MPSC 2022 Prelims Paper-I · Question 45 of 100

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.

• (1)Only (c) and (d) — 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.
• (2)Only (a) and (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.
• (4)None of (a), (b), (c) and (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.

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.

This is an all-of-the-above question and it tests breadth rather than depth: the candidate who has learnt soil as a list of components answers it in seconds, and the candidate who pictures soil only as earth and stones is drawn to option (2). Two general habits are being rewarded. The first is completeness — when a stem asks what a familiar thing is composed of, the answer is likely to include the components that are not visible as well as those that are, and dissolved salts and pigments are as much a part of soil as sand is. The second is not to reject an all-inclusive option merely because it looks too generous; the exact phrase “All of the above” appears ten times among this paper's four hundred options, and several more rows — including this question's own option (3) — say the same thing by listing every item; being offered such an option tells the candidate nothing about whether it is keyed. Soil is examined by MPSC from two directions at once, as school science in the general studies section and as physical geography in the Maharashtra papers, where the black soils of the Deccan plateau, the red soils of the eastern districts, the laterites of the Konkan and the alluvium of the river valleys are all standard material, so a component question of this kind and a soil-type question can appear in the same paper.

• Soil is a mixture of four things: mineral matter weathered from rock, organic matter in the form of humus and living organisms, water held in the pore spaces, and air in the pores not filled by water.
• The mineral fraction is graded by particle size into gravel and stones, sand, silt and clay, and the proportions of sand, silt and clay define soil texture, with loam the most productive balance of the three.
• Plant nutrients are present in soil as dissolved mineral salts and as exchangeable ions held on clay and humus surfaces, so soil chemistry, including its acidity or alkalinity, governs how much of a nutrient a plant can actually take up.
• Soil colour reflects composition: iron oxides give the red and yellow soils, organic matter gives dark brown and black colours, and colour is the property most often used to name Indian soil types.
• The English column of this question glosses the fourth item as 'Colour substances (Dyes)', where the Marathi reads rangadravye, meaning pigments or colouring matter; the item refers to natural soil pigments and not to manufactured dyes.

• Picturing soil as only its visible physical parts and rejecting the chemical and colouring components, which is what makes option (2) the most attractive of the wrong answers
• Rejecting an all-inclusive option because it looks too generous, when this paper offers such options ten times and their being offered says nothing about whether they are keyed
• Reading the English gloss 'Dyes' literally, when the item means the natural pigments that give a soil its colour, chiefly iron oxides and humus
• Assuming that a none-of-these option placed last is a filler, when this paper varies the position of its escape options from question to question

Soil appears in MPSC papers in two registers. In the general science register it is asked as composition, as the role of humus, as the effect of texture on drainage, or as the definition of soil erosion, and the questions are usually direct and drawn from the school textbook. In the geography register it is asked as the distribution of Indian soil types, the parent rock each derives from, the crops each supports and the conservation measure each requires, and here Maharashtra's black soils and the Konkan laterites are the recurring subjects. Because the two registers draw on the same underlying material, a candidate who can say what soil is made of, how it forms, how it is classified by colour and texture and how it is conserved has covered both. Statement-list questions on soil tend to be inclusive rather than exclusive, so the technique is to look for an item that is genuinely not a soil component rather than to assume that a long list must contain a planted error.

No directly related past PYQ was found.

• practice — not a real PYQ

  The dark colour and the high moisture-retaining capacity of the black soils of the Deccan plateau are chiefly derived from which of the following ?

  • (a)The weathering of the basaltic lava of the Deccan traps
  • (b)The deposition of silt by the Ganga and its tributaries
  • (c)The leaching of laterite under heavy rainfall
  • (d)The accumulation of wind-blown sand

  Answer(a) The weathering of the basaltic lava of the Deccan traps — the black soils of Maharashtra, also called regur, are formed in situ from the basalt of the traps, and their clayey texture is what lets them hold moisture through the dry season, which is why they support cotton without irrigation. Alluvial soils are the ones deposited by rivers, laterite is formed by intense leaching under heavy seasonal rainfall, and wind-blown sand gives the desert soils of the north-west.

• practice — not a real PYQ

  Which component of soil is chiefly responsible for binding mineral particles into crumbs and for releasing nutrients as it decomposes ?

  • (a)Coarse sand
  • (b)Humus
  • (c)Soil air
  • (d)Gravel

  Answer(b) Humus — the dark decomposed remains of plant and animal matter, worked over by earthworms, bacteria and fungi. Although it is a small part of the soil by weight, it binds fine particles into crumbs and so improves both aeration and drainage, holds a large amount of water, and releases nitrogen and other nutrients steadily as it breaks down. Coarse sand and gravel form the skeleton of the soil but hold neither water nor nutrients well, and soil air supplies oxygen to roots and soil organisms without contributing nutrients directly.