Introduction And Formation Of Soil

Introduction of soil

The soil is defined as the region of the earth crust where geology and biology meet. The term soil refers to the outer loose materials of the earth surface. It is a layer distinctly different from the underline bed rocks. Agriculturally, soil is the region supporting plant and animal life and from which plants obtain their nutrients and mechanical support. Chemically soil contain a multitude of the organic and inorganic compound, not found in the underline strata. For a microbiologist, soil is unique in several ways. It contains a vast array of the microorganism; bacteria, actinomycetes; fungi, algae, protozoans, viruses and some metazoans including nematodes, insects etc. The soil is one of the most dynamic sites of biological interaction in nature. It is the region where many biological reactions concerned in the distruction of organic matter, weathering of rocks and the nutrition of agricultural crops.

Soil profile

The soil profile is the series of layers of soil different from one another from one another in colour texture and composition. The soil profile is a single vertical cross section of a soil body extending from the surface to the bed rock. The rate of development of a typical soil profile depending upon the climatic and other factors and it is a very slow process. Each layer of soil zones is called soil horizons. A typical soil profile contains following types of horizons;

• O-horizon consists of the layer of undecomposed plant materials.
• A-horizon consists of surface or top soil. It is reached in organic matter found in different stages of decomposition, dark in colour and tilled for agriculture and plant and a large number of microorganism grow here. Microbial activity is high and it is reached in minerals.
• B-horizon consists of sub soil mineral, humus and other substances leached from the soil surface accumulates here. Organic matter is less and microbial activity is present but lower than A-horizon.
• C-horizon consists of soil base. It develops directly underlying bed rocks. Here microbial activity is lower than A and B horizon. It includes weathered minerals including bed rocks.
• D-horizon or bed rock consists of unweathered rock.

Formation of soil

The soil is formed by the weathering of rocks. Weathering processes are divided into those that involved physical activity, chemical activity, and biological activity

physical process do the breaking and the chemical process does the altering.

Physical weathering or mechanical weathering or disintegration:

It refers to processes that break apart rocks without altering their chemical composition. Physical weathering breaks rocks into the smaller fragment and eventually into the individual mineral particle. Each new break in a rock increases the surface area thus creating more surfaces for chemical activity to take place.

Granular disintegration:

The physical separation of individual mineral particles of rocks is termed granular disintegration. It causes the rock to crumble easily.

Exfoliation:

Bed rocks .i.e. brought up to the earth surface is subjected to uneven pressure which causes the rock to fracture. It is because of the pressure release from unloading and thermal effect when relieved of the weight of its former over burden bed rock exposed at the surface expands upward slightly. This upward expansion can be great enough to form sheeting joints parallel to the bodies of bed rock along the sheeting joints is called exfoliation.

Thermal effect:

The effect of temperature on rock are most pronounced where temperature are extreme and fluctuate. The presence of moisture inhences the thermal effect under cold condition where air temperature rich the freezing point of water. Liquid water is transformed into solid ice and its volume expand by 9% and exert a brushing pressure of about 150kg per cm square. The growth of ice crystal causes a lifting of over lying particles and breaking a part of solid bed rocks. Frost wedging is most vigorous at high altitude where alternate freezing and thawing results from temperature fluctuations from above freezing during the day to the freezing range at night. Sheets of rock may break off when the rock is heated and the fire is doused with water. The rocks spalls off in sheets because the fire causes it to expand upward more breaking takes place when cold water causes an abrupt temperature change in the heated rocks.

Chemical weathering or decomposition:

Chemical weathering that changes the composition of rock minerals is called decomposition.

Oxidation:

It is the addition of oxygen to other elements. The elements in rocks that are most easily oxidized are iron and sulfur. The minerals formed during chemical weathering are red, yellow, orange or brownish. If cupper minerals are oxidized, oxidation occurs more rapidly if water is present, so it is very slow in the desert area. Many underground rock surfaces are covered with the thin film of pyrites, it is an iron sulfite which is easily oxidized. During oxidation of pyrites, the released sulfur may collect as the element or combine with water to form sulfuric acid which is a powerful chemical agent. Other sulfite minerals are scattered through rocks or has been concentrated as deposits of sulfite minerals. When cupper minerals are present, oxidation may be responsible for creative ore deposits. Cupper can be released through oxidation in the weathering zone and concentrated as great deapth.

Dissolution:

Water is capable of detaching and surrounding ions from minerals, so it is capable of dissolving some solid rocks. Layers of rock salt are easily dissolved so are seen only in the desert area. The removal of minerals by their dissolution of water called leaching.

Hydrolysis:

Weathering of silicate minerals occurs due to hydrolysis i.e. their combination with water. Some hydroxyl ions of water become part of the crystals of weathered silicate. During the weathering of field part of sodium, calcium and potassium are leached and go into solution. Potassium is immediately taken up by the plant rootlets and sodium and calcium is transported into the solution to the sea.

Carbonation:

Carbon dioxide gas which is present in the atmosphere and is a byproduct of the oxidation of organic matter readily dissolve in water to form a weak acid (carbonic acid).

Co₂ + H₂O H₃Co₃

The reaction between carbonic acid and minerals is called carbonation. Carbonate minerals specially calcite dissolve easily in carbonic acid.

CaCo₃ + H₂Co₃ Ca⁺⁺ + HCo₃^-

Because calcite is not only the chief ingredient of many carbonate rocks but also common cement in sand stone, many kinds of rocks are affected by carbonation.

Exchange reaction:

Exchange reaction is the exchange of positive ion between compounds resulting in the formation of the new compound.

So₂ + H₂o H₂So₄

H₂So₄ + CaCo₃ + H₂o CaSo₄ .H₂o + H₂Co₃

H₂Co₃ + CaCo₃ Ca⁺⁺ + 2 ( HCo₃^-)

H₂So₄ and CaCo₃ in the presence of water exchange ion to produced gypsum and carbonic acid. Because carbonic acid forms in this exchange reaction, further destruction of calcite may take place. Sulphur dioxide from automobiles exhaust fumes oxidizes in the atmosphere and reacts with rain water to form sulfuric acid. This acid reacts with the calcite of the marble monuments to form gypsum and carbonic acid. Carbonic acid inturns destroy more calcite, releasing Co₂ gas. The gypsum is soluble in water so it is easily dissolved.

Biological weathering:

Lichens:

Lichen^,s growth on exposed bed rocks is the first stage in the conversion of that bed rocks into the porous structure.

Bacteria:

Bacteria breakdown into organic materials and liberate acid which attacks the minerals like silicate.

Earthworm:

Earthworm and other burrowing animals such as rodents and termites play an important part in preparing materials for removal by rain and wind. Animal burrow into the lower soil levels turning over and burrowing bring the rock particle to the surface where they can be easily weathered by the action of gases and water. The plants root follow fractures in rocks, widen the cracks and thus cause damage to side walls and foundation. The growing rootlets of shrubs and trees exert incredible force as they work down into the crevices. The cellulose of cell walls is actually stronger than many metals. Plants of all kinds including fungi and lichens also contribute to chemical weathering. Since they extract certain chemical from the rock minerals and liberate others in the process moreover bacteria attacks the minerals of rocks and soil much more vigorously in the presence of water than absence. The death remains of organisms decay in the soil largely as a result of the activities of bacteria and fungi. In this way, carbondioxide and organic acid together traces of ammonia and nitric acid liberated all of which increases the solvent power of soil water. The chief organic product is a complex of brown jelly like substance collectively known as humus. Humus is the characteristics organic constituent of the soil and water containing humic acid can dissolved many substances which are generally insoluble.

Reference:

1. Alexender M (1961). Introduction to Soil Microbiology, Academic press.
2. Rangaswami G and bag ya raja PT (1993). Agricultural Microbiology (second edition), Prentice Hall of India.