Soil: Composition | Formation and Factors in Soil formation

Soil: Composition | Formation and Factors in Soil formation

Meaning and Definition

Soil is usually defined as “any part of earth’s crust in which plants root”. Muddy bottoms of ponds, porous rock surfaces, ravines or glacial deposits, bottoms of lake etc., all are soils. But this is a limited definition, soil is not merely a group of mineral particles, it has also a biological system of living organisms as well as some other components. It is thus preferred to call it a soil complex.

According to Dokuchayev (1889), “the soil is a result of the actions and reciprocal influences of parent rocks, climate, topography, plants, animals and age of the land.” It can be represented by the following formula:

S= (g.e.b)Δt

Where, S = soil; g = geology; e = environment; b = biological influences; t = time

Components of Soil

Soil complex has five categories of components:

1. Mineral matter: It is a matrix of mineral particles derived by varying degrees of breakdown of the parent material, the rock.
2. Soil organic matter (Humus): It is an organic component derived from long and short-term addition of material from organisms growing above and below ground i.e. plants, animals, microorganism.
3. Soil water (Soil solution): It refers to all water contained in soil together with its dissolved solids, liquids and gases. Soil water is held by capillary and absorptive forces both between and at the surface of soil particles. Soil water is a dilute solution of many organic and inorganic compounds, which is the source of plant mineral nutrients.
4. Soil atmosphere: It occupies the pore space between soil particles, which at any time, is not water-filled. Its composition differs from the above ground atmosphere in the sense that it is normally lower in oxygen and higher in carbon dioxide content.
5. Biological system: besides the above four components there is also a biological system. Each soil has distinctive flora and fauna of bacteria, fungi, algae, protozoa, rotifers, nematodes, molluscs and arthropods etc.

Genesis or Formation of Soil

The whole process of soil formation is generally divided into two stages (i) weathering– breakdown of bigger rocks into fine, smaller mineral particles, and (ii) soil development or pedogenesis -modification of the mineral matter through interaction between biological, topographic and climatic effects, which ultimately lead to the development of any of a great variety of potential soil types.

Weathering process

Bare rock surfaces are exposed to various types of physical, chemical and biological processes which lead to physical and chemical disruption of their components. Physical processes of weathering include action of water, temperature, glaciers, gravity etc., which cause weathering of rocks through processes like wetting-drying, heating-cooling, freezing, glaciation, solution and sand blast etc. The chemical processes of weathering include hydration, hydrolysis, oxidation- reduction, carbonation, chelation etc., which are due to chemical composition of rocks, chemical in the atmosphere, as well as those produced as a result of living organisms, such as lichens, fungi, bacteria, blue-green algae, bryophytes etc.

In biological processes lichens are able to extract nutrients from bare rocks. Lichens, fungi and bacteria on rock surface retain the water for a long period during which the chemical processes can proceed, splitting the rock alumino-silicates by hydrolysis and carbonation into the simpler clay alumino-silicates. Algal partner of lichen through photosynthesis, increases the amount of available organic matter at rock surface. Various exudates from these organisms and the respiratory carbon dioxide accelerate the process of weathering, as some of the lichen acids (organic acids) generally dissolve mineral components. Rock weathering is, therefore, for a short time, a physico-chemical process but soon it becomes biogenic, increasing in its rate.

The weathering processes are physical as well as chemical:

Physical weathering

These may be of the following types :

1. Wetting-drying: It is the disruption of layer lattice minerals which swell on wetting.
2. Heating-cooling: It is disruption of heterogeneous crystalline rocks in which inclusions have differential coefficients of thermal expansion. It occurs particularly in dry climates, where due to sun heating large boulders flake at surfaces.
3. Freezing: This is the disruption of porous, lamellar or vesicular rocks by frost shatter due to expansion of water during freezing.
4. Glaciation: Larger masses of snow and ice- glaciers, while falling may cause physical erosion of rocks through grinding process. Solution. Some more mobile components of rocks, such as calcium chlorides, sulphates etc., are simply removed by agents like water.
5. Sand blast: In arid, desert conditions the rocks are disrupted by physical action of wind, sand etc.

Chemical weathering

These processes include the following:

1. Hydration: As a result of taking water, due to reversible change of haematite to limonite (Fe₂O₃= Fe­₂O₃ . 3H₂O), the rock swells. This swelling causes the disruption of sandstones etc.
2. Hydrolysis: In this process, components like alumino-silicate of rock breakdown, during which elements such as potassium and surplus silicon are washed out which gives rise to simpler mineral matter like clay alumina-silicates.
3. Oxidation-reduction: Some oxidation-reduction chemical reactions, such as reversible change of Fe³⁺ to Fe²⁺ cause disruption of rocks, because Fe²⁺ is more stable than Fe³⁺.
4. Carbonisation: Some chemicals, produced in atmosphere and those during the metabolism of microorganisms bring about carbonisation. For example, reversible change of CaCO₃ to Ca(HCO₃)₂ leads to solution loss of limestone or disruption of CaCO_3­ cemented rocks.
5. Chelation: Some chemical exudates, produced through biochemical activity of microorganisms like lichens, bacteria, etc., are able to dissolve out mineral components of the rocks. These metals dissolved with organic products of microbial activity known as chelates.

Pedogenesis

During weathering, the rocks are broken down into smaller particles. But this is not practically possible, soil and plants cannot grow in this manner. The weathered material undergoes further changes, which is a complex process, known as pedogenesis or soil development.

In weathering mostly physical and chemical factors are involved, whereas pedogenesis is largely a biological phenomenon. During this phenomenon, living organisms such as lichens, bacteria, fungi, algae etc. as a result of secretion of organic acids, enzymes. CO_2­ production and addition of organic matter after their death, bring about geochemical, biochemical and biophysical processes. Due to all this the crusts of weathered rock debris are converted to true soils. This soil when fully developed can be seen having number of layers-horizons of a soil, known as soil profile.

Factors of affecting Soil Formation

The nature and type of soil forming at a particular place is largely governed by five soil forming factors, which are as follows:

1. Parent material: Physical constitution of parent material influences aeration, leaching rate and texture of the developing soil, whereas chemical composition influences the chemical characteristics of soil. Some rocks weather easily and others decay at a slower rate. Faster the rate of weathering as in hot and climate, faster is the rate of soil formation.
2. Topography and time: It influences the soil formation through drainage and retention of water. Soil aeration is also governed by topography. The elevation and nature of slope also influence the rate of weathering and drainage of water.
3. Climate: Rainfall, temperature, humidity/evaporation and wind are the climatic factors that strongly influence soil formation. Rainfall determines the direction of solute translocation according to the precipitation/evaporation (P/E) ratio. Temperature governs (P/E) ratio. It also controls the rate of organic turnover in soil. Humidity/Evaporation interaction influences rainfall and amount of water movement. Thus, climate of an area is a major factor in soil formation.
4. Organisms: Plants, animals and microbes exert the biosphere effects on the soil formation. Thus, phytosphere, zoosphere as well as microorganisms are important factors of soil formation.

Joffe (1936) recognized the climate and biological factors as active factors whereas the parent matter, topography and the time as passive factors of soil formation.

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