N.K. Chauhan 
The soil is formed when rocks are broken down by the action of wind, water and climate.
The characteristic features of a soil depend upon the rocks from which it has been formed and the kind of plants that grow on it.
The particles that make up soil are categorized into three groups by size - sand (0.06mm), silt(0.002mm), and clay(<0.002mm).
Sand particles are the largest and clay particles the smallest.
The relative percentages of sand, silt, and clay are what give soil its texture.
In general, the higher the percentage of silt and clay sized particles, the higher the water holding capacity.
The small particles (clay and silt) have a much larger surface area than the larger sand particles. This large surface area allows the soil to hold a greater quantity of water.
Loam is soil composed mostly of sand (40%), silt (40%), and a smaller amount of clay (20%).
Humus is the organic component of soil, formed by the decomposition of leaves and other plant material by soil microorganisms.
At present Indian soils are deficient in zinc (48.1%), iron (11.2%), copper (7%) and manganese(5.1%).
In India, the Indian Council of Agricultural Research (ICAR) has classified soils into 8 categories - Alluvial Soil, Black (Regur) Soil, Red Soil, Laterite Soil, Mountainous (Forest) Soil, Arid (Desert) Soil, Saline (Alkaline) Soil, Peaty (Marshy) Soil.
1) Alluvial Soil
Alluvial soils are soils deposited by surface water (silt deposited by Indo-Gangetic-Brahmaputra rivers), they are found along rivers, in floodplains and deltas, stream terraces, and areas called alluvial fans.Alluvial soil is most productive and the most readily available (~46%) soil in India, which covers an area of 15 lakh sq km.
It is widespread in northern plains and river valleys (Satluj, Ganga, Brahmaputra), and valleys of the Narmada, Tapi.
This soil is distributed in - Punjab, Haryana, Uttar Pradesh, Madhya Pradesh, Bihar, West Bengal and Assam.
In peninsular India, they are mostly found in eastern and western coastal plains, deltas of Mahanadi, Godavari, Krishna, Cauver and estuaries.
Due to their recent origin these soils are immature and have weak profiles, it is also porous in nature because of its loamy (equal proportion of sand and clay) nature.
Porosity and texture provide good drainage and other conditions favorable for agriculture.
They yield splendid crops of rice, wheat, sugarcane, tobacco, cotton, jute, maize, oilseeds, vegetables and fruits.
Humus, lime and organic matters are present, the proportion of nitrogen (N) is generally low, poor in Phosphorous (P), rich in Potash (K), while Iron oxide and lime vary within a wide range.
Potash is the common name given to a group of minerals and chemicals containing potassium (chemical symbol "K"), which is a basic nutrient for plants and an important ingredient in fertilizer.
Geologically, the alluvial soil of the great plain of India is divided into Khadar (younger) and Bhangar(older) soils.
In general, the ratio of bhangar areas to those of khadar increases upstream along all major rivers.
1.1) Bhangar(older)
Bhangar areas are beyond the floodplains, that lie more upland, and compared to Khadar it consists of older alluvial soil.Bangar areas are less prone to flooding but are usually more sandy and less fertile as well.
A few metres below the terrace of the bhangar are beds of lime nodules known as "Kankar".
1.2) Khadar (younger)
Khadar are low-lying areas composed of newer alluvium that are floodplains along the river banks.The banks are flooded almost every year and a new layer of alluvium is deposited with every flood.
Khadar areas are prone to flooding and sometimes include portions of former river-beds that became available for agriculture when a river changes course.
Khadar is enriched with fresh silts, moisture retentive and sticky, and is often very fertile.
Image Source2) Black (Regur) Soil
The black soils are found in the cretaceous lava-covered areas often referred to as regur or black cotton soils, since cotton has been the most common traditional crop in these areas.Black soil is formed by the weathering or breaking of igneous rocks and also by the cooling or solidification of basaltic lava from the volcano eruption.
Because of their high clay content, black soils develop wide cracks during the dry season, but their iron-rich granular structure makes them resistant to wind and water erosion.
Black soil is called as self ploughing soil due to it ability to retain great amount of water.
It swells greatly on accumulating moisture.
They are poor in humus (organic matter) yet highly moisture-retentive, thus needs little irrigation.
Black Soil stretch over 46 lakh sq km (~16.6%) in the parts of Malwa Plateau, Gujarat, Maharashtra (Most), Western Madhya Pradesh, NW Andhra Pradesh, Karnataka, Tamil Nadu, Rajasthan, Chhattisgarh and Jharkhand.
The soil present in the Malva plateau is named as regur soil (black soil) because, it was spread out in that region during volcanic eruption.
Malva plateau: Volcanic upland north of the Vindhya Range, western Madhya Pradesh and parts of south-eastern Rajasthan.
The soil is rich in iron, lime, calcium, potash, magnesium, and aluminium and deficient in Nitrogen, Phosphorous and organic matter, has high water retaining capacity and good for Cotton (most suitable), Tobacco, Citrus fruits, Castor, and linseed cultivation.
Lime is a soil amendment made from ground limestone rock, which naturally contains calcium carbonate and magnesium carbonate. When lime is added to soil, these compounds work to increase the soil's pH, making soil less acidic and more alkaline.
In general, black soils of uplands are of low fertility while those in the valleys are very fertile.
The black colour is due to the presence of a small proportion of iron (titaniferous magnetite) and black constituents of the parent rock.
3) Red Soil (Omnibus group)
Red Soil occupy around 3.5 lakh sq km (~10.6%) and found in low rainfall area in Tamil Nadu, parts of Karnataka, south-east of Maharashtra, eastern parts of Andhra Pradesh and Madhya Pradesh, Chhattisgarh, Orissa and Chota Nagpur in Jharkhand.Most of the red soils have come into existence due to weathering of iron-rich crystalline and metamorphic rocks.
The top layer of the soil is red and the horizon below is yellowish.
The presence of ferric oxides makes the colour of soil red.
Red Soil are acidic, usually poor growing soils, low in phosphate, lime, magnesia, humus, and nitrogen and difficult to be cultivated because of its low water holding capacity.
But fairly rich in potash and potassium and good for the cultivation of wheat, cotton, pulses, tobacco, millets, orchards, potato and oilseeds.
4) Laterite Soil
Laterite (brick) soils are mostly the end products of weathering, formed under conditions of high temperature and heavy rainfall with alternate wet and dry periods - Humid tropical region.Heavy rainfall (~200 cm) promote leaching of soil whereby lime and silica are leached away and a soil rich in oxides of iron and aluminium compounds is left behind.
It become very soft when wet and very hard when dried, can be easily cut and indefinitely durable beacuse of being already weathered - these qualities makes this soil a valuable building material.
Laterite soils are rich in Iron and Aluminum and very poor in Nitrogen, Potash, Potassium, Lime, Humus.
Numerous varieties of laterite have bauxite and an indefinite mixture of ferric oxides, the Red colour of this soil is due to the presence of ferric oxides.
Due to intensive leaching and low base exchange capacity, typical laterite soils generally lack fertility (due to excess iron) and are of little use for crop production.
But when manured and irrigated some laterites are suitable for growing plantation crops like tea, coffee, rubber, cinchona, coconut, arecanut, etc.
In low lying areas rice, ragi, sugarcane, tapioca and cashew nuts are also grown.
Lateritic soils are the subsoils of the equatorial forests, of the savannas of the humid tropical regions.
In some areas, these soils support grazing grounds and scrub forests.
Laterite Soil covers an area of 2.48 lakh sq km and continuous stretch of laterite soil is found in Western Ghats at 1000 to 1500 m above the sea level, Eastern Ghats, the Rajmahal Hills (Jharkhand), Vindhyan, Satpuras and Malwa Plateau.
It is predominant in malabar coastal region and also found in south Maharashtra, parts of Karnataka, Andhra Pradesh, Orissa, West Bengal, Kerala, Jharkhand, Assam and Meghalaya.
5) Mountainous (Forest) Soil
Mountainous or Forest soil occupy about 2.85 lakh sq km (~8.67%) land area and mainly found on the hill slopes covered by forests.These are heterogeneous soils formed by the deposition of organic matter derived from forests and their character changes with parent rocks, ground-configuration and climate.
Mountainous soil is found in the Himalayan region (valley basins, depressions and inclined slope), Western and Eastern Ghats amd in some parts of the Peninsular plateau.
They are generally infertile for the production of field crops, but useful for supplying forest products, such as timber and fuel.
They are rich in humus, but are deficient in potash, phosphorus, and lime, thus, they require a large amount of fertilizers for plantations of tea, coffee, spices, and tropical fruits.
6) Arid (Desert) Soil
Desert soil is mostly sandy (aeolian sand) soil found in low-rainfall regions. It has a low content of nitrogen and organic matter with very high calcium carbonate and phosphate, thus making it infertile.Using fertilizer and proper irrigation it can be used to grow crops like barley, rapeseed, cotton, wheat, millets, maize, and pulses.
Around 1.42 lakh sq km (~4%<) area of the country is covered by desert soil in the arid and semi-arid regions of Rajasthan, areas of Punjab and Haryana between the Indus and the Aravallis, the Rann of Kuchchh in Gujarat, and coastal regions of Orissa, Tamil Nadu, and Kerala.
7) Saline (Alkaline) Soil
Alkaline soils (sweet soil) are clay soils with high pH (> 8.5), a poor soil structure and a low infiltration capacity.Often they have a hard calcareous layer at 0.5 to 1 metre depth.
They are formed by the weathering of rocks rich in minerals, like calcium (Calcium Carbonate), magnesium and sodium, and in acids like sulfurous acid.
These minerals are transported by rivers to subsoils of plainer region.
These soils have poor drainage and the lowest fertility, rendering them unfit for agriculture.
Saline and alkaline soils occur in the areas having little rainfall (more than areas of desert soils), they are found in the drier areas of Uttar Pradesh, Punjab, Andhra Pradesh, Karnataka, Haryana, Rajasthan, Maharashtra, and Gujarat.
These soils are found in canal irrigated areas and in areas of high sub-soil water table.
In regions with high sub-soil water table, injurious salts are transferred from below by the capillary action as a result of evaporation in dry season.
One of the best ways to "treat alkaline soil (increase acidity in soil)" is to add sulfur, i.e. Gypsum (CaSO₄.2H₂O).
8) Peaty (Marshy) Soil
Peaty (Marshy) Soil are black, heavy and highly acidic soil with large amount of organic matter and considerable amount of soluble salts.This is found in humid regions like Kottayam and Alappuzha districts of Kerala, here it is called "kari".
Marshy soils with a high proportion of vegetable matter is also found in coastal areas of Orissa and Tamil Nadu, Sunderbans of West Bengal, in Bihar and Almora district of Uttaranchal.
They are deficient in potash and phosphate.
Most of the peaty soils are under water during the rainy season but as soon the rains cease, they are put under paddy cultivation.
Related Information
Plants require nine different macronutrients to survive: Carbon (C), Nitrogen (N), Hydrogen (H), Oxygen (O), Phosphorus (P), Potassium (K), Sulfur (S), Calcium (Ca) and Magnesium (Mg).Acidic soil (Sour Soil)
Soil acidity or acidity in general is measured on a scale of 1 to 14. Everything below 7 is considered to be acidic, erverything above is considered to be alkaline.The optimum pH range for most plants is between 6 and 7.
Plants grown in acid soils can experience a variety of stresses including aluminium (Al), hydrogen (H), and/or manganese (Mn) toxicity, as well as nutrient deficiencies of calcium (Ca) and magnesium (Mg).
Aluminium toxicity is the most widespread problem in acid soils.
In order to produce a better crop yield on acid soils, farmers are recommended to apply alkaline materials such as lime (primarily calcium carbonate) to increase the soil pH and thus eliminate Al (aluminium) toxicity, and to apply P (Phosphorus) fertilizer to increase the bioavailable P in soil.
Tea plants prefer a soil pH level between 4.0 to 6.5 (acidic).
Soil salinity (alkalinity)
Salinization of soil is an excessive accumulation of water-soluble salts.Typically, it is table salt NaCl.
The list is far more extensive and includes various compounds of sodium, potassium, calcium, magnesium, sulfates, chlorides, carbohydrates, and bicarbonates.
Salinization occurs when the irrigation water accumulated in the soil evaporates, leaving behind salts and minerals.
If soil has a high salinity content, the plants growing there will not be as vigorous as they would be in normal soils.
Gypsum (calcium sulfate) can be used to help leach salt from the soil
Largest area under saline soils (71.2%) occurs in the state of Uttar Pradesh.
Lime is a carbonate, oxide or hydroxide of calcium - It is used to increase soil pH and provide calcium ions in the soil.
Gypsum is calcium sulphate - It is also used to provide calcium ions in the soil, but does not have the effect of increasing soil pH.
Soil Erosion
Soil erosion is the process of detachment and transportation of soil particles from the soil mass due to:1) Natural factors such as strong winds, heavy rains, flowing rivers, glaciers.
2) Human activities like deforestation, over grazing, shifting cultivation, over-ploughing of land.
3) Faulty agricultural practices.
4) Removal of top soil for industrial or infrastructural purposes.
Besides Shivalik, barren lands of Chambal (Morena) and Yamuna, Chotanagpur Plateau, Malwa Plateau, a large part of Peninsular India, including the Western Ghats and the black cotton soil regions, and coastal zones are the most vulnerable.
Plant trees on barren lands to limit erosion of soil.
Legumes
Legumes revive the soil fertility because the root nodules of leguminous plants contain certain nitrogen fixing bacteria.These nitrogen fixing bacteria absorb nitrogen gas from the atmosphere and convert it into compounds which help in fixing the fertility of the soil.
Legumes plants also lower the emission of carbon dioxide.
Well-known legumes include beans, soybeans, peas, chickpeas, peanuts, lentils, lupins, mesquite, carob, tamarind, alfalfa, and clover.
