Soil Formation| Class 11 Geography Notes
Last Updated : 29 Apr, 2024
Soil is a dynamic medium with constant chemical, physical, and biological activities. Soil is a result of decay, it is also the medium for growth. It is a changing and developing body. It has many characteristics that fluctuate with the seasons. Organic matter increases when leaves fall or grasses die. The process of soil formation is a continuous one, and it is constantly being affected by the factors that contribute to it. Soil is a valuable resource, and it is important to protect it from erosion and other forms of degradation.
- Soil formation or pedogenesis depends first on weathering. It is this weathering mantle (depth of the weathered material) that is the basic input for soil to form.
- Weathering is defined as the mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate.
- First, the weathered material or transported deposits are colonized by bacteria and other inferior plant bodies like mosses and lichens. Also, several minor organisms may take shelter within the mantle and deposits.
- The dead remains of organisms and plants help in humus accumulation. Minor grasses and ferns may grow; later, bushes and trees will start growing through seeds brought in by birds and wind.
- Plant roots penetrate down, burrowing animals bring up particles, the mass of material becomes porous and sponge-like with a capacity to retain water and to permit the passage of air, and finally a mature soil, a complex mixture of mineral and organic products forms.
Five basic factors control the formation of soils:
- Parent material
- Topography
- Climate
- Biological activity
- Time.
Soil-forming factors act in union and affect the actions of one another:
Soil Forming Factors | Characteristics |
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Parent Material | Parent material is a passive control factor in soil formation. Parent materials can be any in-situ or on-site weathered rock debris (residual soils) or transported deposits. |
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Topography | Topography like parent materials is another passive control factor. The influence of topography is felt through the amount of exposure of a surface covered by parent materials to sunlight and the amount of surface and sub-surface drainage over and through the parent materials. |
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Climate | The climate is an important active factor in soil formation. Temperature and moisture are the various factors within climate that add up to the formation of soil. |
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Biological Activity | The vegetative cover and organisms that occupy the parent materials from the beginning and also at later stages help in adding organic matter, moisture retention, and nitrogen to the soil, which helps in improving the fertility of the soil. |
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Time | Time is the third important controlling factor in soil formation. The length of time the soil-forming processes operate determines the maturation of soils and profile development. |
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Parent Material
- Parent material is a passive control factor in soil formation. Parent materials can be any in-situ or on-site weathered rock debris or transported deposits.
- Soil formation depends upon the texture and structure as well as the mineral and chemical composition of the rock debris/deposits.
- Nature and rate of weathering and depth of weathering mantle are important considerations under parent materials. There may be differences in soil over similar bedrock and dissimilar bedrocks may have similar soils above them.
- But when soils are very young and have not matured these show strong links with the type of parent rock.
- In the case of some limestone areas, where the weathering processes are specific, soils will show a clear relation with the parent rock.
Topography
- Topography like parent materials is another passive control factor.
- The influence of topography is felt through the amount of exposure of a surface covered by parent materials to sunlight and the amount of surface and sub-surface drainage over and through the parent materials.
- Soils will be thin on steep slopes and thick over flat upland areas. Over gentle slopes where erosion is slow and percolation of water is good, soil formation is very favorable.
- Soils over flat areas may develop a thick layer of clay with a good accumulation of organic matter giving the soil a dark color.
Climate
The climate is an important active factor in soil formation. The climatic elements involved in soil development are
Moisture in terms of its intensity, frequency,, and duration of precipitation - evaporation, that and humidity
- Precipitation gives soil its moisture content which makes chemical and biological activities possible. Excess water helps in the downward transportation of soil components through the soil (eluviation) and deposits the same down below (illuviation).
- In climates like wet equatorial rainy areas with high rainfall, not only calcium, sodium, magnesium, potassium, etc. but also a major part of silica is removed from the soil. Removal of silica from the soil is known as desilication.
- In dry climates, because of high temperatures, evaporation exceeds precipitation and hence groundwater is brought up to the surface by capillary action, and in the process, the water evaporates leaving behind salts in the soil.
- Such salts form into a crust in the soil known as hardpans. In tropical climates and areas with intermediate precipitation conditions, calcium carbonate nodules (kanker) are formed.
Temperature in Terms of Seasonal and Diurnal Variations
- Temperature acts in two ways — increasing or reducing chemical and biological activity.
- Chemical activity is increased in higher temperatures, reduced in cooler temperatures (with the exception of carbonation), and stopped in freezing conditions.
- That is why, tropical soils with higher temperatures show deeper profiles, and in the tundra regions, soils contain largely mechanically broken materials.
Biological Activity
- The vegetative cover and organisms that occupy the parent materials from the beginning and also at later stages help in adding organic matter, moisture retention, nitrogen, etc. Dead plants provide humus, the finely divided organic matter of the soil.
- Some organic acids that form during humification aid in decomposing the minerals of the soil's parent materials. The intensity of bacterial activity shows differences between soils of cold and warm climates. Humus accumulates in cold climates as bacterial growth is slow.
Nitrogen Fixation
- With undecomposed organic matter because of low bacterial activity, layers of peat develop in subarctic and tundra climates. In humid tropical and equatorial climates, bacterial growth and action are intense, and dead vegetation is rapidly oxidized leaving very low humus content in the soil.
- Further, bacteria and other soil organisms take gaseous nitrogen from the air and convert it into a chemical form that can be used by plants. This process is known as nitrogen fixation.
- Rhizobium, a type of bacteria, lives in the root nodules of leguminous plants and fixes nitrogen beneficial to the host plant. The influence of large animals like ants, termites, earthworms, rodents, etc., is mechanical, but, it is nevertheless important in soil formation as they rework the soil up and down.
- In the case of earthworms, as they feed on soil, the texture and chemistry of the soil that comes out of their body changes.
Time
- Time is the third important controlling factor in soil formation. The length of time the soil-forming processes operate determines the maturation of soils and profile development.
- A soil becomes mature when all soil-forming processes act for a sufficiently long time developing a profile. Soils developing from recently deposited alluvium or glacial till are considered young and they exhibit no horizons or only poorly developed horizons.
- No specific length of time in absolute terms can be fixed for soils to develop and mature.
Conclusion
Hence, we can conclude that soil is a mixture of rock debris and organic materials that develop on the earth's surface. Various agents of gradation and weathering have acted upon the parent rock material to form a thin layer of soil.
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