Soil: Definition, Components and Role of Soil Organisms (With Diagram)

The word soil is derived from a Latin word ‘solum’ meaning earthy material in which plants grow.

The study of soil is known as Soil science or Pedology (pedos = earth) or Edaphology (edaphos = soil).

The study of soil is important in many respects. Soil is natural habitat for Plants and animals. It provides water and nutrients to the living organisms.

“Soil is a natural body developed by natural forces acting on natural materials. It is usually differentiated into horizons of minerals and organic constituents of variable depths which differ from the parent materials in morphology, physical constitutions, chemical properties, composition and biological characteristics”- Joffe and Marbut.

Knowledge of soil is helpful in agricultural practices, such as cultivation, irrigation, artificial drainage and use of fertilizers. It is also important from geological, petro-logical, mineralogical and paleobotamcal points of view.

The earth is more or less, spherical and its surface is highly irregular, marked by deep oceans, high mountain ranges and plains in between. The internal composition of earth is not known exactly. According to a generally accepted interpretation, the earth has three zones, viz., core, mantle and crust (Fig. 21.1). The core is the central fluid or vapounsed sphere having diameter of about 2 500 kms from the centre and is possibly composed of nickel-iron (Urey, 1952). The mantle extends about 2,900 kms above the core.

This is in molten state. The outermost solid zone of the earth is called crust which is about 8 to 40 kms above the mantle. The crust is very complex. We live on its surface. Soil developed from the pre-existing rocks, the regolith of the crust. Here soil means the loose, friable, unconsolidated top layer of the earth crust. The soil is differentiated into several layers or horizons which can be distinguished from one another by their colours, textures and other characteristics.

It contains water, gases, complex minerals organic substances and micro-organisms. The dead remains of plants and animals are degraded by micro-organisms and after degradation a number of organic substances, generally called humus, are contributed to the soil. The mineral component of the soil is derived from the rocks.

Definition of soil:

Soil may be defined as “the part of earth crust in which humus is present”.

According to R.F. Daubenmire, “soil is the upper part of earth crust in which plants are anchored.” He defines soil as weathered superficial layer of earth crust with which are mingled living organisms and products of their decay.

According to Hilgard, 1917 (American school), “it is, more or less, loose, friable material in which, by means of their roots, plants may or do find a foothold, nourishment as well as other conditions of growth.”

According to Raman, 1928 (German school), “soil is the upper weathering layer (i.e., layer subjected to physical and chemical changes) of the solid earth crust.”

Joffe and Marbut, two well-known American soil scientists, have defined soil in the following way “soil is a natural body developed by natural forces acting on natural materials. It is usually differentiated into horizons of minerals and organic constituents of variable depths which differ from the parent materials in morphology, physical constitutions, chemical properties, composition and biological characteristics.”

According to Russian school, “soil is natural body differentiated into horizons of usually unconsolidated minerals and organic constituents of variable depths.”

According to Wadia (1945), “soil is the topmost layer of earth crust capping the rock.” It is natural body of variable thickness, composed of disintegrated rock materials together with variable proportions of organic matters, generally differentiated into zones or layers and mostly unconsolidated.

In brief, soil can be defined as that region on the earth surface where geology and biology meet each other.

Components of Soil:

The soil is made up of the following components:

(1) Mineral particles,

(2) Dead organic matter or humus,

(3) Soil atmosphere,

(4) Soil water, and

(5) Biological system or soil micro-organisms.

1. Mineral Components:

The mineral constituents of the soil are derived from the parental rocks or regolith. They may be found in the form of particles of different sizes; from clay (.0002 mm or less in diam) to large pebbles and gravels. The minerals represent about 90% of the total weight of the soil. Important elements which are found in compound state are Oxygen, Si, Fe, Al, N, P, K, Ca, Mg, C, H, etc. In soil, nitrogen comes from atmosphere in the form of nitrogen salts.

2. Organic Matter or Humus:

Besides inorganic minerals, some organic residues derived either from dead remains of plants and animals or through metabolic activities of living organisms are present in the soil. When the plants and animals die, their dead remains are acted upon by a number of microorganisms and are finally degraded or decomposed into simple organic compounds. A product of this microbial decomposition is humus which is a dark coloured, jelly-like amorphous substance composed of residual organic matters not readily decomposed by soil microorganisms. The process of humus formation is called humification.

The chief elements found in humus are carbon, hydrogen, oxygen, sulphur and nitrogen. The important compounds found in it (humus) are carbohydrates, phosphoric acid, some organic acids, fats, resins, urea, etc. Tree litter (very little decomposed dead matter) also contains some inorganic substances as lime, potash, Mn, Mg, silica, Cu, Al, Ga, Na, K, etc. Humus is a dynamic product and is constantly changing because of its oxidation, reduction and hydrolysis. Hence, it has no definite chemical composition. It has much carbon content and less nitrogen.

Humus is not soluble in water. It is present in soil in the form of organic colloids. The amounts of humus in different soils vary greatly. Humus percentage in the soil is affected by climatic and biological factors. It is less in arid soils and very high in humid soils. In the top layer of the soil, humus quantity is greater than in the deep layers.

In dark humid areas which are thickly covered with vegetation, the humus may be found in the following three stages of degradation:

(i) The top floor is covered with dead organic parts showing low degree of decomposition. These poorly decayed dead parts of plants form litter. (Fig. 21.2).

(ii) Below the litter may be found a layer of partially decomposed organic matter which is known as duff layer.

(iii) When the duff is decomposed completely into organic substances, the decomposition products, generally called leaf moulds, are accumulated below duff layer.

Sometimes under anaerobic conditions, the dead remains are not at all acted upon by the microorganisms. Accumulation of such un-decomposed organic remains is termed as peat.

Humus plays many important roles in the soil, such as:

(a) It makes the soil fertile.

(b) It provides nutrients to the plants and microorganisms.

(c) On complete decomposition, it forms several organic acids which serve as solvents for soil materials. Thus humus increases the availability of minerals in dissolved state to plants.

(d) Because it is porous, it has got high capacity for retaining water.

(e) Humus makes the soil porous, thus increases the aeration and percolation which make the soil more suitable for the plant growth.

(f) It also acts as weak cement thus binds the sand particles.

(g) Presence of humus in the soil increases the rate of absorption in plants.

The factors which influence the rate of humifications are outlined below:

(i) Nature of plants, animals or soil organisms.

(ii) Rate of decomposition.

(iii) Temperature (increase in temperature up to a certain limit increases the rate of humification).

(iv) Aeration and moisture. These increase the rate of humification.

3. Soil Atmosphere:

Gases found in soil profiles are said to form the soil atmosphere which is one of the most important components of the soil. The spaces between soil particles and soil organisms are called pore spaces. These are filled with moisture and air in varying quantities which account for approximately half of the total volume of soil. In dry soils, percentage of moisture is lesser than that in wet soils.

The soil atmosphere contains three main gases, namely oxygen, carbon dioxide and nitrogen In soil atmosphere, oxygen is 20%, nitrogen is approximately 79 per cent and carbon dioxide IS 0.15 to 0.65 per cent by volume. In the cultivated land, percentage of CO₂ is much higher than that of atmospheric CO₂, but oxygen content in such soil is poorer than the percentage of oxygen in atmospheric air.

Oxygen of soil is absorbed by plant roots and soil micro organisms in respiration and CO₂ is given out which accumulates in spaces. The amount of CO₂ increases with the increase in depth of the soil due to decomposition of accumulated organic matter and abundance of plant roots. Heavy accumulation of CO₂ in the soil is harmful for the plant growth Presence of oxygen in the soil is important in the sense that it helps in the process of breakdown of resoluble rocky mass into soluble minerals and also in the humification (a process in which insoluble minerals and organic nutrients locked up in the dead remains of plants and animals are converted into soluble forms).

The accumulation of soluble nutrients in the soil makes it more productive. If the soil is deficient in oxygen, the rates of microbial activities are slowed down and may be eliminated. Under such conditions, several undesirable processes, such as evolution of nitrogen, methane, accumulation of sulphides, ferrous, manganous ions and organic inhibitors and so many other processes may come into play which may be injurious to plants.

The important factors which bring about changes in the soil atmosphere are temperature atmospheric pressure, wind and rainfall. Temperature and atmospheric pressure cause expansion and contraction of the soil air. Wind helps the soil in sucking the air in and rain water displaces the soil air. Any considerable change in the soil atmosphere affects the size and function of micro-flora and other biological populations.

4. Soil Water:

Soil water plays very important role in the plant growth. Plants absorb a small quantity of ram water and dew directly from their surfaces but most of water absorbed by them comes from the soil. Soil water maintains the soil texture, arrangement and compactness of soil particles. It is good solvent for minerals and it makes the concentration of nutrients low so that nutrients may be absorbed by plants easily.

Water affects the plant growth and other physiological activities In plant growth, water forms a major part of the plant itself It is essential for the process of photosynthesis, it maintains the turgidity of the plants and acts as a medium by which mineral salts essential for plant growth enter the plants from the soil. In brief, water regulates the physical, chemical and biological activities in the soil.

Water in the soil comes mainly through infiltration of precipitated water (rain, sleet, snow and hail) and irrigation whereas it is lost from the soil chiefly through evaporation, percolation stream flow and transpiration. ‘ The quantity of water available in the soil varies from place to place. The amount also depends upon the quality of soil. In loamy, silty and clay soils, the amount of water is greater than that in coarse sandy soil.

Water is held in the soil in the following forms:

(i) Gravitation water,

(ii) Capillary water,

(iii) Hygroscopic water,

(iv) Water vapour, and

(v) Combined water.

(i) Gravitational water:

After complete water saturation of soils the excess water displaces air from the pore spaces between soil particles and percolates downwardly under gravitation influence and finally it is accumulated in the pore spaces. This excess water is called gravitational water. The amount of water held in the soil, when all pores are filled and when drainage is restricted is maximum water holding capacity.

When the gravitational water percolates down and reaches to the level of parental rock it is called ground water.

(ii) Capillary water:

The amount of water present around the soil particles at saturation stage, when gravitational water has drained away through capillaries or channels, is called capillary capacity or field capacity and the water which is held by surface tension and attraction force of water molecules as thin film around soil particles in the capillary spaces is called capillary water. It moves in the direction where capillary tension is more.

(iii) Hygroscopic water:

Water which is adsorbed on the soil particles and held on the surface of soil particles by forces of attraction and cohesion of its molecules is called hygroscopic water.

(iv) Water vapour:

This is the water present in the soil atmosphere in the vapour form.

(v) Combined water:

It is water of chemical compounds held by chemical forces of molecules (as for example, CuSO₄.5H₂O). It can be driven out from the compounds only at bright red heating.

Water requirement of plants varies from individual to individual. Some absorb large quantity, while some others require very small quantities of water for their normal growth. A major fraction of total water absorbed from the soil is transpired by the plants and only a small quantity of it enters the composition of protoplast.

The availability of soil water to plants depends primarily on its diffusion pressure deficit, often termed the soil moisture stress. The total of all the forces which the plants must overcome to take up water from soil is called soil moisture stress. Water lost from the soil surface by evaporation and through absorption by plants is replaced by rise of capillary water from root zone.

The continuous loss of water may finally result in a stage at which water content of the soil becomes so poor as it (soil) cannot supply water to growing plants rapidly enough to maintain them turgid. Under such conditions, permanent wilting occurs in the plants. At permanent wilting stage, the percentage of moisture in the soil is termed as wilting coefficient or permanent wilting percentage (Fig. 21.3 C, G).

The difference between field capacity and wilting coefficient is termed as maximum available water and the water content of the soil at any time over and above the wilting coefficient is referred to as available water The amount of water to be added to a soil at the wilting point to reach the field capacity is called the available water capacity.

It need not be emphasized here that the actual moisture content of a soil has little meaning in respect to plant growth unless:

(i) The field capacity and

(ii) The permanent wilting percentage of the soil are also known.

Water contents above field capacity displace so much of the soil air that the plant roots usually suffer from inadequate aeration and serve to be detrimental. Although plants usually continue to absorb water in the soil drier than at permanent wilting stage, absorption is too slow to replace water losses and the resulting water deficit causes cessation of growth and finally results in death from dehydration. The moisture at the field capacity is held with a force of one-third atmosphere and that at permanent wilting stage is held with a force of 1.5 atmosphere.

5. Biological System of the Soil or Soil Microorganisms:

Organisms present in the soils are called soil organisms. Important group of soil organisms are given below (Fig. 21.4).

Many of these soil organisms are stable, many are mobile, but some are held in the colloidal films of the soil particles. Protozoa, mites and insects are example of moving organisms. They move in or on the surface of soil in search of food. Earthworms by the burrowing habit make the soil loose and fertile. They are found in abundance. In some forests their number may reach up to 10,000 per square foot. These soil organisms feed on the organic matter of the soil.

The majority of soil fungi are found in acidic soils. Actinomycetes prefer saline soils and soil bacteria grow fairly well in the neutral soils richly supplied with organic nutrients. These microorganisms are found in the soil at variable depths. Algae are found in the top layer of soil under the conditions of constant shade and moisture.

It is estimated that in soil micro flora bacteria form about 90 per cent of the total microbe population. Fungi and algae together represent only one per cent and actinomycetes cover 9 per cent. Density of microbial population is actually governed and influenced by climatic conditions, physical and chemical nature of soil and vegetation cover. The greatest amount of microbe (10, 00 000 per cubic cm) is found in the top layer of soil at a depth of 5 to 15 cm.

In deeper layer (1.5 to 5 m) individual microbes are found. However, they have been discovered at a depth of 17.5inin coal, oil and artesian water. It has been calculated that in the ploughed layer of cultivated soil over an area of one hectare there may be from 5 to 6 tons of microbial mass and one gram of ploughed soil contains 1-10 thousand million bacteria

Role of Soil Organisms:

Soil organisms take part in a number of processes in the soils. Some of their important roles are as follows:

(1) Decompose the dead organic matter and increase plant nutrients in available forms,

(2) Production of toxins,

(3) Production of growth stimulating substances,

(4) Nitrogen fixation in the soil,

(5) Mixing of soil,

(6) Improvement in soil aeration,

(7) Improvement in the aggregation of soil particles or soil binding, and

(8) Cause injury to the plants.

1. Decomposition of dead organic matter:

A number of soil microbes attack the dead remains of plants and animals and cause decomposition. In the process of decomposition, complex organic matters are converted into simple organic compounds. Compounds like sugars, starch and proteins are decomposed first in the decomposition process and then cellulose, fatty substances and lastly lignin and woody substances are degraded.

Proteins when acted upon by microbes are converted into ammo acids, ammonium salts, nitrates and nitrites. Humus, an intermediate product of decomposition process, is formed by micro-organism in optimum physical conditions. In the decomposition process, a number of complex mineral compounds are also converted into simpler and soluble compounds. Organic acids and carbon dioxide that are released by decomposition make insoluble phosphates and other unavailable compounds more easily available to plants.

Decomposition of dead organic matter primarily helps in the feeding and growth process of these micro-organisms and secondly, increases the nutrient contents of the soil. Bacteria and soil fungi are main agents which bring about the process of decomposition in the soil.

2. Production of toxins:

In the absence of oxygen some soil microbes secrete chemicals such as, aldehydes, organic acids, etc. which may show toxic effects on many plants. Examples of toxin secreting organisms may be found in fungi, bacteria and algae. Fusarium lini which causes wilt of flax (Alasi) secretes HCN, a deadly poisonous substance and Fusarium udum a fungus causing wilt of pigeon pea (Arhar) secretes fusaric acid in the roots of the host plants These toxic chemicals secreted by fungi may be responsible for causing wilt in the flax and arhar (Cajanus cajan).

3. Production of growth stimulating substances:

Many soil organisms including soil fungi and bacteria produce growth stimulating substances such as 3-Indol acetic acid, Gibberellins and Gibberellic acid in the soil. Fusarium species too have been found to secrete Gibberellin and Gibberellic acid (C₁₉H₂₂O₆).

4. Nitrogen fixation:

Many bacteria inhabiting the root nodules of leguminous plants (Rhizobium), nitrogen-fixing bacteria living free in the soil (Azotobacter, Clostridium pasteurianum) actinomycetes, fungi, purple bacteria and a number of blue-green algae are known to fix free atmospheric nitrogen gas into nitrogenous compounds, such as nitrates and nitrites etc and thereby increase the fertility of the soil.

It has been established that in each hectare of ordinary soil every year 25-50 kg of nitrogen are fixed and in cultivated soil and in soil containing legume plants 35 to 60 kg and 100 to 400 kg of nitrogen are fixed respectively. Anabaena Nostoc Microcystis are important nitrogen fixing blue-green algae. De and Fritsch (1938) have found that certain blue-green algae are able to fix 20 lbs of atmospheric nitrogen per acre in a rice field. They increase the yield of rice from 15 per cent to 25 per cent. Singh, R.N. and Relwani and others have also shown experimentally that some blue-green algae fix nitrogen in the paddy soils.

5. Soil mixing:

Many organisms by their mechanical activities help in mixing and weathering of soil. Roots of the higher plants take active part in the disintegration of rocky mass and also make the compact soil loose. Many rodents, insects and earthworms turn over the soil and sometimes also expose the rock surface for physical and chemical weathering. Burrowing animals such as rodents, bring soil from deeper regions to the surface. The excreta of soil animal is deposited on the surface of soil in the form of casts which increases the fertility of the soil.

6. Soil aeration improvement:

Soil micro-organisms improve aeration of soil. Burrowing worms are also helpful in improving the aeration and percolation.

7. Improvement in aggregation of soil particles:

Bacteria, blue-green algae, and some other micro-organisms secrete mucilaginous substances which bind the soil particles into soil aggregates.

8. Injury to plants:

Not all the soil organisms are beneficial in their properties and behavior. Some microbes become parasites of higher plants and cause considerable damage. Nematodes are important animals which cause a number of diseases in plants. Besides these, many soil bacteria and fungi cause many diseases, such as damping off, seedling blight, root rot, mildew diseases in a number of crops.