Soil properties (Biological, Physical, And Chemical Properties Of Soil)


The main constituents of soil are mineral particles, produced as a result of the weathering or breakdown of rocks, and plant and animal tissues in various stages of breakdown or decomposition. Within this complex is the soil moisture, which serves as a reservoir to supply plant roots with water and also acts as a carrier for minerals in solution which are again needed by plants  for normal growth and development. Many living organisms also inhabit soils, these include bacteria, nematodes and earthworms, some of which are beneficial to crop growth while others are harmful.

Soils also contain a properties soil composition  of which varies with the amount of oxygen used by roots in respiration and the amount of carbon dioxide which is either produced or absorbed by the activity of soil organisms such as bacteria. The physical nature of the soil, the water content and the degree of compaction will also determine, to some extent, the volume of the soil atmosphere.

The physical and chemical composition of soils will depend largely on the nature of the parent material in which the mineral content of the soil has been formed, but the vegetation which has existed on the soil surface will also have contributed to the final product. Soils which have been covered with firriga vegetation, for example, will have a higher content of organic material or humus than soils which have been exposed to the direct effects of high temperatures

The past effects of sun, water and vegetation cover on soils in tropical areas has therefore been of considerable importance in determining their present constitution, particularly if these have been accompanied by more recent changes brought about by cultivation measures which include forest clearing, alteration of the water content by draining or irrigation, excessive cropping or over-grazing by animals.

Biological content of soils

The roots of plants have a significant effect on soil structure by penetrating into hard soils; the subsequent death of the roots leaves cracks which encourage the penetration of water and oxygen.

Some crops, such as the pigeon pea (Cajanus cajan) are particularly
effective in breaking up hard subsoil layers. In addition to plant roots, the soil is inhabited by many types of micro-organism including: bacteria; protozoa; fungi; algae and nematodes. In various ways, these organisms affect the fertility of the soil, the beneficial effects being the production of materials which can be absorbed by plant roots as a result of their biological processes. The Rhizobium bacteria
which inhabit the roots of leguminous plants and are capable of absorbing and 'fixing' the soil nitrogen to form nitrates which can be used by plants can also be included in this category. Some nematodes, however, are harmful to plant roots which they use as a  source of food.

Other larger organisms such as earthworms have a more mechanical effect on the soil constitution since
they tunnel through the soil, allowing for the penetration of water and soil
Most of the biological activity in soils occurs in the upper layers, where there is an adequate amount of organic material to provide a source of food material gases.

Physical properties of soils

Soil texture

Soils are mixtures of sand, silt and clay in various proportions depending upon the nature of the material from which the soil was formed and the various processes it has been subjected to during parent weathering and formation. The term 'texture' indicates the relative content of these three components in any given soil sample. A basic classification which is commonly used to describe the texture of soils is:
clay, light clay, heavy loam, loam, light loam, loamy sand and sand. The term 'loam' refers to a soil which is neither a clay nor a sand but which has characteristics intermediate between these category.

A test which can be carried out to obtain an approximate appreciation of these categories is as follows.
1 Take a sample of finely sifted soil, sufficient to fill a matchbox.
2 Add water slowly until the soil becomes sticky, this point is reached when the soil can be moulded into a ball about 2.5 cm in diameter.
3 The ball should then be rolled out with the hand on a wooden surface to form a cylinder until it is about 15 cm in length.

A sandy soil cannot be moulded or rolled out, it will remain in a small heap. If the soil can be made into a ball, but will not roll into a cylinder, it is a loamy sand, samples which can be rolled out to form a cylinder but which break when curved are likely to be loam soils.

If a sample is bent to form a U-shape, but will not form a circle, it is likely to be a heavy loam. When a sample is bent to form a circle and does not crack it can be classified as a clay soil.
Many of the soils of West Africa have been subjected to considerable weathering and therefore have a relatively high clay content while the percentage of silt is generally low.

Chemical properties of soil

The effects of high temperatures in breaking down rock particles into smaller fragments, due to alternately heating and cooling, have resulted in a larger areg of the parent material being in contact with water.

The weathering of parent soil material by water has therefore determined, to a large extent, the chemical composition of the soil which has been ultimately produced by these weathering processes. Some of the chemical products of weathering will have been leached or washed down into the lower layers of the subsoil, leaving other more insoluble products in the upper layers or horizons of the soil.
The most rapidly removed compounds are the chlorides and sulphates, followed by calcium, sodium, magnesium and potassium. The silicates and oxides of iron and aluminium (sesquioxides) decompose very slowly and are rarely leached. When some of these products come into contact with the oxygen from the soil atmosphere, chemical reactions occur, particularly oxidation,

resulting in the formation of compounds which may be more soluble than the original products. There may also be a physical change in the products of oxidative reactions; the original material, for example, may disintegrate or break up as a result of the change in its chemical composition.
The carbon dioxide in the soil atmosphere also has an effect on the chemical constitution of soils, since it
can form a weak acid

This will also react with some of the soil constituents to form new chemical compounds, some of which can be absorbed by plant root systems.
Soils may have either an acid or alkaline reaction, some may be neutral, with a pH value of approximately 7. Acid soils are most likely to occur where the drainage is poor, this accumulation of excessive amounts of soil water, accompanied by a reduction in the percentage of oxygen present, frequently results in a lowering of the pH level of the soil. The activity of
some soil organisms under these conditions also contributes to the accumulation of organic acids.

Alkaline soils are often produced where the rate of evaporation of the water from the upper layers of the soil is excessive, leading to an accumulation of calcium, magnesium, sodium and other bases in these layers.
The low rainfall which is characteristic of the savanna areas also reduces the rate of leaching of many compounds which are alkaline in nature.

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