Constituents Of Soil

_{Constituents of soil}

Inorganic matter or minerals:

Compound of silicon, alumminium and iron are the major minerals found in soil. Other minerals including calcium, magnesium, potassium, sodium, nitrogen, manganese, phosphorous and sulfur is present in lesser amount in most soil. The size of these minerals constituent may be 0.002mm or less as in clay or as big as large pebbles and gravels (2mm). These different types of particles are formed by weathering of the rock and the degradative metabolic activities of the microorganism. The proportion of these particles in the soil vary and this variation determines the physical structure, aeration, water holding capacity and availability of nutrients. Soil is classified broadly into two types:

Minerals soil:

It has solid matter which is largely inorganic materials.

Organic soil:

It has very less inorganic materials.

Soil contain varying proportions of clay, slit and sand particles. According to the composition of these particles, soil is classified into following types:

Sandy soil:

It contain more or less 60% of sand particles with a small proportion of clay and slit, usually not exceeding 10% of each. It is well aerated being very porous. It allows easy percolation of water to large pore space, quickly dries up and often remain dry. These soil is loose and light and has no cohesive power. Capillarity decrease in this soil and it can hold only 25% of water of its own weight, when saturated. It contains very little plant food. It can be improved by the addition of clay lime or humus. Being loose and porous, It helps in seed germination and root growth but is not suitable for subsequent growth.

Clay soil:

It contains more than 50% of clay particles. It is compact and heavy. It becomes easily water logged and is badly aerated. It is hard and often cracks when dry but becomes soft and sticky when wet. Capillarity increases in the soil and has great capacity of holding water 40% or more of its weight. It always contains a considerable amount of plant food but the root cannot penetrate it. Addition of lime or sand improves the soil and makes It more suitable for plant growth.

Loam soil:

Loam contain 30-50% of slit and a small amount of clay the rest being sand. It is the best soil for plant growth and is most suitable for agriculture crops because all important physical condition are satisfied. It can hold 15% or more water of its own weight. It is rich in plant food or plant nutrient.

Some other types of soil are:

Calcareous soil:

Contain greater than 20% of carbonate, neutralized organic acid formed from humus.

Laterite soil:

Contain high % of iron and alumminium oxide and reddish and yellow in color.

Peat soil:

Contain high % of humus, is dark in color, porous and light.

The soil texture is important because it determine the surface area available for microbial growth. The proportion of various clay minerals also affect the binding properties and chemistry of soil. The physicochemical differences in temperature, moisture and pH greatly influence which microorganism occur within a indigenous microbial communities of different soil habitat. The determine the soil texture, the proportion of sand, silt, clay are determined. The intersection of lines drawn perpendicular to the side of the soil triangle indicates the soil texture.

Organic matter:

Organic matter include the plant and animals remains deposited on or in the soil. After decomposition of these remains by the synthetic activities of the microflora, humus is formed. Humus is a dark brownish black, waxy materials , amorphous in nature and also known as the "fat of the land". humus is an important food source for plant and microbes. It improve the texture and structure of soil, contribute to the buffering capacity, increases its water holding capacity and aerates the soil. The organic matter of the soil generally include a large population of microorganism; both living and dead cells. Humus also influence the chemical properties of the soil such as the solubility of soil or rock minerals and the oxidation reduction potential. It is the great store house of food materials for higher plants. It holds and slowly release nitrogen, phosphorous, minerals and also Co₂ under the action of microbes. Organic fraction of soil is derived from:

1. Plant constituent which are modified by the micro flora.
2. constituents of microbial cell and products of microbial metabolism which are relatively resistant to decay and therefore persist for some time even after the death of microorganism.

In terms of specific elements, the organic fraction contains compound of carbon, hydrogen, oxygen, nitrogen, phosphorous, sulfur and small amount of other elements. Humus contains polymerize substances, aromatic molecules, different polysaccharides bound amino acid, polymer of uronic acid and phosphorous containing compounds.

Soil solution:

Soil water is derived from atmospheric precipitation. The amount of soil water depends on the other climatic condition, drainage, soil composition and the living population of the soil specially plant cover. The supply of soil water is variables and fluctuation are seen in different soil types. Part of soil water move with the gravitational pull and is called free or gravitational water. Free water exist in thin sheet of film between the soil particles, such water is situated within the larger soil pores. Some water is retained against the pull of gravity. It is due to the adsorption of water and other soil constituent. All of the soil water is not available biologically, only some part of the water retained against the gravitational pull is used by the living system. Non biological constituents of soil have a great binding power for water. Various organic and inorganic components are dissolved in soil water to form soil solution which provide nutrients for the soil inhabitant. Soil solution is quite concentrated in arid regions. The downward movement of water removes the substances essential for microorganism; nitrogen, potassium, magnesium, sulfur, calcium but less amount of phosphorous and organic matter are leached in this ways.

Gases:

Air and water together comprise half of the soil volume. The volume thus occupied is called pore space. Amount of pore space depends on the texture, structure and organic matter content. In clay soil, the pore are generally small where as in sandy soil, the pores are larger. In sandy soil, the total quantity of pore space is less in comparision to soils reach in fine particles like clay. Aeration and moisture are directly related because the pore space is not filled with water, is filled with gas. Air moves into those pores which are free of water. Water on the other hand, displace the air. The gas found on the soil profile constituent the soil atmosphere. The subterranean atmosphere is different from the atmosphere above the earth. Though the soil atmosphere is derived from the air above its composition is different because of the biological process occurring in the soil. The gases present in the soil mostly include Co₂, O₂ and No₂. A small amount of gases in soil is related to the amount of moisture. The amount of Co₂ in soil atmosphere is more than in the atmosphere above the earth 10-1000 times. But the amount of O₂ in the soil atmosphere is less than in the atmosphere above the earth. This is because O₂ in the soil is used by microorganism and plant roots for respiration and during this process they release Co₂ into the soil atmosphere. Changes in soil atmosphere also change the size and function of micro flora as both O₂ and Co₂ are needed for growth. Therefore, in terms of microbiology, a well aerated soil is the one in which microbial process requiring O₂ occur at a very rapid rate. Because gas movement cannot occur easily into small pores and microenvironment, soil can satisfy all its inhabitants. If a soil is properly aerated for higher plant, it may not be well aerated for microbial flora. Improper aeration leads to poor drainage and water logging. The aeration status of heavy soils having a lots of micro pores is often poor and large volume of pore is occupied by liquid rather than gas. It is because small pores have a greater tenacity for water than large pore. If the oxygen content in the soil atmosphere is less the rates of many microbial transformation are reduced and some microbial processes may be eliminated as well. In this condition, new microbial processes may occur, some of which may even be harmful for the development of plant . for eg. N₂ gas or methane is evolved, organic inhibitors appear and sulfide ferrous and Mn ions accumulates during the condition of oxygen deficiency. In well drained soil air penetrates readily and O₂ concentration can be high. However, in water logged soil the only oxygen is present that dissolved in water and it is quickly consumed by microorganism. Such soil quickly become anoxic and show great changes in biological properties. eg. more dominated by anaerobes and change in metabolic activities.

Reference:

1. Alexender M (1961). Introduction to Soil Microbiology, Academic Press.
2. Rangaswami G and Bagyaraja PT(1993). Agricultural Microbiology(second edition), prentice Hall Of India.