SOIL may be defined as a thin layer of earth's crust which serves as a natural medium for growth of plants. It is the unconsolidated mineral matter that has been subjected to, and influenced by, genetic and environmental factors-- parent material, climate, organisms and topography all acting over a period of time. Soil differs from the parent material in the morphological, physical , chemical and biological properties. Also, soils differ among themselves in some or all the properties, depending on the differences in the genetic and environmental factors. Thus some soils are red, some are black; some are deep and some are shallow; some are coarse textured and some are fine-textured. They serve as a reservoir of nutrients and water for crops, provide mechanical anchorage and favourable tilth. The components of soil are mineral matter, organic matter, water and air, the proportions of which vary and which together form a system for plant growth; hence the need to study the soils in perspective.

Rocks are the chief sources for the parent materials over which soils are developed. There are three main kinds of rocks:
(i)igneous rocks,
(ii)sedimentary rocks, and
(iii)metamorphic rocks.

Igneous rocks. They are formed by the cooling, hardening and crystallizing of various kinds of lavas and differ widely in their chemical composition. They chiefly contain feldspars, maphic minerals and quartz. Rocks containing a high proportion of quartz (60-75%) are classified as acidic, whereas those containing less than 50% quartz are classified as basic. The common igneous rocks found in India are the granites(acidic) and basalts or the Deccan Trap (basic)

Sedimentary rocks. They are derived from igneous rocks and are formed by the consolidation of fragmentary rock materials and the products of their decomposition deposited by water. The common sedimentary rocks are conglomerate, sandstone, shale and limestone. Alluvial, glacial and aeolian deposits form the unconsolidated sedimentary rocks.

Metamorphic rocks. They are formed from the igneous or sedimentary rocks by the action of intense heat and high pressure or both resulting in considerable change in the texture and mineral composition. The common metamorphic rocks are gneis from granite, quartzite from quartz or sandstone, marble from limestone and slate from shale.

The rocks vary greatly in chemical composition. Table1 gives the average composition of four different kinds of rocks

Weathering refers to the physical and chemical disintegration and decomposition of rocks which are not under equilibrium under temperature, pressure and moisture conditions on the earth's surface. In the beginning, weathering precedes soil formation, more so in hard rocks. In other words, weathering creates the parent material over which soil formation takes place. Later, weathering, soil formation and development proceed simultaneously. The weathering may be physical or chemical.

Physical Weathering. The principal agents of physical weathering are given below.

1. TEMPERATURE. The differential expansion and contraction of minerals in the rocks due to variations in temperature set up internal tensions, form weaker zones and gradually break them apart. High temperatures accelerate the process of chemical weathering, especially in warm humid regions.

2.WATER. Torrential rains and flowing waters dislocate the solid particles on the rocks and expose the inner portion to the agents of weathering. The dislodged particles are carried down and deposited elsewhere as alluvium. Similiarly, the sea-waves wear off the rocks on the shore, and the glaciers in the high mountains exert an erosive and transporting influence on the rocks and their fragments.

3.WIND. Wind exerts abrasive action, detaches the particles from the rocks and acts as a carrying agent. Sand-storms in the deserts and high winds on the sea shore have both erosive and transportive action.

4.PLANTS AND ANIMALS. Lichens and mosses growing on bare rocks cause their gradual disintegration. Grasses, shrubs and trees growing in rocks' crevices help to extend the cracks by the growth of their roots. The decomposition of litter and decayed matter accelerates the chemical weathering owing to the release of organic acids.

Chemical weathering. The chemical decomposition of rocks is brought about by solution, hydration, hydrolysis, carbonation, oxidation and reduction. Chemical weathering taking place in the lower layers may be termed as geochemical weathering whereas that taking place at the surface and below the surface may be termed pedochemical weathering.

1.SOLUTION. The solvent action of water is an important means of weathering rocks containing soluble salts; e.g.gypsum and limestone. The solvent action is increased in the presence of carbon dioxide and organic acids released during the decomposition of organic matter. Salts of sodium, potassium, calcium and magnesium are readily removed in solution.

2.HYDRATION. Hydration implies the association of water molecules with minerals. it provides a bridge or entry for the hydronium ions to attack the structure.

3.HYDROLYSIS. It is the most important process of chemical weathering. During hydrolysis, the hydronium ions from water attack the weatherable minerals and alter them completely or modify them drastically. In the process, hydroxides of potassium, iron, magnesium, calcium, etc. are formed.

4.CARBONATION. The hydroxides produced during hydrolysis react with the dissolved carbon dioxide to form corressponding carbonates which may either leach out or accumulate according to drainage or weather conditions.

5.OXIDATION AND REDUCTION. Oxidation is an important reaction in well-aerated rocks and soil material where oxygen supply is high and biological demand is low. For example the oxidation of iron is a disintegrating weathering process in minerals containing ferrous iron as part of their structure and the ferric iron due to its size and structure breaks up the mineral.


During reduction (the reverse of oxidation), when the supply of oxygen is low but the biological demand is high, the ferric ion is reduced to the ferrous ion, which being more mobile , may leach downwards. In this manner, oxidation and reduction are responsible for weathering minerals. <.font>

The mineral weathering combined with the associated physical and chemical phenomena constitutes the processes of soil formation. Soil formation is a complex of events. One process is distinguished from another on the basis of the chemical nature of events. The processes of soil formation include:
(i)the addition of organic and mineral materials,
(ii)the loss of these materials from the soil,
(iii)the translocation of materials from one point to another within the soil column, and (iv)the transformation of mineral and organic substances within the soil.

Weathering and soil formation showing the influence of genetic and environmental factors lead to soil development. The kind and intensity of weathering and processes of soil formation indicate the degree of soil development, as judged from the examination of a soil profile. Successive stages of soil development may be called infancy, youth, maturity and old age, introducing the time factor in them. For attaining maturity and old age, the positonal stability of soil over a long period to show the influence of intensive processes is essential. In the steep Himalayan region, landslides and avalanches frquently move large masses of soft rocks and their weathered products. Similiarly in deserts, the region is subjected to fresh wind-borne deposits. In these situations therefore the soil may never reach maturity or old age. The soils in the active flood plains which receive frequent fresh deposits also remain remain youthful in terms of soil development. Differences in relief play an important part in soil formation by draining away water and and finer fractions from high levels to depressions and low-lands. Thus the soils formed at higher levels under low moisture conditons are coarse grained as compared with those of lowlands which are finer in texture. Though soil differences in the initial period arise from differences of parent materials, yet in a large measure, they are profoundly modified by other factors of soil formation, namely climate, organisms, topography and time.

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