Soils of India: Formation, Types, Properties, Erosion and Conservation

These soils of India notes for OPSC OAS cover the factors of soil formation, the ICAR classification of eight primary and twenty-seven secondary soil types, the physical properties of texture and water retention, each major soil type from alluvial to peaty, soil pH and fertility, the processes and types of soil erosion, conservation measures, and the soil groups of Odisha. The chapter is heavily factual, so the notes are built around tables that can be revised quickly, and the page closes with previous year practice questions.

Soil Formation and ICAR Classification

Soil is formed by the interaction of five factors: climate, parent material, relief, time and vegetation and organisms. Climate and organisms are active formers, while parent material, relief and time are passive. In the Northern Plains, the soil has been formed by aggradation, the deposition of alluvium by rivers, rather than by in-situ weathering.

The Indian Council of Agricultural Research (ICAR) recognises 8 primary soil types and 27 secondary soil types.

The ICAR classification has 8 primary and 27 secondary soil types. Alluvial soil covers the largest area in India, about 40% of the land.

Soil Texture, Water Retention and Nutrients

Soil texture is determined by the proportion of three particle sizes. The smallest particles give the highest water retention and the lowest drainage.

Water-holding capacity therefore decreases in the order clay > silt > sand, and drainage increases in the same order. Loam is a balanced mixture of all three particles and is the most fertile textural class. Regosols are weakly developed soils of loose, unconsolidated material with only the beginning of topsoil formation.

The most deficient micronutrient in Indian soils is zinc, whose deficiency causes khaira disease in paddy. Among the major nutrients, Indian soils are generally deficient in nitrogen and phosphorus.

BPSC 2011 asked which soil holds the most water — clay. UPPSC Mains 2006 asked which soil particle is smaller than 0.002 mm — clay. BPSC 2011 asked what particles make up loam — all types.

Alluvial Soils

Alluvial soils are transported and deposited by rivers, making them azonal. They cover the largest area in India, about 40%, chiefly in the Northern Plains and the coastal deltas. They are rich in potash and lime, generally fertile and the most productive soil group of the country.

SSC 2019 asked which soil group Khadar and Bhangar belong to — alluvial. BPSC 1996 asked what old Kachhari clay of the Gangetic plain is called — Bhangar. UPRO/ARO 2014 asked the most productive soil in India — alluvial.

Black Soils (Regur)

Black soils are derived from the weathering of Deccan basalt and are locally called regur. They are the classic cotton soil and are most widespread in Maharashtra, followed by Madhya Pradesh, Gujarat, Andhra Pradesh and Tamil Nadu. They are intrazonal soils.

The soil becomes sticky and difficult to work when wet. The Malwa Plateau and the Deccan lava country are its heartland. It is not found in the Himalayan region.

NDA 2015 asked what Regur means — black cotton soil. UP Lower Sub 2009 asked the self-ploughed soil — black soil. CDS 2011 asked whether black soil is rich in nitrogen and phosphorus — it is not.

Red Soils

Red soils develop from the weathering of ancient crystalline and metamorphic rocks of the peninsular shield. Their colour comes from ferric oxides (haematite). They are zonal soils and are found in western Tamil Nadu, southern Karnataka, north-eastern Andhra Pradesh, parts of Madhya Pradesh and Jharkhand, and Odisha.

They are generally shallow, porous and poor in phosphorus, nitrogen and lime. In Tamil Nadu they occupy over two-thirds of the state's area. The red soils of Bardhaman, Birbhum and Bankura in West Bengal are transported from the Chota Nagpur Plateau, and the light sandy red and yellow soils of Balaghat, Durg and Raipur are of alluvial origin.

IAS 2000 asked the reason for the red colour — ferric oxides. The standard statement question lists the correct distribution and nutrient deficiency of red soils.

Laterite Soils

Laterite soils form in the humid tropics under heavy rainfall and high temperature. Intense leaching removes silica, lime and bases and leaves a residue rich in iron and aluminium oxides. The name comes from the Latin later, meaning brick, because the moist soil can be cut into blocks that harden on exposure.

The Malabar coastal region is the classic laterite area of India. Laterites are not well developed in Rajasthan and Uttar Pradesh, and they are not rich in lime.

IAS 1994 asked which soil is rendered infertile by excess iron — lateritic. UPPSC 2001 asked which statement is not true — that laterites are rich in lime. CDS 2016 asked whether laterites are rich in nitrogen and potash — they are not.

Arid and Desert Soils

Arid soils occur in Rajasthan, Gujarat and the dry parts of Haryana and Punjab. They are sandy, low in organic matter and often calcareous, with high calcium carbonate content. They are low in nitrogen and phosphorus. In western Rajasthan the soils have a high calcium content, and irrigation without drainage leads to salinisation.

IAS 1993 asked the high content of western Rajasthan soils — calcium.

Saline and Alkaline Soils

Saline and alkaline soils occur in drought-prone and arid regions where evaporation exceeds rainfall, or where irrigation without drainage raises the water table and brings salts to the surface. In India they are locally called Usar, Reh or Kallar. The largest area of saline soil is in Gujarat, and the largest area of alkali (sodic) soil is in Uttar Pradesh.

Salinisation makes soils impermeable because salt crusts block the pores. The long-term remedy is gypsum application, which supplies calcium that displaces sodium and allows leaching. Drainage is essential. These soils are often sandy loam in texture in the Indo-Gangetic plain.

IAS 2011 asked the effect of salinisation — it makes soils impermeable. RAS/RTS 1997 asked the long-term remedy — gypsum. UPPSC Mains 2012 asked the state with the largest saline area — Gujarat.

Peaty, Marshy and Forest Soils

Peaty and marshy soils form in waterlogged coastal areas with heavy organic accumulation. They are acidic and rich in organic matter. In Kerala they are called Kari soils, and they also occur in the Sundarbans and coastal Odisha. The Karewas of the Kashmir valley are lacustrine deposits famous for saffron cultivation, and they are rich in organic matter and well drained.

In the USDA taxonomy, histosols are the organic-rich soils corresponding to peats, while vertisols are clay-rich, gelisols are permafrost soils and spodosols are podzols.

BPSC 2018 asked where Karewas are found — the Kashmir Himalayas. CDS 2020 asked which soil has very high organic matter — histosol.

Soil pH, Acidity and Alkalinity

Soil pH determines nutrient availability. A pH of about 6 to 7 is the most favourable for most crops. Strongly acidic soils (pH 3–4) and strongly alkaline soils (pH 9–10) both reduce fertility.

Tea requires acidic soils of pH about 5.0–5.5, which is why it grows on the well-drained hill slopes of Assam, Darjeeling and the Nilgiris. Applying lime makes the soil less acidic, not more acidic, which is a frequent trap.

IAS 1993 asked the pH of fertile soil — six to seven. NDA 2015 asked which statement is not correct — that applying lime makes the soil acidic. CDS 2012 asked why farmers mix lime — to decrease soil acidity.

Soil Erosion: Types and Causes

Soil erosion is the removal of the topsoil by water or wind. In India, water erosion is the main cause of topsoil loss, with wind erosion significant in arid and semi-arid regions. The sequence of water erosion is splash → sheet → rill → gully.

The Chambal Valley is India's most severely eroded area, where gully erosion has created badland ravines. The Chhos of northern Haryana and Punjab are gullied, water-eroded tracts, not wind-eroded areas. In the Chota Nagpur Plateau, excessive erosion is caused mainly by large-scale felling of trees. Kerala has the most serious coastal erosion because of heavy rainfall and undulating lateritic terrain.

Crop choice affects erosion: sorghum, a wide-row crop, erodes fastest because it leaves soil bare between rows, while close-grown crops such as clover protect the soil. Soil impoverishment is the depletion of plant nutrients, distinct from erosion.

IAS 2001 asked the order of erosion — splash, sheet, rill, gully. IAS 1994 asked which crop erodes fastest — sorghum. CDS 2019 asked what soil impoverishment means — nutrient depletion.

Soil Conservation

Conservation combines biological and mechanical measures. Afforestation is the most effective biological measure because roots bind soil and improve infiltration. Crop rotation maintains fertility and cover, while terracing, contour ploughing, strip cropping and check dams control runoff on slopes.

Mulching protects soil from sheet wash and wind erosion and helps retain moisture and nutrients, but it does not control gully erosion, which needs structural measures such as check dams. Terrace cultivation is a conservation measure, not a cause of erosion.

UPRO/ARO 2014 asked how soil erosion can be prevented — afforestation. IAS 2010 asked which practices conserve soil — crop rotation, sand fences, terracing and windbreaks. IAS 2017 asked about water conservation — reduced tillage, gypsum and residue retention.

Odisha Angle: Soil Groups and Erosion

Odisha's soils are heterogeneous because of the interplay of climate, topography and parent material. The state's soils are conventionally divided into eight broad groups.

Laterite soils of Odisha are naturally poor in nitrogen and phosphorus and need heavy fertilisation, while the deltaic alluvial soils are the state's most fertile. The coastal saline belt of Balasore, Bhadrak, Kendrapara and Jagatsinghpur needs drainage and gypsum for reclamation, and salinity ingress also affects the periphery of Chilika lagoon, reducing paddy yields.

The red and laterite uplands of the Eastern Ghats are prone to erosion because of their undulating terrain and heavy monsoon runoff. Contour bunding, afforestation and gully plugging are the standard remedies. The state's soil health card programme and the promotion of millets in tribal districts are responses to the nutrient-poor upland soils.

The Odisha Economic Survey and state agriculture department data show red soil as the largest group and deltaic alluvium as the most fertile.

Practice Questions