STRUCTURE OF CLAY MINERALS
Clay mineral properties are fundamental to many soil functions including water and nutrient retention, contaminant (pesticides, heavy metals) attenuation, carbon storage, the maintenance of soil structure and the filtering of both ground and surface waters, (Bennett, 2004).
On the basis of the number and arrangement of tetrahedral and octahedral sheets contained in the crystal units or layers, silicate clays are classified into two different groups, (Brady and Weil, 1999) i.e. 1:1 type minerals and 2:1 type minerals:
(i) 1:1 Type Minerals
The layers of 1:1 type minerals are made of one tetrahedral sheet and one octahedral sheet bonded together to form a layer. In soils, kaolinite is the most prominent member of this group, which includes halloysite, nacrite and dickite. No expansion ordinarily occurs between layers of these minerals when wetted, because the structure is fixed. Cations and water do not enter between structural layers of 1:1 type mineral particle. There is also a little isormophous substitution in this 1:1 type mineral. In contrast with other silicate groups, kaolinite exhibits less plasticity, stickiness, cohesion, shrinkage or swelling. Kaolinite-containg soils make good bases for roadbeds and building foundations, and are commonly used in making bricks. They are easy to cultivate for agriculture and with some nutrient supplementation from manure and fertilizer, can be very productive.
Most of the 1:1 type minerals are considered to exhibit the properties of low activity clays. Low activity clays are more highly weathered, (Van der Merwe et al. 2002). Thus, due to their lesser surface area, low activity clays have a lower capacity to retain and supply nutrients. In addition to CEC, low activity clays can also have anion exchange capacity (AEC), depending upon the pH of the soil. The AEC causes these clays to retain and supply nutrients, such as phosphate, sulfate, and nitrate, rather than the base cations, under acidic conditions. Yet, under neutral and alkaline conditions, these low activity clays generate a CEC. Low activity clays have an AEC under acidic conditions and a CEC under alkaline conditions. In order to provide adequate amounts of the base cations, proper management of pH is crucial. If soil pH is low, additions of lime and/or organic matter may increase the CEC for soils high in low activity clays. Low activity clays have a low shrink and swell potential. With additions of nutrients, these soils may be very productive soils, (Brady and Weil, 2002).
(ii) 2:1 Type Minerals
The crystal units of these minerals are characterized by an octahedral sheet sandwiched between two tetrahedral sheets. Four general groups have this basic crystal structure. Two of them are smectite and vermiculite, including expanding-type minerals; the other two are fine-grained micas (illite) and chlorite and are relatively nonexpanding type minerals.
Expanding minerals
The smectite group is noted for interlayer expansion, which occurs by swelling when minerals are wetted, the water entering the interlayer spaces and forcing the layers apart, (Brady and Weil, 1999). Motmorillonite is the most prominent member of this group in soils, although beidenite, nontonite and saponite are also found. In turn these layers are loosely held together by weak oxygen-oxygen and cation-oxygen linkages. Exchangeable cations and associated water molecules are attracted between layers (interlayer space), causing expansion of the crystal lattice. The internal surface thus exposed by far exceeds the external surface area of these minerals. Smectites are also noted for their high plasticity and cohesion and their marked swelling when wet and shrinkage when drying. Wide cracks commonly forms as smectite dominated soils (e.g. vertisol) are dried. The dry aggregates are very hard making such soils too difficult to till.
Vermiculites are also 2:1 type minerals, an octahedral sheet being found between two tetrahedral sheets, (Brady and Weil, 1999). In the tetrahedral sheet of most vermiculite, considerable substitution of aluminum for silicon takes place. This accounts for most of the very high net negative charge associated with this mineral. Water molecules along with magnesium and other ions including Al-hydoxy ions, are strongly adsorbed in the interlayer space of vermiculites. However these interlayer constituents act primarily as bridges holding the units together rather than as wedges driving them apart. The degree of swelling is therefore considerably less for vermiculites than for smectites. For this reason vermiculites are considered to be limited-expansion clay minerals, expanding more than kaolinite but much less than the smectites. The cation adsorbing capacity of vermiculites usually exceeds that of all other silicate clays, including montmorillonite and other smectites, because of the very high negative charge in the tetrahedral sheet. Vermiculite crystals are larger than those of the smectites but much smaller than those of kaolinite.
Nonexpanding Minerals
Micas and chlorites are the type of minerals in this group. Weathered minerals similar in structure to these micas are found in the clay fraction of soils and they are called fine-grained micas, (Brady and Weil, 1999). Like smectites, fine-grained micas have a 2:1 crystal; however the particles are much larger than those of smectites. The major source of charge is in the tetrahedral sheet, where about 20% of the silicon sites are occupied by aluminum atoms, (Olis et al., 1990). This result in a net negative charge in the tetrahedral sheet, even bigger than that found in the vermiculites. To satisfy this charge, potassium ions are strongly attracted in the interlayer space. The potassium being strongly attracted to the adjacent sheets, acts as a binding agent, preventing expansion of the crystal. Hence, fine-grained micas are quite nonexpansive. Such properties as hydration, cation adsorption, swelling, shrinkage and plasticity are much less intense in fine-grained micas than in smectites. The specific surface area of fine-grained micas varies from 70 to 100 m2/g, about one-eighth that for a smectites.
Most of the 2:1 type mineral exhibits the properties of high activity clays. Generally, soils with large amounts of high activity clays are not highly weathered, (Van der Merwe et al. 2002). High activity clays have a high ‘cation exchange capacity’ (CEC), due to their large surface area. This means that these clays have a great capacity to retain and supply large quantities of nutrients, such as calcium, magnesium, potassium, and ammonium. Not only do these clays have a large CEC, but they also generate CEC under all soil conditions regardless of soil pH. As a result, these clays tend to produce highly fertile soils. Examples of these clays are montmorillonite (and other smectites), vermiculite, illite, and mica. Some high activity clays, such as montmorrillonite, have a shrink and swell potential, (Mathe et al, 2007). This means that the clays will shrink and crack when dry, and expand and swell when wet. With little additions of nutrients, these soils may be very productive. However, the shrink and swell potential will result in poorer drainage, and so, proper management of irrigation is required.
2.2.3. ROLE OF CLAY MINERALS IN SOIL FERTILITY
Soil mineralogy is closely related to soil fertility as knowledge of mineralogy helps to determine the appropriate nutrient management strategy for soils, (Tucker, 1999). Soil clay minerals play a very important role in the chemical reaction which take play in soil and influence the movement and retention of contaminants, metals, and nutrients in the soil, (Tucker, 1999). The magnitude of the contribution of clay minerals to soil cation-exchange capacity, buffer capacity, cation fixation, and various physical properties depends upon the nature and amount of the various mineral species present, (Pal, 2000) .
Clay minerals are increasingly recognized as an important fraction in soils influencing the nutrient retention capacity and their mobility in specific environments (Boonfueng et al., 2005). The influence of common clay minerals such as kaolinite (Zhou and Gunter, 1992), smectite (Jongmans et al., 1998), and illite (Post and Borer, 2002) on soil properties including cation exchange capacity (CEC) (Asadu et al., 1997), organic matter character (Peinemann et al., 2000) and dispersivity (Singer, 1994) have been studied in details. CEC varies according to the type of clay whereby it’s usually highest in montmorillonite clay, lowest in heavily weathered kaolinite clay and slightly higher in the less weathered illite clay, (Bergaya and Vayer, 1997).
According to Bain and Griffen (2002), the clay fraction of the soil and in particular the kind and amount of the respective clay minerals present, determines in large measure the chemical and physical properties of the soil. The physical effects manifest themselves in soil structure and aggregation and in the movement and retention of soil moisture. By reason of its predominant role in cation and anion exchange, the clay fraction is the primary factor which controls soil acidity and alkalinity, and as such must be taken into account in any program of reclamation of alkaline and saline soils, (Bohn et al., 2001). To the extent that it influences the ease or difficulty of release of plant nutrient ions, the clay fraction is important in plant nutrition, (Tucker, 1999).
Cation exchange capacity (CEC) and anion exchange capacity (AEC) are properties that can help differentiate soil minerals, (Landon, 1991). CEC and AEC values are important measurements that provide important information regarding the soil’s ability to retain and supply certain nutrients to the plant, (Landon, 1991). In addition to nutrient retention, CEC and AEC help in predicting the leaching potential of certain nutrients in areas with high rainfall. When the soil has a very high CEC, negatively charged nutrients such as nitrate are not retained by the soil, (Hassett and Banwart 1992). Instead, nitrate leaches through the soil profile in areas with high amounts of precipitation. Likewise, soils with high AEC experience leaching of positively charged nutrients, such as calcium and potassium, (Hassett and Banwart 1992).
The surfaces of less-weathered clay minerals (such as montmorillonite) generally have a negative charge, (Righi et al., 1995). Much like a magnet, negatively charged soil surfaces attract positively charged cations. However, under acidic conditions, the soil will also have a tendency to attract aluminum and hydrogen cations. The presence of aluminum and hydrogen contributes to soil acidity. In contrast, under alkaline conditions, the soil attracts sodium which contributes to soil alkalinity.
For soils with AEC, proper management of pH is crucial in order to provide sufficient amounts of the nutrient cations (calcium, magnesium, ammonium, and potassium). Minerals that exhibit AEC are highly weathered kaolinite, aluminum and iron oxides, (Bennett, (2004). Highly weathered Ultisols and Oxisols, volcanic Andisols, and organic Histosols all have AEC under acidic conditions. The pH at which these soils develop AEC differs depending upon the clay minerals within the soil.
You cannot end your literature review so abruptly without making a statement that summarizes the significance of knowledge of soil physicochemical and mineralogical properties in soil fertility.
In addition, the section on mineralogy has not highlighted the role of soil mineralogy in fixation of soil nutrients, decomposition/accumulation of organic matter, and acidity and alkalinity of soils. You therefore need to beef up this section accordingly
Your write-up is still fragmented. There needs to be logical flow of material presented
3. RESEARCH PROBLEM
The focus of this research will be based mainly on subsistence farms, small-size farms mainly owned by black farmers. Generally, these farms are characterized by fragmented plots and concentrated in the former homelands. Farming under smallholder system in Waterberg district of Limpopo Province is characterized by low level of production and small farm sizes with production being primarily for subsistence and little marketable surplus (White Paper on Agriculture, 1995). They are largely utilised for production of food crops for domestic consumption, with limited surpluses sold in local markets. These farms lack adequate production, adequate agricultural extension services and market infrastructure. The use of modern inputs on these farms is very limited, and household’s members provide the main source of labour. The clay mineralogical data of agricultural Limpopo soils and their impact on soil fertility is deficient, especially to the small scale farmers who can’t afford to pay for the soil survey of their fields. This study therefore forms part of an initiative to develop a stable agricultural environment for the smallholder farmers of the Waterberg district, to ensure that there is maximum level of production. The results obtained from the study would therefore assist farmers and individuals in developing a soil fertility management strategy in their farming systems to increase the level of crop production on a sustainable basis.
You have still not articulated the research problem adequately
What about the need information of the physicochemistry of the soil?
Establish the importance of knowledge of soil physic chemistry and mineralogy on soil productivity and how this information is lacking
4. AIM AND OBJECTIVES OF THE STUDY
The aim of this study is to determine soil physicochemical and clay mineralogical characteristics of selected agricultural soils of Waterberg district in Limpopo Province with a view of understanding how they may be affecting fertility.
The specific objectives of the study are:
To identify spatially dominant agricultural soil types of selected areas of Vhembe in Limpopo Province
To determine clay minerals dominant in each of the identified soil type
To determine physicochemical properties of identified soil types
Determining the fertility index of the selected soils
Understanding the role of mineralogy and physico-chemistry on the fertility status of the soils
5. RESEARCH QUESTIONS
These questions will assist in the research in terms of attaining the objectives of the study:
What are the dominant agricultural soil types of Waterberg in Limpopo Province?
What is the clay mineralogical composition of those soils?
What are the physicochemical characteristics of those soils?
What is the fertility index of those soils?
6. HYPOTHESIS
This study will be guided by the following hypothesis:
Soil fertility in the Waterberg district is affected by soil physicochemical and clay mineralogical properties
7. SCOPE OF THE RESEARCH
This study will focus on agricultural soils of selected areas of Waterberg district in Limpopo Province and determination of their physicochemical and clay mineralogical characteristics as to investigate how they may be affecting soil fertility for agricultural purpose.
8. RATIONALE OF THE RESEARCH
South Africa has a wide range of soils of different physical and mineralogical composition. Limpopo province alone has a diversity of soils and climatic conditions permitting a variety of different forms of agriculture, (White Paper on Agriculture, 1995). The clay mineralogy of Limpopo Province also varies due to indifference of the geological composition of the province i.e. the soils are mainly developed on basalt, sandstone and biotite gneiss and are generally of low inherent soil fertility. Information on the mineralogy and nutrient status of soils in the Province is deficient, especially of soils found in the communal areas where smallholder agriculture is practiced. Such information is crucial for any strategy that seeks to improve the productivity of arable agriculture in the Province.
This study will therefore generate results on the clay mineralogical and soil physicochemical implication on the fertility of selected representative agricultural soils in Limpopo Province as a first step toward developing a soil fertility management strategy for the farming sector in the region.
9. STUDY SITE
9.1. Location of the site
Limpopo is South Africa’s northernmost province, lying within the great curve of the Limpopo River. The province borders the countries of Botswana to the west, Zimbabwe to the north and Mozambique to the east, (DBSA, 1998). Limpopo Province is divided into five municipal districts: Capricorn, Mopani, Sekhukhune, Vhembe and Waterberg , subdivided in 24 local municipalities, (Limpopo Province Natural Resource Maps, 2003). The province occupies the total surface area of 125 755 km2 ,about 10.3% of the country’s land area .The population is about 5 355 172 which is about 11.3% of South African population, (Statistics SA, 2009). The waterberg district is the largest in the Limpopo province and is located in the western side of the Limpopo Province. According to the Gross Geographic Product, (1994), the five sectors within the Waterberg district that contributes to the economy are mining, electricity/water, services and agriculture. It lies between 24°42′S and 28°24′E»¿ / »¿24.7°S and 28.4°E»¿, (Limpopo Province Natural Resource Maps, 2003).Given the fact that 89% of the population of Limpopo Province is classified as rural, agriculture plays a major role in the economic development of rural areas of the province. The contribution of agriculture to the economy of Limpopo Province has been summarized in DBSA Report (1994). The study area is shown in the map on figure1.
9.2. Geology and landscape of the site
The geology of the Province varies from Palaeo-Archaean mafic, ultramafic and felsic extrusives to Mesozoic sedimentary rocks and flood basalts (RSA Geological Map series, 1984). The geology of Limpopo is complex and diverse. The rock formations in the Province can be considered in four main divisions based on time and general homogeneity namely: the Archaean, generally known as the ‘Basal’ or ‘Fundamental’ Complex; the Pre-Cambrian, or Algonkian Systems; the Palaeozoic, pre-Karoo Formations; and the Mesozoic, the Karoo System. The topography varies from relatively flat areas to mountainous terrain, (Barker et al., 2006).
Millions of years after the intrusion of the Bushveld complex, sediments began to accumulate across the Waterberg system giving rise to distinctive red shales and quartzite masses that make the region so distinctive., (White Paper on Agriculture, 1995). It is these cliffs, valleys and buttresses that have protected the fragile vleis and vegetation from over exploitation. Today, the mining of chrome, lead, iron ore, tin, nickel and platinum, mostly on the periphery of the mountains, provide much needed revenue to the economy of the region.
9.3. Climate and rainfall
Limpopo falls in the summer rainfall region with the western part semi-arid, and the eastern part largely sub-tropical, (Limpopo Province Natural Resource Maps, 2003). The western and far northern parts experience frequent droughts. Winter throughout Limpopo is mild and mostly frost-free. The average annual temperatures for the southern to central plateau areas of the province is generally below 20°C, in the Lowveld and northern parts average annual temperatures are above 20°C. The largest portion of the province has a mean annual rainfall of between 300 and 500mm.
The southwestern part has an annual rainfall of up to 700mm a year and in the Lowveld the rainfall can exceed 1 000mm a year in places. Waterberg district is generally arid to semi-arid with rainfall ranging from 540mm in up to 700mm per year. Temperature varies from 2. 50C and 400C. Experience of hail storms 1 – 3 days a year. These varied climates allows Limpopo Province to produce a wide variety of agricultural produce ranging from tropical fruits such as banana, mangoes to cereals such as maize, wheat and vegetables such as tomatoes, onion and potatoes.
9.4. Soils of the study area
Soil is the product of the weathering of rocks. The type of soils that occur in the province is therefore related to the parent material, the surface character of the area in which it is deposited, climate, rainfall and hydrological systems, (Limpopo Province Natural Resource Maps, 2003). There are wide varieties of soils that occur in the province, tending to be sandy in the west, but with a clay content toward east. The soils are differentiated based on depth, the nature of diagnostic horizons and materials. Those soils are mainly developed on basalt, sandstone and biotite gneiss and are generally of low inherent soil fertility.
Considering that this study is about soils, more details of the soils in the district/province should be given
9.5. Agricultural activities
Maize is the staple food of majority of people in Limpopo Province. It is also used as animal feed. On the basis of area and volume of production, it remains the most important dominant cereal grain despite the dry and drought prone agro-ecology of much of the district.. The Limpopo Province has large quantities of land suitable for dry-land production of the Maize crop. Climatic variation could lead to variations in yields. As a staple food, maize has a large and stable market and is the most important agricultural product in South Africa.
Wheat is another crop (winter crop) that is grown in Waterberg district. Consumption patterns suggest that wheat can be a substitute to maize. An increase in wheat production could lead to reduction in production of maize. However, availability of water for irrigation would limit the expansion of area under wheat.
Figure1: Map of South Africa and Limpopo Province showing also the study area
Where are the coordinates of this map?
The map showing South Africa should be in a box with its own coordinates as well
10. METHODOLOGY
This research will involve mainly fieldwork and laboratory analysis. The results obtained from the analyses will help in concluding on the hypothesis of the study.
10.1. Field work
The field work will be done in Waterberg district of Limpopo Province. Different soil types will be identified using Soil Database from Food and Agriculture Organization (FAO) of which further analysis would be done i.e. Analysis of soil physicochemical and clay mineralogical properties. Soil samples will be collected in maize fields of smallholder in the study area mainly,
What will be the main activities that will be carried out during fieldwork?
10.2. Sample collection
Systematic sampling which involves the use of grid lines (i.e. it divides the samples into equal portions) will be used as a sampling method. To minimize sampling error, all the units of the study area would be included to cover the inherent variations among soil units. Soil samples will be taken each to a depth of 0-30 cm. A soil auger and geological hammer will be used for collection of the samples; and GPS for taking coordinates of the sampling plots.
How many samples will be collected?
How many maize fields will be sampled?
Are you going to collect soil samples from all maize fields?
If not, state how you will identify fields from where you will collect samples
10.3. Laboratory analysis
You cannot begin a section with a table.
Make a statement that highlights the different parameters that will be analysed in the lab and justify which these parameters and not others will be analysed.
Then present the table
Each parameter should be on a separate row. According to your table, now, soil texture will be determined by hydrometer while bulk density will be determined by method. This is because you have not formatted your table appropriately
Your table should be formatted as indicated below
Property
Parameter
Method
Reference
Physico-chemical
Particle size distribution
Hydrometer
Bulk density
Soil cores
Chemical
CEC
Ammonium acetate
pH
potentiometric
Table 1: Standard methods to be used for the soil analyses: