Fig. Geologic Cycle of Soil
Soils are formed from materials that have resulted from the disintegration of rocks by various processes of physical
and chemical weathering. The nature and structure of a given soil depends on the processes and conditions that
Breakdown of parent rock: weathering, decomposition, erosion.
Transportation to site of final deposition: gravity, flowing water, ice, wind.
Environment of final deposition: flood plain, river terrace, glacial moraine, lacustrine or marine.
Subsequent conditions of loading and drainage: little or no surcharge, heavy surcharge due to ice or
overlying deposits, change from saline to freshwater, leaching, contamination.
All soils originate, directly or indirectly, from different rock types.
Physical weathering reduces the size of the parent rock material, without any change in the original composition of
the parent rock. Physical or mechanical processes taking place on the earth's surface include the actions of water,
frost, temperature changes, wind and ice. They cause disintegration and the products are mainly coarse soils.
The main processes involved are exfoliation, unloading, erosion, freezing, and thawing. The principal cause is
climatic change. In exfoliation, the outer shell separates from the main rock. Heavy rain and wind cause erosion of
the rock surface. Adverse temperature changes produce fragments due to different thermal coefficients of rock
minerals. The effect is more for freeze-thaw cycles.
Chemical weathering not only breaks up the material into smaller particles but alters the nature of the original parent
rock itself. The main processes responsible are hydration, oxidation, and carbonation. New compounds are formed
due to the chemical alterations.
Rain water that comes in contact with the rock surface reacts to form hydrated oxides, carbonates and sulphates. If
there is a volume increase, the disintegration continues. Due to leaching, water-soluble materials are washed away
and rocks lose their cementing properties.
Chemical weathering occurs in wet and warm conditions and consists of degradation by decomposition and/or
alteration. The results of chemical weathering are generally fine soils with altered mineral grains.
The effects of weathering and transportation mainly determine the basic nature of the soil (size, shape, composition