Five soil-forming factors
- Active factors: Which represent agents that supply energy that act upon the mass for process of soil formation – They are climate and biotic organisms.
- Passive factors: Which represent the source of the soil-forming mass and conditions affecting it – They are parent material, relief, and time.
Climate:
- Temperature and rainfall are the most important factors in soil formation.
- They determine the effectiveness of weathering of the parent material, the quantity of water seeping through the soil and the type of micro-organisms present therein.
- Two different parent materials may develop the same soil in the same type of climate. Similarly, the same parent material may produce two different types of soils in two different types of climates.
- The crystalline granites produce laterite soil in relatively moist parts of the monsoonal region and non-laterite soil in drier areas.
- Hot summer and low rainfall develops black soil as is found in some parts of Tamil Nadu irrespective of the parent rock.
- In Rajasthan, both granite and sandstone give birth to sandy soil under arid climate.
- In arid and semi-arid regions, evaporation always exceeds precipitation. There is little vegetation and the soils badly lack humus content. Hence the soils are invariably of light colour.
- In Rajasthan and the adjoining arid and semi-arid regions, excess of evaporation makes soils lime accumulating. Hence the soil is pedocal in nature [Pedocal is a subdivision of the zonal soil order. It is a class of soil which forms in semiarid and arid regions. It is rich in calcium carbonate and has low soil organic matter].
- In cold climates of the Himalayan region, the process of vegetation decay is very slow and the soils are acidic in nature.
Biotic organisms:
- Natural vegetation reflects the combined effects of relief and climate.
- The formation and development of soil is very much influenced by the growth of vegetation.
- The decayed leaf material adds much needed humus to soil thereby increasing its fertility.
- The densely forested areas contain some of the best soils in India. There is a close relationship between the vegetation types and soil types in India.
- Vegetation aids control of soil erosion
- They facilitate percolation and drainage and bring about a greater dissolution of minerals through the action of carbon dioxide and other acidic substances.
- Fauna: Burrowing animals, rodents, earthworms, ants etc., are highly important in soil formation. Burrowing animals cause constant mixing within the soil profile.
- The role of microorganisms as soil formers is intimately related to humification and mineralizations.
Parent material:
- The rocks from which soils are formed are called parent materials.
- In most of the cases, the parent material determines the colouration, mineral composition and texture of the soil.
- In some cases, the soil formed may or may not have the same physical properties of the parent rock.
- Climatic factors induce chemical changes which also affect physical properties of the soil.
- The surface rocks are exposed to the process of weathering. In this process, the rocks are converted into fine grains and provide a base for the soil formation.
- For example, In Indian Conditions, parent material is generally categorized into:
- Ancient crystalline and metamorphic rocks – red soils_ the red colour of the soils is due to the presence of iron oxides.
- Cuddapah and Vindhyan rocks- calcareous and argillaceous soils_as the rocks are ancient sedimentary rocks devoid of metalliferous minerals.
- Gondwana rocks- less mature soils with low fertility_as the rocks are younger sedimentary rocks
- Deccan basalts- regur or black soils_ as basalts are rich in titanium, magnetite, aluminum and magnesium giving the soil its dark colour
Relief:
- The relief is the most important factor for soil formation in places with steep slopes like the hilly regions, edges of plateaus etc.
- Soil erosion on barren slopes is rampant and it hinders soil formation. Example:Chambal ravines, higher reaches of Himalayas where there is minimal or no forest cover (most on the steep southern slopes) etc.
- The areas of low relief or gentle slope generally experience deposition and have deep soils. Example: Indo-Gangetic plain.
- The exceptions in the plateau are river basins where the soil layers are sufficiently deep.
Time:
- The length of time required for a soil to develop horizons depends upon many interrelated factors, such as climate, nature of the parent material, burrowing animals and relief.
- Certain soils are termed mature or immature which give some idea of the time factor.
