GEOG 301: SOILS AND BIOGEOGRAPHY

LECTURE 1 SOIL FORMATION AND SOIL HORIZONS

Definition and Composition of Soils

Soil has been defined differently by various scholars. Foth (1978:1) defines it as “the loose surface of the earth”. Soil is comprised of minerals, organic matter, air and water. Another useful definition given by MacDonald (2003:32) is that: “Soil is the uppermost layer of mineral and organic matter found on the earth’s surface”. Soil is comprised of minerals, organic matter, air and water. Minerals and organic mater constitute the solid material part. Air and water form the pore space. Although the proportion of these components may vary slightly from one soil to another, minerals generally comprise about 45% of the total volume of a typical surface soil. About 5% of the volume of a good surface soil is made up of organic matter. The remaining 50% soil pore space contains roughly equal amounts of water (25%) and air (25%).

Soil Formation

Soil formation (also known as pedogenesis) begins with the weathering of rocks or transported sediments into small particles. Physical weathering occurs during the early stages of soil formation as freeze-thaw processes and differential heating and cooling breakdown parent material. After underlying rock or rock fragments are broken down into smaller particles that can retain sufficient water and support plant life, the soil formation process now continues more rapidly. The decomposition of organic materials leads to the production of carbon dioxide, which dissolves in water to form carbonic acid. Carbonic acid, in turn, reacts with and alters many of the primary minerals in the newly formed soil to form smaller soil particles of sand, silt, and clay minerals. Thus, chemical weathering is very important at the latter stages of soil formation.

It is important to stress that the transformation of parent materials into soil is affected by a number of factors, notably climate, topography, living organisms, and time. Depending on the environment, soils produced from weathering of rocks are further affected by various pedogenic (soil forming) processes. For instance, in the warm moist climates, heavy rainfall may cause the leaching of some ions from the surface soil. This pedogenic process is known as laterization.

Soil Horizons A soil horizon can be defined as a layer in the soil which measures parallel to the soil surface and possess properties which differ from the layers beneath and above. The vertical sequence of soil horizons found at a given location is collectively called the soil profile. In other words, soil profile is a “vertical cross-section of the

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soil showing the various horizons of which it is composed and extending into the parent material” (Ahn, 1993: Xi). The processes involved in the formation of soil profiles are (a) Transformations, such as weathering and organic matter breakdown; (b) Translocations or movement of inorganic and organic materials from one horizon to another, the materials being moved mostly by water but also by soil organisms; (c) Additions of materials to the soil profile from outside sources, such as organic matter from leaves, dust from the atmosphere, or soluble salts from the ground water; (d) Losses of materials from the soil profile by leaching or erosion of surface materials.

Soil scientists normally use the capital letters O, A, B, C, and E to denote the main or “master” horizons. Not all soils have all these horizons. For instance, only unmanaged forest soils have an organic O horizon on the surface and a leached zone (E horizon) below the A horizon. In any soil, each “master” horizon may have subdivisions. There are also transitional zones between master horizons (see Figure 1.1). In some classification systems, these capital letters (i.e. O, A, B, C and E) are followed by several alphanumerical modifiers highlighting particular outstanding features of the horizon. The main characteristics of each of the “master” horizons of a typical soil profile are presented below:

O Horizon: This lies at the top of the soil profile and it is primarily composed of organic matter. The surface of the layer is normally made up of fresh litter. However, at depth, litter may have been destroyed by decomposition. As shown in Figure 1.1, the horizon may be further divided into three layers, based on the state of the organic matter (whether it is fully decomposed or partially decomposed). The decomposed organic matter enriches the soil with nutrients, such as potassium and nitrogen. This horizon usually occurs in forested areas, but are generally absent in grassland regions

A Horizon: The A horizon lies beneath the O horizon. This horizon marks the beginning of the true mineral soil. It is, therefore, sometimes referred to as the surface soil or topsoil. The horizon is usually coarser than the subsoil layer (B horizon), and contains more organic matter than the other soil layers beneath it. The colour of this topsoil is brownish or black, due to the presence of organic matter. The A horizon is generally more fertile than the horizons beneath it.

E Horizon: The E horizon, found just beneath the A horizon, is generally light coloured. Some scientists see this as part of the surface soils or topsoil (see, for instance, Daniels and Haering, 2006). Eluviation (the movement of dissolved chemicals and minerals downward through the soil due to the movement of ground water) is the dominant process in this horizon. Leaching of organic matter, clay particles, and oxides of aluminium and iron is active in this horizon.  Soil in this layer tends to be acidic because of the intense leaching that occurs in the zone.

B Horizon: Beneath the E horizon is the B horizon, which is also referred to as the subsoil. This is a zone of illuviation where downward moving chemicals and fine materials are accumulated. This layer is generally denser and finer in texture than the surface soil. Organic matter content of the B horizon is also much lower than that of the A horizon. Due to the accumulation of iron coated clays in this layer, the soil colours are often brighter, with shades of yellow, brown, and red.

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C Horizon: This horizon comprises partially weathered parent material. The characteristics of materials here are more like the parent material from which they have been formed. Beneath the C horizon is the bedrock, which is often denoted by the letter R.

Figure 1.1 A typical soil profile

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Source: Daniels and Haering (2006: 36)

It must be emphasised that various soil types have different degrees of horizon development. While some “older or mature” soils may display the full profile of the horizons described above, younger or “immature” soils may only have an O, A, C sequence. For instance, relatively new deposits of soil parent material, such as alluvium and volcanic ash, may not have well developed horizons.

An important term that is related to soil horizon is “solum”. The solum (plural, sola) consists of the surface and subsoil layers that have undergone the same soil forming conditions. Some soils may not have any solum. For instance, a soil that consists only of recently deposited alluvium does not have a solum. In terms of soil horizon designations, a solum generally comprises of A, E, and B horizons and their transitional horizons. In some cases, the O horizon is also part of the solum. In other words, the solum is that part of the soil which contains well weathered parent materials.

Diversity of Soil Profiles

As hinted already, soil profiles differ from place to place. While some soils may show all the master horizons discussed above (i.e. O, A, E, B and C horizons), some soil profiles may be characterized by only a few horizons. Again, while the profiles of some soils may be several metres deep before they reach the consolidated rocks, some other profiles may be so shallow that plants do not have enough root room, with rock or ironstone at less than 1 m. What causes these differences in the nature of soil profiles? Basically, the variations in the nature of soil profiles are due to differences in the soil-forming factors (i.e. climate, parent material, biota, topography and time).

In relation to climate, variations in temperature and precipitation influences weathering and profile development factors. In the hot, wet tropics, for instance, rock weathering is deep and extensive. Hence, soils formed in such regions tend to have deeper profiles. For instance, the forest oxysols in the South Western part of Ghana generally have deeper profiles. In contrast, soils formed in arid regions (eg. Aridisols) have thin profiles due to as weathering is not rapid in such regions. Precipitation also influences horizon development factors, such as the translocation of dissolved ions through the soil. Leaching tends to be more pronounced in places with high amount of rainfall. Precipitation also determines the extent of erosion. In some regions, rainfall may erode the top soil.

With regards to parent material, it must be understood that soils that develop over very hard rocks may have shallow profiles. In contrast soil materials that weather faster are likely to produce deeper soil profiles. Relatively new deposits of soil parent material, such as alluvium, sand dunes, or volcanic ash, may not have distinct horizons. Vegetation influences horizon development in various ways. In places with low levels of plant residues, the topmost soil may be lighter than places with high amount of organic matter, where the top soil may be darkened. Again, in some places burrowing animals such as earthworms may help to move soil from one horizon to another.

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Time also affects soil horizon formation. In fact, the mature of a soil’s profile depends on how long the soil formation processes have been occurring. The longer a soil surface has been exposed to soil forming agents, the greater the development of soil horizons. Older/mature soils tend to have well-developed sequence of horizons. As the soil’s age increases, horizons generally are more easily observed. However, in a few cases, older soils may undergo so much leaching that distinct layers are not clearly visible. It is important to note that the age of soils does not only depend on time, it also depends on the parent materials and other factors. Soils that develop on very hard rocks may take more years to mature than those developing on very soft rocks.

Topography also affects soil profile development. Soils on stable surfaces are usually characterised by well defined horizons. In contrast, those developed over steep slopes may not have enough time to develop deep profiles, as a result of pronounced erosion on steep slopes.

LECTURE 2 PHYSICAL AND CHEMICAL PROPERTIES OF SOILS

Physical Properties of Soils

We shall discuss four important soil physical properties, namely texture, structure, colour and porosity.

Soil Texture

Soil texture is a term used to indicate the proportionate distribution of the different sizes of mineral particles in a soil. In other words, soil texture refers to the relative amounts of the different mineral particles in the soil. It can also be seen as a measure of the coarseness or fineness of the mineral particles that a soil contains. Organic matter is not considered in soil texture analysis. Based on their sizes, mineral particles are grouped into “separates”. A soil separate is a group of mineral particles that fit within definite size limits expressed as diameter in millimetres” (Brown 2003:1). In soil texture analysis, mineral particles over 2 mm in diameter (i.e. rock fragments) are normally not considered. Only three particles, namely clay, silt and sand, are considered in soil texture analysis. Soil texture, therefore, precisely refers to “the relative proportions of sand, silt and clay in a soil” (Ahn, 1993:16). Below are the characteristics of each of these particles.

Characteristics of different soil particle as copied directly from Daniels and Haering (2006:37)

• Sand particles vary in size from very fine (0.05 mm) to very coarse (2.0 mm) in average diameter. Most sand particles can be seen without a magnifying glass. Sands feel coarse and gritty when rubbed between the thumb and fingers, except for mica flakes which tend to smear when rubbed.

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• Silt particles range in size from 0.05 mm to 0.002 mm. When moistened, silt feels smooth but is not slick or sticky. When dry, it is smooth and floury and if pressed between the thumb and finger will retain the imprint. Silt particles are so fine that they cannot usually be seen by the unaided eye and are best seen with the aid of a strong hand lens or microscope.

• Clay is the finest soil particle size class. Individual particles are finer than 0.002 mm. Clay particles can be seen only with the aid of an electron microscope. They feel extremely smooth or powdery when dry and become plastic and sticky when wet. Clay will hold the form into which it is moulded when moist and will form a long ribbon when extruded between the fingers.

* Please note that for the same soil, textural class names and limits can differ depending on the soil classification system used. For instance, clay is defined as less than 0.002 mm in diameter by some classification systems, but others define it as soil with particles less than 0.005 mm (Tabor, 2001).

Determination of Soil Textural Class

In the field, the percentages of clay, silt, and sand particles in a soil can be estimated by feel. The soil is rubbed between the thumb and the fingers and an estimate of its texture is made based on the characteristics observed. It is sometimes difficult to use this traditional method for the determination of texture of a soil that has roughly equal proportions of different particles. A Textural Triangle (Figure 1.2) can be used to determine the exact textural class of any soil. To determine the textural class of any given soil, you will need to know the percentages of clay, silt and sand in your soil. To do this, you will have to take a sample of the soil to the laboratory for soil particle size analysis. Once the percentages of clay, silt and sand are determined in the laboratory, you can now use the textural triangle to determine the textural class of your soil. As shown in Figure 1.2, the sides of the soil textural triangle are scaled for the proportions of clay, silt, and sand. Clay percentages are shown on the left side of the triangle and are read from left to right. Silt percentages are indicated on the right side and are read from the upper right to the lower left. Sand is read along the base of the triangle from the lower right towards the upper left. The bold lines indicate the boundaries of the soil textural classes. The intersection of the three sizes on the triangle indicates the textural class of the soil. In this case, a soil with 20% sand, 60% silt, and 20% clay falls within the “silt loam” class.

Please note that although we usually sketch all the three lines and use their intersection to determine the textural class, any two properly sketched lines (e.g clay and sand or sand and silt) will correctly indicate the textural class name of any soil sample (Brown, 2003).

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Figure 1.2 Textural Triangle

Source: Soil Survey Division Staff (1993).

Influence of soil texture on soil properties and plant life (Significance of soil texture)

Soil texture has a significant influence on other soil properties. First, it affects plant life as it determines the availability and movement of water and other nutrients in the soil (Gurevitch et al., 2002). Plants cannot obtain nutrients from very coarse soil particles. Soils that contain high proportion of sand tend to be low in organic matter and fertility. They also have low water holding capacity. Consequently, coarse sandy soils may require irrigations and application of fertilizers to meet the needs of specific crops. In contrast, soils containing relatively higher percentage of silt and/or clay particles are more fertile, contain more organic matter and are able to retain more water and nutrients. Thus, to some extent, fine-textured soils are good for plant growth. However, when soils are so fine-textured to be classified as clayey, they are likely to exhibit properties which are difficult to manage. Clay-dominated soils are usually too sticky when wet and too hard when dry. This makes them difficult to cultivate (Brown, 2003). So which soil type is best for plants? There is no straight forward answer to this question. As you will learn later, various plants have different

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water and nutrient requirements. Therefore, different soils are good for different plants. In general, however, loam soils, with a relatively equal mixture of sand, silt, and clay, are best for plant growth because they hold and exchange water easily and supply mineral nutrients in a form useful to plants (MacDonald, 2003).

It is clear from the above that a good knowledge of soil texture can help a farmer in his/her choice of the type of crop that will grow well on a given parcel of land. Adequate knowledge of soil texture can also help farmers to choose the type of soil treatments that will improve farming success. Again, given that the rate at which water runs through the soil is partly a function of texture, knowledge about the exact soil texture can help us design appropriate irrigation and drainage systems. Soil texture also determines its suitability for construction works. For instance, clayey soils tend to exhibit “shrink-swell” characteristics, which make them unsuitable for the construction of roads and buildings.

Soil Structure

According to McCauley et al, “soil structure is the arrangement and binding together of soil particles into larger clusters, called aggregates or peds” (McCauley et al. 2005: 3). Generally, organic matter and iron oxides act as adhesives to bind soil particles into peds. There is a relationship between a soil’s structure and its texture. Fine-textured soils, with a higher proportion of clay, tend to have a stronger, more defined structure than coarse-textured soils. This is due to more cohesive strength between clay particles.

Types of soil structure

The major soil structural types are: Granular, Blocky, Platy, Prismatic and Structureless. These are summarised in Figures 1.3 and table 1.1 below.

Figure 1.3 Major types of soil structure

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Table 1.1 Characteristics of the major types of soil structure

Structure type Description Granular Soil particles are arranged in small, rounded units.

Granular structure is very common in surface soils (A horizons) and is usually most distinct in soils with relatively high organic matter content.

Blocky Soil particles are arranged to form block-like units, which are about as wide as they are high or long. Some blocky peds are rounded on the edges and corners; others are angular. Blocky structure is commonly found in the subsoil, although some eroded fine-textured soils have blocky structure in the surface horizons.

Platy Soil particles are arranged in plate-like sheets. These plate-like pieces are approximately horizontal in the soil and may occur in either the surface or subsoil, although they are most common in the subsoil. Platy structure strongly limits downward movement of water, air, and roots. Platy structure may occur just beneath the plough layer, resulting from compaction by heavy equipment, or on the soil surface when it is too wet to work satisfactorily.

Prismatic Soil particles are arranged into large peds with a long vertical axis. Tops of prisms may be somewhat indistinct and normally angular. Prismatic structure occurs mainly in subsoils, and the prisms are typically much larger than other typical subsoil structure types such as blocks.

Structureless Either: • Massive, with no definite structure or shape, as in some C horizons or compacted material.

Or: • Single grain, which is typically individual sand grains in A or C horizons not held together by organic matter or clay.

Source: Daniels and Haering (2006:40)

Effects of Soil Structure on other Soil properties

Soil structure has some effects on water and air movement within the soil. According to Bandel (2002), the greater the degree of aggregation, the more pore space available to supply water and air to plant roots. Cracks and channels between peds are important for water and air movement and deep water drainage. Since water and air are both important for plant life, it can be stated that soil structure also affects its ability to support plant life. Again, since plant roots move through the same channels in the soil as air and water, well-developed structure also promotes extensive root

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development. In a nutshell, soil aggregation is important for increasing stability against soil erosion, for maintaining soil porosity and soil water movement (Nichols et al., 2004).I want to stress that even stable structures can be lost by careless handling of the soil. Cultivating the soil when it is too wet or subjecting it to the stresses and compaction of vehicles and heavy machinery can destroy its structure.

Soil Colour

One of the most important physical properties of soil is its colour. Colour of surface soil tends to reflect a strong imprint of biological processes, particularly those influenced by the ecological origin of organic matter. Surface soils that contain high amount of organic matter or humus tend to have black or dark-brown colours. A surface soil with bright-light colour can be associated with the horizon of eluviation (usually the E horizon), where organic matter, carbonates and clay minerals have been leached out.

Subsoil colour is more strongly influenced by physical and chemical processes, especially the redox status of iron (Fe). Well aerated soils contain Fe3+ which gives soil a red or yellow colour. This is normally the situation in a well drained soil. In poorly drained soils (i.e. under anaerobic conditions), iron compounds are reduced, and the soil exhibits the grey colours of Fe2+ or the bluish-green colours of iron sulphides and iron carbonates. While iron generally exerts the greatest influence on subsoil colour, manganese can also influence the colour of subsoil. A black colour in the subsoil may reflect an accumulation of manganese.

In some cases, the colour of soils may be inherited from the minerals of the parent materials from which those soils have developed. For instance, light grey colours in the subsoil can be inherited from parent material quartz. Again, basalt can imprint a black colour to the subsoil horizons. 

Soil Colour and Drainage

Soil drainage is the rate of vertical or horizontal water removal from the soil . Generally, bright red and yellow subsoil colours are associated with well-drained soils where iron is present in its oxidized form. Soils that are extremely poorly drained usually have grey colours. The reason is that when soils are saturated for a long period of time, the oxidized (red or yellow) forms of iron are reduced to soluble forms which are easily moved with drainage water. Through this process, the soil colour eventually changes from red/yellow to grey.

While many soils have a dominant colour, others in environments where soil forming factors vary seasonally (e.g. wet and dry seasons), tend to exhibit a mixture of different colours. The term mottling is used when several colours are present in a given soil. Soils which are wet in their upper layers for a long period of time (e.g wetlands) are referred to as hydric soils. Drainage mottles in these soils are termed redoximorphic features. Subsoil layers with mixtures of red/yellow and grey mottling are indicative of seasonally fluctuating water tables, where the soil is saturated during the rainy season and unsaturated in the dry season.

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Importance of Soil Colour

Soil colour is used to classify various soils of the world into groups (e.g black soils, red soils etc). Soil colour also gives us an indication of the type of minerals that are likely to be present in any given soil. For instance, a bright red/yellow colour in the subsoil may indicate the presence of iron minerals. It has also been explained that soil colour gives us an indication of the drainage condition of an environment. It can also tell us something about the organic matter content of a soil. A black colour in the surface soil may indicate the presence of humus and for that matter a high fertility.

Soil Porosity

Soil porosity refers to the amount of pore space between soil particles. In other words, soil porosity is to the volume percentage of the total soil that is not occupied by solid particles. Generally, pores smaller than 0.05 mm in diameter are termed micropores, while those larger than 0.05 mm in diameter are called macropores. There is a relationship between texture and porosity. Coarse-textured soils have many macropores because of the loose arrangement of larger particles. Conversely, fine-textured soils have more micropores because the fine particles are often more tightly arranged than large particles (McCauley et al. 2005). Macropores in fine-textured soils exist between soil aggregates or peds. Since fine-textured soils have both micropores and macropores, they tend to have a greater total porosity (or sum of all pores) than coarse-textured soils.

Soil porosity and other properties, such as texture and structure, have some effects on soil-water relationships. Macropores allow the free movement of water, air and roots of plants. On the other hand, micropores in wet soils are generally filled with water, and this makes it difficult for much air to move into or out of the soil. Internal water movement is also very slow in micropores. The movement of water and air through a coarse-textured sandy soil can be very fast despite its low total porosity because of the dominance of macropores. In contrast, fine-textured clay soils, especially those without a stable granular structure, may have reduced movement of water even though they have a large volume of total pore space. The reason is that micropores are dominant in these fine-textured clay soils. Stated differently, sandy soils hold little water since their large pore spaces allow water to drain easily from the soils. Fine-textured clay soils, on the other hand, absorb relatively large amount of water, and their small pore spaces do not allow the water to drain freely. This explains why fine-textured clay soils hold more water than coarse-textured sandy soils. It must be stressed that clayey soils hold water much more tightly than sandy soils. In effect, not all the moisture retained in clayey soils is readily available to plants. Consequently, moisture stress can become a problem in clayey soils even though they have high water-holding capacity (Daniels and Haering, 2006). In addition, because micropores in fine-textured clay soils are often full of water, aeration, especially in the subsoil, can be inadequate for root development.

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CHEMICAL PROPERTIES OF SOILS

Two important chemical properties will be discussed here. These are soil pH and Cation Exchange Capacity.

Soil pH

Soil pH refers to the degree of acidity or alkalinity of the soil. In other words, soil pH is a measure of hydrogen ion (H+) concentration in the soil (McCauley et al. 2005). The measurement of hydrogen ion is based on the pH scale. The pH scale ranges from 0 to 14. The pH of pure water is 7 and this is considered neutral. pH values below 7 are considered as acidic, while those above 7 are said to be alkaline or basic. Generally, good agricultural soils have a soil pH between 5 and 7 (Ritter, 2006).

Causes and Effects of Soil Acidity

According to Daniels and Haering (2006), soils become acidic when basic cations (e.g calcium) are leached from the soil, and are replaced by aluminum ions. This process of acidification is generally accelerated by the decomposition of organic matter which also releases acids into the soil. Thus, high rate of leaching and high amount of organic matter are likely to produce acidic soils. Soils that develop under high rainfall conditions and abundant vegetative cover tend to be acidic because of high rate of leaching and high organic matter content.

Soils with very high levels of hydrogen ions (i.e. acidic soils) are generally not good for crop production, since they lack important nutrients. Alkaline soils are also sometimes not very good for crop production, since they sometimes contain significant amounts of sodium that may exceed the tolerances of plants.

Soil Cation Exchange Capacity

Cation Exchange Capacity (CEC) refers to the net ability of a soil to hold, retain, and exchange cations (positively charged ions), such as calcium, potassium, magnesium and sodium (Daniels and Haering, 2006: 47). The finest clay and organic matter particles are referred to as soil colloids (McCauley et al. 2005). They generally possess negative surface charges, which are present in excess of any positive charges that may be present in the soil. Consequently, soil surface has a net negative charge (Daniels and Haering, 2006). The negatively charged colloids attract cations (positively charged ions), such as calcium (Ca2), potassium (K+), magnesium (Mg2+) and sodium (Na+), and prevent their leaching. The cations that are retained are often weakly held to the colloids and can therefore be replaced or exchanged for other cations, in the soil solution (Ritter, 2006). For instance, Ca2+ can be exchanged for aluminum (Al3+). Thus, the ability of a soil to attract, retain and exchange cations is its Cation Exchange Capacity. The higher a soil’s Cation Exchange Capacity, the more cations it can retain. Since cations are attracted by colloids (humus and clay particles), a soil with a higher amount of clay and humus (colloids) is likely to have a higher Cation Exchange Capacity.

Effects of Cation Exchange Capacity on Soil Properties

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A soil with a relatively high Cation Exchange Capacity is likely to have high organic matter content. CEC also provides an estimate of nutrient storage and release from soil particles. Soils with higher CEC values have high amount of nutrients to support plant life. The CEC also provides an approximation of soil texture. Soils with a higher CEC values tend to have higher clay and silt particles. Conversely, coarse–textured sandy soils generally have low CEC value (Mackowiak, 2007). Also, soils with high CEC values generally have a relatively higher water-holding capacity than those with low CEC values.

Good Luck!

Prepared by Dr. J.K. Teye

Further Reading

Daniels, W. L. & Haering, K. C. (2006) “Concepts of Basic Soil Science” The Mid-Atlantic Management Handbook. February 2006, Chapter 3.

MacDonald, G.M. (2003) - Biogeography: Introduction to Space, Time and Life. Read pages 32-34.

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