Soils & Agriculture — PPSC previous year questions

Humus is decomposed organic matter, so it is not a mineral (1 is correct). It acts like a sponge and improves the soil's moisture-holding capacity (2). It releases nutrients and, through the colloidal complex, helps roots take up the materials the plant builds its body from (3). Its dark colour gives fertile topsoil its dark shade (4). All four statements are correct.

Alluvial soil is deposited by rivers. It is India's most widespread and fertile soil. It covers the whole Indo-Gangetic–Brahmaputra plains, the coastal strips and river deltas — about 40% of the country's area. The Deccan plateau is dominated by black (regur) and red soils. The Western Ghats have laterite and red soils. The north-eastern hills have laterite and forest soils.

Subsistence crops are grown mainly for the farmer's own consumption or for sale in the local market, e.g. millets, pulses and coarse grains on small holdings. Cash crops such as cotton, sugarcane, tea and jute are grown for sale in distant markets or for industry, so option (b) alone is correct.

All four are regional names for shifting (slash-and-burn) cultivation: Jhum in north-east India, Chena in Sri Lanka, Milpa in Mexico and Central America, and Ladang in Indonesia and Malaysia. In this primitive subsistence farming a patch of forest is cleared and burnt, cropped for a few years and then abandoned as fertility falls. Other names include Roca (Brazil), Masole (Congo), Conuco (Venezuela) and Ray (Vietnam).

All three practices check soil erosion on slopes. Contour bunding builds small banks along contour lines to slow run-off. Trees bind the soil with their roots and break the force of rain. Mulching covers the surface, so rain cannot loosen soil particles.

Alluvial soil covers the Indo-Gangetic and Brahmaputra plains (2); black soil covers the Deccan lava region of Maharashtra, M.P. and Gujarat (4); red soil is found in Tamil Nadu, Chota Nagpur, Odisha, Manipur and Mizoram (1); laterite soil in Tamil Nadu, Meghalaya, western Karnataka and eastern Odisha (3).

Statement 1 is wrong. Alluvial soils are rich in potash and lime, but poor in nitrogen, phosphorus and humus. Statement 2 is correct. Black (regur) soils are rich in lime, iron, magnesia and alumina, but poor in nitrogen, phosphorus and organic matter. Statement 3 is correct. Laterite soils form by intense leaching (rain washing soluble nutrients out of the soil). They are rich in iron and aluminium oxides, but poor in nitrogen, potash, lime and organic matter. So 2 and 3 only.

Karewas are thick lacustrine (lake-bed) deposits of clay, silt, sand and glacial material found on the flanks of the Kashmir valley and in the Pir Panjal range. This fertile soil formation is famous for the cultivation of saffron, almonds, walnuts and apples. They formed when the Pleistocene lake that once filled the valley drained and left flat-topped terraces.

Pulses such as masoor, gram and arhar need a soil that holds moisture but drains freely. Loamy soil suits them best. It is a balanced mixture of sand, silt and clay, rich in humus. Clayey soil holds too much water and cakes hard on drying, so it rots the shallow roots of pulses. Sandy soil drains too fast and is poor in nutrients.

Pedology is the branch of soil science that studies the formation, classification and distribution of soils (Greek pedon, 'soil'). The study of water is hydrology and of the atmosphere is meteorology; the study of soils in relation to plant growth is called edaphology.