Soils, Rocks and Minerals

Laterite forms under a hot and seasonally very wet climate in which alternating wet and dry periods drive intense leaching. Silica and the soluble bases are washed out and oxides of iron and aluminium are left behind and concentrated, which is the process of laterisation.

• Statement 1 is correct: the residual iron oxide gives the soil its red to reddish brown colour, and the same enrichment is what allows lateritic material to be cut into blocks that harden on exposure and be used as building stone.
• Statement 2 is incorrect and states the opposite of the truth. Leaching removes the bases, so laterite is poor in nitrogen, phosphate, potash, lime and organic matter, and its acidity and low humus are why it needs heavy manuring and liming to be productive at all.
• Statement 3 is incorrect. Laterite occurs on the summits of the Western and Eastern Ghats, in Karnataka, Kerala, Tamil Nadu, Maharashtra, Odisha, parts of Madhya Pradesh and the hills of Assam and Meghalaya, all areas of high seasonal rainfall. Rajasthan is arid and carries desert and saline soils, and the greater part of Uttar Pradesh carries alluvium, and neither has the rainfall regime laterisation requires.
• Statement 4 is correct: tapioca, cashew, tea, coffee, rubber and coconut are the crops associated with lateritic tracts, cashew on the coastal laterites of Kerala, Karnataka and Konkan being the standard example. The official answer (c) follows. The elimination route is the rainfall requirement, which defeats statement 3 and with it options (a), (b) and (d) together, so a single correct idea about how laterite forms disposes of the whole question.

The question tests the association between geological systems and the resources found in them.

• Statement 1 is incorrect. The Gondwana system, of Upper Carboniferous to Jurassic age, holds about ninety eight per cent of India's coal, in the Damodar, Son, Mahanadi, Godavari and Wardha valleys, but petroleum and natural gas in India occur in the Tertiary sedimentary basins, Assam, Gujarat, the Cambay basin, Mumbai High and the Krishna Godavari and Cauvery offshore basins, where marine organic matter was buried in younger sediments. Coal in the Gondwana, hydrocarbons in the Tertiary, is the standard pairing.
• Statement 2 is correct. Koderma in the Hazaribagh plateau of Jharkhand is the best known mica belt in India and was long the largest producer of sheet mica in the world, the mica occurring in pegmatite veins intruded into the Archaean gneisses and schists, with the Nellore belt of Andhra Pradesh and the Bhilwara belt of Rajasthan the other two producing tracts.
• Statement 3 is incorrect. The Dharwar system is Archaean and Proterozoic, the oldest metamorphosed sedimentary system in India, and it is the great repository of metallic minerals, iron ore, manganese, gold at Kolar and Hutti, copper, lead and zinc. Petroleum is a hydrocarbon of organic origin trapped in porous sedimentary rock; it cannot occur in a highly metamorphosed Archaean system, and the mismatch is one of kind rather than of degree. The official answer (b) follows. The elimination route is statement 3, which removes option (c), after which statement 1 decides between options (a) and (b).

• Statement 1 is incorrect and is the whole of the question. Terrace cultivation is a soil conservation measure, not a cause of erosion. Cutting a slope into a series of level or gently backward-sloping steps reduces the effective slope length and gradient, which reduces the velocity and therefore the erosive energy of surface runoff, and allows water to infiltrate rather than concentrate. The confusion the examiner is inviting is with shifting cultivation, which does accelerate erosion by exposing bare soil on slopes after burning.
• Statement 2 is correct. Removal of forest cover exposes soil to direct raindrop impact, ends the interception and litter protection that dissipate that impact, removes root reinforcement of the soil mass and increases the proportion of rainfall that becomes overland flow. The official answer is (b), and the option set makes the item decidable on statement 1 alone: (b) is the only option that omits terrace cultivation, so a candidate who rejects statement 1 need never adjudicate statement 3.

• Statement 1 is incorrect and it is the only statement needed, since it appears in options (a), (c) and (d) and rejecting it leaves (b) at once. Organic matter increases water holding capacity rather than reducing it, and it does so in two ways: humus is colloidal and hydrophilic and holds several times its own weight in water, and organic matter binds mineral particles into stable aggregates, creating the pore structure in which water is retained against gravity. Raising soil organic carbon is the standard recommendation for drought resilience, which makes the statement not merely wrong but the reverse of settled practice.
• Statement 2 is incorrect. Soil is central to the sulphur cycle. The bulk of soil sulphur is held in organic matter and is released to the plant available sulphate form by microbial mineralisation, sulphur oxidising bacteria such as Thiobacillus convert reduced forms to sulphate, and under waterlogged and anaerobic conditions sulphate reducing bacteria run the reaction the other way to produce hydrogen sulphide. Sulphur deficiency in Indian soils, aggravated by the shift to high analysis fertilisers that carry no incidental sulphur, is a live agronomic problem.
• Statement 3 is correct. Irrigation water carries dissolved salts, and where drainage is poor and evaporation high the water leaves while the salts remain and accumulate in the root zone; a rising water table brought on by canal irrigation carries salts up by capillary action to the surface. The salinity and waterlogging of parts of the canal commands in Punjab, Haryana and western Uttar Pradesh is the standard Indian illustration.

Hence (b).

Black cotton soil, or regur, is the weathering product of the Deccan basalts, erupted as flood basalts through fissures rather than central vents during the late Cretaceous, so option (b) is correct. The parent rock explains every property of the soil: it is rich in iron, magnesium, calcium and aluminium and poor in nitrogen and organic matter; it is clayey and montmorillonitic, so it swells when wet and develops deep cracks when dry, giving it self ploughing character and high moisture retention that suits cotton in a rainfed setting.

• Option (a) is wrong because brown forest soils are Himalayan.
• Option (c) is wrong because granite and schist weather to red and lateritic soils.
• Option (d) is wrong because shale and limestone give calcareous and alluvial derivatives. Hence (b). The geographical correspondence between the Deccan Trap outcrop and the regur belt of Maharashtra, Madhya Pradesh and Gujarat is the confirming check.

This question turns entirely on the distinction between porosity and permeability, and rewards the candidate who holds the two apart. Porosity is the proportion of void space in a rock; permeability is the ability of fluid to pass through it, which depends on whether those voids are large and interconnected.

• Statement I is correct: chalk is a highly permeable rock and forms important aquifers, while clay is the classic aquiclude.
• Statement II is correct: chalk is porous, with well-connected pore spaces and fissures that both hold and transmit water, and this explains the permeability asserted in Statement I.
• Statement III is incorrect, and this is the decisive point. Clay is in fact highly porous, often with porosity of forty to fifty per cent, which is comparable to or greater than many permeable rocks. It is impermeable not because it lacks voids but because its pores are extremely fine and poorly connected, so capillary and surface tension forces hold water in place rather than transmitting it. Only Statement II is correct and it does explain Statement I, giving (c).
• Options (a) and (b) both require Statement III to stand, and option (d) wrongly rejects the porosity of chalk.