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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.
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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.
Soil profile. It is a vertical secton of soil through all its horizons and extends upto the parent materials. A study of soil profile is important both from the standpoint of soil formation and soil development(pedology) and crop husbandry(edaphology). In deep soils the soil profile may be studied upto one metre and a quarter and in others upto the parent material. The layers (horizons) in the soil profile which vary in thickness may be distinguished from the morphological characteristics which include colour, texture, structure, etc. Generally, the profile consists of 3 mineral horizons 'A', 'B' and 'C'.
The 'A' horizon may consist of sub-horizons richer in organic matter intimately mixed with mineral matter 'A', the horizon that has lost clay, iron or aluminium with the resultant concentration of quartz (A2--eluvial horizon) or an intermediate or transitonal horizon.
The 'B' horizon is below the'A' horizon showing dominant features of concentration of clay, iron, aluminium of humus alone or in combination.
The 'C' horizon excludes the bedrock from which 'A' and 'B' horizons are presumed to have been formed. The 'C' horizon is itself little affected by pedogenic processes. It is the layer of consolidated bedrock which, upon weathering, may be presumed to have given rise to the 'C' horizon above.
Not all the profiles show the sequence mentioned above. profiles developed in situ under intense pedogenic processes over a sufficient period shoe the presence of the horizons 'A', 'B' and 'C'. Sincce the formation of soil and the development of profile are dependant on the genetic and environmental factors which vary considerably within and between regions, variations in horizonation are frequent and common. Thus, for example, the soils developed in a recent flood plain may have only AC profile without any A2, whereas those in the red and laterite soils area may have 'A1', 'B2' and 'C'.In some forest soils organic layers may also be found on the surface above 'A'. It will be beyond scope of this brief chapter to deal at length the nomenclature, implication and significance of horizons. It may be mentioned that a study of morphology of the soil profile gives a clear expression of the influence of the pedological processes involved.
A study of the soil profile is important from crop husbandry point of view, since it reveals the surface and the subsurface characteristics and qualities, namely depth, texture, structure, drainage conditions and soil-moisture relatonships. A study of the soil profile in the field, therefore, firnishes a base which has to be supplemented by physical, chemical and biological properties of the soil briefly discussed seperately.
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