Soil

Soil is the topmost layer of the earth’s surface. It consists of a mixture of minute particles of disintegrated rocks, minerals, organic matter and bacteria. Soil is formed when forces of nature such as temperature, rain, wind, waves, animals and plants act on rocks and break them into tiny pieces over a long period of time.

Soil can be simply defined as a mixture of small rock particles/debris and organic materials/ humus which develop on the earth surface and support growth of plants.

These are certain factors that affect soil formation:

• Parent Material
• Relief
• Climate
• Natural Vegetation

This is the material from which the soil has developed. The parent material can influence the soil in a number of ways: colour; texture; structure; mineral composition and permeability/drainage.

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.

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.

In areas of heavy rainfall and high temperature, the soils are red or lateritic. Why?

Torrential rainfall during the rainy season washes the upper soil and leaches the materials into deeper horizon. During the dry summer season the evaporation exceeds precipitation and through capillary action iron and aluminium oxides are transported to the surface making the soil red. In areas of alternate wet and dry climate, the leached material which goes deep down in the horizon is brought up and the blazing sun bakes the top soil so hard that it resembles a brick. Therefore, this soil is called lateritic which literally means brick.

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.

A vertical section through different layers of the soil is called the soil profile. A soil horizon is a layer generally parallel to the soil surface, whose physical characteristics differ from the layers above and beneath. Horizons are defined in most cases by obvious physical features, chiefly color and texture.

Soil formation begins with the breakdown of rock into regolith. Continued weathering and soil horizon development process leads to the development of a soil profile, the vertical display of soil horizons.

The top layer of the profile is the O horizon which is composed of organic matter.
Decomposition of organic matter enriches the soil with nutrients (nitrogen, potassium, etc.), aids soil structure (acts to bind particles), and enhances soil moisture retention.

Next layer is the A horizon in which organic material mixes with inorganic products of weathering. The A horizon typically is dark coloured horizon due to the presence organic matter.
Eluviations, the removal of inorganic and organic substances from a horizon by leaching occurs in the A horizon due to the downward movement of soil water.
The E horizon generally is a light-colored horizon with eluviations being the dominant process. Leaching or the removal of clay particles, organic matter, and/or oxides of iron and aluminum is also active in E horizon. The E horizon has a high concentration of quartz under coniferous forests, giving the horizon an ash-gray appearance.
The B horizon is a zone of illuviation where downward moving, especially fine material, is accumulated.  The accumulation of fine material leads to the creation of a dense layer in the soil.
The C horizon represents the soil parent material, either created in situ or transported into its present location. Beneath the C horizon lays bedrock.
Eluviations are significant in humid climates where ample precipitation leads to the downward movement of minerals in the soil.
Illuviation are found closer to the surface in semiarid and arid climates where precipitation is scarce because due to capillary action cations like calcium and sodium dissolved in soil moves upward.

Sandy soil is a light soil that consists mainly of sand, i.e., grains of quartz with considerable air spaces between them. The sand may either be ‘coarse’ where the particles are between 0.2 and 2 mm in diameter, or ‘fine’ where the grains between 0.05 and 0.2 mm are just visible to the naked eye.  These light soils allow water to drain through rapidly, taking soluble plant foods with it. Sandy Soils are known as ‘hungry’ soils, which not only need constant manuring but may dry out completely during a period of drought so that shallow-rooted crops fail and pastures ‘burn’. They are good for horticulture (vegetables and fruits), legumes (such as moth and pulses), ground nut and bajra.

Clayey soil is an exceptionally fine grained soil, very retentive of moisture. It often becomes plastic when mixed with water. The individual grains of clayey soil are 0.002 mm in diameter. These particles consist mainly of hydrated aluminium silicates.

Clay contains little air and can hold more water, so forming a sticky mass, but when it dries out completely, it forms a hard, concrete like surface, seamed with numerous cracks. Sometimes, a compacted solid layer of clay in the subsoil is formed, which is known as clay pan, and is often hard and difficult to dig or plough. Clayey soils are often rich in plant food and give much better yields than that of sandy soils.  They are devoted to rice; perennial grasses other crops such as clover. Efficient drainage methods, modern machinery and careful liming enable clayey soils to grow roots, green crops and cereals.

Silty soil is finer than sand but coarser than clay. Its particles are assumed to have a diameter between 0.02 and 0.002 mm. These soils are rich in humus contents and are devoted to numerous cereal and non-cereal crops.

It is highly fertile soil consisting mainly of a mixture of sand and day, together with silt and humus. It has the good qualities of both sand and clay, but not their bad qualities. It comprises an almost equal mix of sand and silt with less than 30 per cent clay. It can retain some moisture and plant food even under the adverse weather and climatic conditions. It is well· aerated and drained, and can be readily worked.  It is generally devoted to wheat, barley, legumes, sugarcane, sugar beet, maize, millets, rice, grasses, vegetables and orchards.

‘Soil erosion’ has been defined as the gradual removal of the top soil by running water, wind, glacier, sea-waves, anthropogenic agents and animals.

– Soil erosion is a universal phenomenon.
– According to one estimate about 75, 000 million tones of soil is removed by these agents annually.

Normal erosion: This is caused by the gradual removal of topsoil by natural processes. The rate of erosion is slow.
Accelerated erosion: This is caused by manmade activities. In this case, the rate of erosion is much faster than the rate of formation of soil.

a) Uniform removal of soil
b) Rill erosion
c) Gully erosion (e.g. northern Punjab, Haryana, Rajasthan, M.P. U.P)

Mainly in the arid and semi -arid regions.

Farmland can be degraded in several other ways besides erosion. Physical degradation from mechanical tilling can lead to compaction and crusting. Repeated cropping without sufficient fallow periods or replacement of nutrients with cover crops, manure or fertilizer can deplete soil nutrients. In addition over application of chemical fertilizers, insecticides and pesticides can kill beneficial soil organism.

– Poor water management on irrigated crop land is a leading cause of degraded farmland.
– Inadequate drainage can lead to water logging of the soil or to Salinization, in which salt levels built up in the soil to toxic levels. About 15 to 20 percent of the irrigated land is suffering from some degree of water-logging and salinization.

Overgrazing, mining and deforestation are the major biotic agents causing soil erosion. These processes disturb the top soil thereby exposing the soil to various physical forces inducing erosion.

‘Deforestation’ is the conversion of forest to another land use or the long-term reduction of tree canopy cover below the 10% threshold. Deforestation defined broadly can include not only conversion to non-forest, but also degradation that reduces forest quality – the density and structure of the trees, the ecological services supplied, the biomass of plants and animals, the species diversity and the genetic diversity. Deforestation includes cutting and felling of trees, removal of forest litter. Browsing and trampling by livestock, forest fires, also leads to cause deforestation etc. Deforestation leads to erosion. Deforestation further leads to land degradation, nutrient and the disruption of the delicate soil plant relationship.

• During the rainy season, there is plenty of vegetation and animals get enough fodder.
• But during the dry period, there is shortage of fodder and the grass is grazed to the ground and torn out by the roots by animals.
• This leads to loose structure of the soil and the soil is easily washed away by rains.
• Moreover, soil is pulverized (reduce to fine particles) by the hoofs of animals, and thus proves detrimental to top soil when heavy showers fall on it.
• Soil erosion due to overgrazing is a common site in the hilly areas.

Loss of fertile top soil leading to gradual loss of soil fertility and agricultural productivity. Loss of mineral nutrients from soil through leaching and flooding.
Loss of soil ability to hold water and sediment. Sediment runoff can pollute water courses and kill aquatic life. Lowering of the underground water table and decrease in the percentage of soil moisture. Drying of vegetation and extension of arid lands.
Increase in frequency of droughts and floods. Silting of river and canal belts.
Recurrence of landslides. Adverse effect on economic prosperity and cultural development.

Desertification refers to degradation of land in arid, semi-arid, and dry sub-humid areas due to an array of factors. The direct impact of desertification is reduced biodiversity. The reasons are many such as climatic changes such as drought, or human such as overgrazing. Desertification is recognized as a major threat to biodiversity and consequently leads the countries to develop plans to counter its impacts.

• Drifting of sand and its accumulation on fertile agricultural land.
• Excessive soil erosion by wind and to some extent by water.
• Deposition of sand in rivers, lakes and other water bodies, thereby decreasing their water containing capacity.
• Lowering of water table leading to acute water shortage.
• Increase in area under wastelands.
• Decrease in agricultural production.
• Increase in frequency and intensity of droughts.

• Indiscriminate felling of trees should be banned.
• Intensive tree plantation programme should be initiated.
• Mulching the shifting sand dunes with different plant species serves as an effective physical barrier to the moving sands.
• Grazing should be controlled and new pastures should be developed.
• Alternative sources of fall can reduce the demand for fuel wood.
• Sandy and wasteland shuld be puit to proper use by judicious planning.

Water logging refers to the saturation of soil with water. Soil may be regarded as waterlogged when the water table of the ground water is too high to conveniently permit an anticipated activity, like agriculture. Crops need air to a greater or lesser depth in the soil. If there is no proper drainage system in the land, the land will become waterlogged.

This results in the saturation of crops. The normal circulation of air will not be possible due to water logging and the amount of oxygen in the soil will decline. Construction of proper drainage system to allow water flow away from the land is the best way to reduce water logging.

“Soil conservation is the prevention of soil loss from erosion or reduced fertility caused by over usage, acidification, salinization or other chemical soil contamination. Slash-and-burn and other unsustainable methods of subsistence farming are practiced in some lesser developed areas.