Which of the following substances are components of soil ?
(a) Stones and sand
(b) Fine soil and decomposed matter
(c) Chemical substances
(d) Colour substances (Dyes)

Question

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Correct — option (3), all four. Soil is not a substance but a mixture, and the question is asking whether the candidate can recognise the full range of what that mixture contains rather than only its most obvious part. Take the four in turn. Statement (a), stones and sand, names the coarse mineral fraction. Soil begins as rock broken down by weathering, and the fragments that result are graded by size — gravel and stones at the top of the scale, then sand, then silt, then clay — so the coarse particles are not an impurity in soil but a part of its definition, and the proportion of them decides how freely a soil drains. Statement (b), fine soil and decomposed matter, names the two components that make soil fertile: the fine mineral particles of silt and clay, which hold water and nutrients on their surfaces, and humus, the dark decomposed remains of plants and animals worked over by soil organisms, which supplies nutrients and gives the soil its crumb structure. Statement (c), chemical substances, is correct because a soil is chemically active throughout. Mineral salts of nitrogen, phosphorus, potassium, calcium and magnesium are dissolved in the soil water and held on the clay and humus particles, and it is in this dissolved chemical form that plants take up everything they need from the ground; the acidity or alkalinity of a soil, the salts that accumulate in it, and the fertilisers added to it are all matters of soil chemistry. Statement (d) names the colouring substances of soil, and it is correct as well, although the English printing of this paper glosses the item as 'Dyes' where the Marathi column reads rangadravye, which means pigments or colouring matter rather than dyes in the manufactured sense. Soil does contain pigments, and they are the reason soils are classified by colour in the first place: iron oxides give the red and yellow colours of the red and lateritic soils, humus gives the dark brown and black colours of soils rich in organic matter, and the black soils of the Deccan take their colour from the iron-bearing minerals of the basalt they weathered out of together with their organic content. Because every one of the four items is genuinely a component of soil, the option that includes all of them is the answer, and option (3) is that option.

Why the other options are wrong — (a) This option keeps the chemical substances and the colouring matter and throws away the physical body of the soil, which is the larger part of it by far. Stones, sand, silt and clay are the mineral skeleton that soil is built on, and the fine particles together with the decomposed organic matter are what make it capable of supporting a crop; a mixture consisting only of dissolved chemicals and pigments would not be soil at all. The option is worth pausing over only because it shows how a partial answer is constructed in this format — two of the four items are retained and the two that a hurried reader would treat as too obvious to be part of a question are dropped. In an all-of-the-above question the obvious items are usually included precisely because they are correct, and discarding an item because it seems too plain to be worth asking about is a reliable way to lose the mark.  (b) This is the most attractive of the three wrong options and the one that will collect most of the candidates who miss the question. It gives the physical account of soil — the coarse mineral fragments, the fine particles and the decomposed organic matter — and stops there, which is how soil is usually pictured and how a hurried reader will summarise it. What it leaves out is that soil is a chemical system as well as a physical one. The nutrients a plant absorbs are taken up as dissolved mineral salts, held on the surfaces of the clay and humus particles and released into the soil water; a soil is described by its reaction, whether acidic or alkaline, and by its salinity; and the whole practice of manuring and fertilising rests on adding chemical substances to it. The colouring matter left out by this option is equally real, since the colour of a soil is used to name and to classify it. Both omissions are of things the question expects the candidate to recognise as belonging to soil.  (d) This option asserts that not one of the four items is a component of soil, which cannot be sustained against even the first of them, since stones and sand are visibly present in any handful of soil taken from the ground. It is the escape option of the set, and it is placed there in the position a candidate expects the all-inclusive answer to occupy — this paper puts 'none of the above' at position (4) here and elsewhere places it at (3), so its position carries no information. The general point is worth making because this paper offers escape options freely — the exact phrase “All of the above” appears ten times among its four hundred options and “None of the above” six times, besides rows like this question's own that spell the same idea out in full: being offered such an option is not evidence about whether it is the right one, and it has to be tested against the statements like any other. Here it fails against every one of them.

Concept: Soil is the loose surface layer in which plants root, and it is a four-part mixture: mineral matter derived from weathered rock, organic matter in the form of humus and living organisms, water held in the pore spaces, and air occupying the pores that the water does not fill. The mineral fraction is graded by particle size into gravel, sand, silt and clay, and the proportions of the last three define the texture of the soil — a sandy soil drains fast and holds few nutrients, a clay soil holds water and nutrients but drains poorly, and a loam, which balances the three, is the most productive. The organic fraction is small by weight but decisive in effect, since humus binds mineral particles into crumbs, holds many times its own weight of water, and releases nutrients as it decomposes. The chemistry of the soil is carried in the soil solution and on the surfaces of the clay and humus particles, which hold exchangeable ions of calcium, magnesium, potassium and other nutrients and release them to plant roots; soil reaction, measured as acidity or alkalinity, controls how available those nutrients are. Colour is the property most used to name soils in India, and it reflects composition directly — iron oxides give red and yellow, organic matter gives dark brown and black, and poor drainage with waterlogging gives grey. Maharashtra's characteristic black soil, weathered from the basalt of the Deccan traps, is the state's most extensive soil type and is prized for cotton.

State PCS 2022 Q45 — Science & Technology PYQ with Answer

State PCS 2022 Q45 — Science & Technology PYQ with Answer