Soil : Soil is the uppermost layer of the earth’s crust in which plants grow.
Pedology : The study of soil is known as Pedology or Edaphology or soil science.
Formation of Soil
Soil is formed from the parent rock by the process of disintegration into smaller particles. This is a slow and gradual process. This is known as pedgenesis. The process is called weathering.
Weathering is of three types—
(a) Mechanical Weathering
(b) Chemical Weathering
(c) Biological Weathering.
Soils are of two types—
(i) Residual Soils and
(ii) Transported Soils
Transportation takes place through water (Alluvial), through gravity (Colluvial), through wind (Eolin) and through glaciers (Glacial).
Soil Profile : The vertical section of the soil as seen in a trench is called soil profile. It has A, B, and C—three sections namely;—A– Horizon or top soil, B–Horizon or sub-soil and C–horizon or Regolith zone.
Components of soil
(i) Mineral matter
(ii) Organic matter or humus
(iii) Soil water
(iv) Soil atmosphere
(v) Biological systems
Mineral matter
Quartz, Feldspars, Mica, Calcite, iron oxide, magnesite, etc. are minerals present in the soil. Sand, silt minerals, clay etc. are also present.
Aggregations of soil particles define soil structure.
The size, proportion and arrangement of mineral particles is known as soil texture.
Loam soils : Loam soil is a mixture of sand, silt and clay particles in equal proportions. This is most suitable for the growth of the plants. Loam soils are of two types sandy and clay.
Clay soils : These soils contain high proportion of clay particles and hence referred to as heavy soils. Aeration is poor due to little pore spaces. Water holding capacity is high.
Sandy soils : They contain high proportion of sandy particles. Water holding capacity is poor due to percolation of water.
Silt soils : These soils contain high proportion of silt particles. Due to considerable water holding capacity and aeration, these soils are suitable for vegetation.
Gravel soils : Aeration and percolation of water is unrestricted in these soils.
Humification : Process of humus formation from dead remains of plants and animals by the action of saprobiotic soil organisms, is called humification.
Humus : Decomposed organic matter mainly derived from the dead remains of plants and animals is called humus.
Litter : Fresh organic matter with fallen leaves, twigs, animal excreta and dead bodies of animals.
Mineralisation : Formation of inorganic substances from humus by the action of soil microbes.
Chresard : Chresard is the amount of soil water available to plants.
Echard : Water in the soil which is not available to plants.
Hollard : Total amount of water present in the soil.
Importance of Humus—
(i) Humus makes soil fertile.
(ii) Provides nutrients.
(iii) Has high water holding capacity.
(iv) Humus makes soil porus.
Different types of Soil water
(i) Runaway water.
(ii) Gravitational water
(iii) Hygroscopic water
(iv) Capillary water
(v) Chemical combined water.
Soil Erosion : Removal of top fertile soil by which productivity is reduced, is called soil erosion. Soil erosion is of two types—
(a) Normal Soil Erosion : The erosion which takes place under natural conditions, is called normal erosion.
(b) Accelerated Soil Erosion : The erosion caused by interference of an agency is called accelerated erosion. Main agents are men, animals, etc.
Agents of Soil Erosion
(i) Water Erosion : Caused by the action of water.
(a) Sheet erosion.
(b) Rill erosion.
(c) Gully erosion.
(d) River erosion–caused by river water.
(e) Ice erosion–caused by ice and snow.
(f) Wave erosion caused by waves.
(ii) Wind erosion—caused by wind.
(iii) Land slides—caused by land slides.
(iv) Stream bank erosion—caused by streams.
(v) Deforestation—caused by removal of plants.
Soil Conservation
Soil conservation refers to methods which protect the soil against erosion. Soil conservation methods are—
1. Countour farming : In hilly areas the land is ploughed with alternate furrows and ridges. The water held in furrows by erosion is reduced lands ploughed with alternate furrows and ridges. The water is held in furrows and stored, which reduces soil erosion.
Crop rotation : In crop rotation alternative sowing of legumes and cereal crops is done.
Strip cropping : It involves planting of crop in rows and strips to check the flow of water. It prevents soil erosion.
Mulching : This is a very effective method against wind and water erosion.
Reclamation : Process of converting uncultivable land to cultivable land.
Scrub Jungle : Forests with small trees or shrubs which are mostly drought resistant.
Reforestation : The creation of a forest on previously cleared land is called reforestation.
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