Which of the following soil contain the maximum proportion of iron and bauxite minerals ?

Question

UnlockIAS · Accepted Answer

Correct — option (3), 'Laterite soil'. Laterite is the one Indian soil whose defining chemistry is precisely an enrichment in iron and aluminium, and it is enriched in them by subtraction rather than by addition. It forms where high temperatures are combined with heavy seasonal rainfall and a sharp alternation of wet and dry periods, as on the summits of the Western Ghats. During the wet season, rainwater percolating through the soil dissolves and carries away the more soluble constituents — silica first, along with lime, and with them most of the bases and the nutrients. What cannot be carried away are the oxides and hydroxides of iron and aluminium, which are almost insoluble under those conditions, so they stay behind and steadily come to dominate what is left. This process, laterisation, is the reason the soil is red to reddish-brown, since the residue is largely iron oxide, and it is also the reason laterite is the geological setting of bauxite. Bauxite is the principal ore of aluminium and is essentially a lateritic deposit in which the aluminium hydroxides have been concentrated far enough to be worth mining, which is why India's bauxite is found in lateritic country on the plateaus and hill summits of the Western Ghats, of Odisha and of the north-east. No other option on the list is produced by this mechanism. Red soil owes its colour to iron too, but it is the product of the weathering of old crystalline rocks under lower rainfall, where leaching never goes far enough to concentrate an ore. Regur carries iron and alumina inherited from basalt, but they remain bound up in its clay minerals rather than concentrated as free oxides. Alluvial soil is not a weathering residue at all but material transported and dropped by rivers. Laterite alone answers both halves of the stem — the maximum proportion of iron and the association with bauxite — so option (3) is the answer.

Why the other options are wrong — (a) Red soil is the most attractive wrong answer, because its colour comes from exactly the same source as laterite's: iron oxide diffused through the soil. The trap is the reasoning 'red means iron, so the reddest name must hold the most iron'. Red soils develop on the ancient crystalline and metamorphic rocks of the peninsula in areas of comparatively low rainfall, and it is the rainfall that makes the difference. Leaching there is partial, so silica and the bases are not stripped out and the iron and aluminium oxides are never concentrated into anything approaching an ore; the soil is red without being lateritic. Red and yellow soils are also relatively porous and are cultivated widely across the Deccan margins, whereas bauxite mining follows the lateritic crusts. Colour, in short, tells you an iron compound is present; it does not tell you how much has been left behind by leaching.  (b) Regur, the black cotton soil, is a tempting choice for a Maharashtra candidate because it is the soil under his feet across most of the state, and because it genuinely does contain iron along with lime, magnesia, alumina and potash — all inherited from the Deccan trap basalt it weathers from. But the constituents of black soil are for the most part bound up in its clay minerals, which is what gives the soil its remarkable capacity to hold moisture, its deep cracking when it dries and its reputation for self-ploughing. They are not concentrated as free oxides, and bauxite is not derived from regur. The soil's defining qualities are physical rather than chemical: fine texture, high moisture retention and the ability to carry a cotton crop through a dry spell. A question about ore-forming enrichment is asking about a different process altogether.  (d) Alluvial soil is not a residue at all, which rules it out before its composition is even considered. It is transported material — silt, sand and clay carried down by rivers and laid on their plains and deltas — so its make-up reflects the rocks of the distant catchment rather than any chemical change happening where it now lies. Nothing in its history involves the prolonged in-situ leaching that concentrates iron and aluminium oxides. Alluvial soil is generally well supplied with potash, phosphoric acid and lime and deficient in nitrogen and humus, and it is India's most extensive and most productive soil, covering the northern plains and the deltas of the peninsular rivers. Its importance in the syllabus makes it a natural guess for a candidate reaching for the soil he has read most about, but fertility and mineral-ore association are unrelated properties.

Concept: Soils are classified in Indian geography by how they were formed as much as by what they contain, and the two commonest formative stories are transport and weathering in place. Alluvial soils are transported: rivers strip material from their catchments and lay it down across plains and deltas, so the soil is young, layered and chemically close to its source. Everything else in the standard list is residual, formed where it lies, and there the controlling variable is climate acting on parent rock. Black soil, or regur, is what the Deccan trap basalt weathers into under a semi-arid climate: fine, clayey, moisture-retentive, cracking deeply in the dry season. Red soils form on the old crystalline and metamorphic rocks of the peninsula where rainfall is moderate, and take their colour from diffused iron oxide. Laterite forms at the wet extreme, where high temperature and heavy seasonal rain drive leaching so far that silica and lime are carried away and a crust dominated by iron and aluminium oxides is left behind, which is why laterite and bauxite occur together. Arid soils are sandy and saline with a low humus content, and forest or mountain soils vary with altitude and slope. Read down that list and a pattern emerges: as rainfall rises, more is removed from the soil and what remains is progressively less fertile but progressively more concentrated in the insoluble oxides. Laterite is the end of that road — a poor soil and, for the same reason, an ore-bearing one.

State PCS 2023 Q24 — Geography PYQ with Answer

State PCS 2023 Q24 — Geography PYQ with Answer