Which chemical compound is used to neutralize the acidity of soil increased due to excessive use of Chemical Fertilizers ? (a) Potash (b) Lime water (c) Milk of magnesia (d) Ammonium hydroxide Answer options :
- (1)Only (c)
- (2)Only (a) and (d)
- (3)Only (b)
- (4)All the above
Correct — option (3), 'Only (b)', that is lime water. Soil turned acidic by the heavy use of chemical fertilisers is corrected by adding a base, and the base used in the field is lime — quicklime, which is calcium oxide, slaked lime, which is calcium hydroxide, or ground limestone and chalk, which are calcium carbonate. Lime water is simply a solution of slaked lime, calcium hydroxide, in water, so statement (b) names the standard remedy. The chemistry is the ordinary neutralisation of an acid by a base: the hydrogen ions that make the soil solution acidic are taken up, the pH rises towards neutrality, and the calcium left behind is itself a plant nutrient that improves the crumb structure of the soil. The reason the problem arises in the first place is worth understanding, because it explains why two of the other statements are the opposite of a remedy. Nitrogenous fertilisers supply nitrogen mostly in the ammonium form, and when soil bacteria oxidise ammonium to nitrate in the process of nitrification, hydrogen ions are released; year after year of heavy application therefore drives the soil pH down. Adding a calcium base restores it, which is why liming is a routine operation on acid soils and why agricultural extension advice is framed in terms of lime requirement, the quantity of lime needed to raise the pH of a given soil to the desired level.
- (1)Only (c) — Milk of magnesia is a suspension of magnesium hydroxide in water, and it is indeed a base that neutralises acid — but its use is medicinal, as an antacid taken to relieve excess acid in the stomach, not agricultural. It is neither cheap enough nor available in the tonnage that field application demands, and it is not what is meant by soil treatment in any standard account. The option is attractive because milk of magnesia is the neutralisation example most students remember, and this question deliberately places the familiar antacid beside the correct field remedy to see whether the two contexts have been kept apart.
- (2)Only (a) and (d) — Neither statement in this option is a remedy for soil acidity. Potash in agricultural usage means potassic fertiliser, supplied chiefly as muriate of potash, and it is applied to feed the crop with potassium, not to correct pH — adding another fertiliser is not the answer to a problem caused by excessive fertiliser use. Ammonium hydroxide is worse than useless here: although ammonia solution is itself alkaline, ammonium nitrogen added to soil is oxidised by nitrifying bacteria to nitrate with the release of hydrogen ions, so an ammonium source pushes the soil further towards acidity over time rather than away from it.
- (4)All the above — 'All the above' requires every one of the four printed statements to be a correct answer to the stem, and three of them are not. Potash is a nutrient rather than a pH corrective, milk of magnesia belongs to the medicine cabinet rather than the field, and ammonium hydroxide supplies the very form of nitrogen whose nitrification acidified the soil. An escape option of this kind is safe to take only when each substantive statement has been positively confirmed, and here a single confirmed failure — the ammonium source — closes it.
Soil pH governs how readily a plant can take up nutrients, and most crops do best in soil that is near neutral, roughly between pH 6 and 7.5. Soils become acidic through natural leaching in high-rainfall regions and through agricultural practice, chiefly the long-continued use of ammonium-based nitrogenous fertilisers such as urea and ammonium sulphate: when soil bacteria nitrify ammonium to nitrate they release hydrogen ions, and repeated applications drive the pH down. Acid soil harms crops in more than one way — phosphorus becomes locked in unavailable forms, aluminium and manganese become soluble enough to be toxic to roots, and the bacteria that fix nitrogen and decompose organic matter work poorly. The correction is liming, the application of a calcium base: quicklime (calcium oxide), slaked lime (calcium hydroxide, whose solution is lime water) or ground limestone and chalk (calcium carbonate). The reverse problem, an alkaline or saline-alkali soil of the kind found in parts of the Deccan and in irrigated tracts of northern India, is treated with gypsum, calcium sulphate, which is an acid-side amendment. Holding the pair together — lime for acid soil, gypsum for alkaline soil — is the compact form in which this topic is worth remembering.
This question is a good example of MPSC's habit of testing school chemistry through an applied situation rather than as abstract theory. The neutralisation reaction is Class 10 material, but the paper wraps it in agriculture, which is close to the concerns of the Maharashtra syllabus and to the everyday world of the state's cultivators. The distractors are chosen from the same chapter of the textbook: milk of magnesia is the antacid example, potash is the fertiliser example, and ammonium hydroxide is the weak-base example, so a candidate who has memorised the chapter without attaching each substance to its use can be drawn to any of them. The habit that pays is to ask what a substance is actually used for in practice, not merely whether it is chemically a base — and to remember that the question specifies acidity caused by fertiliser use, which points at the field and not the stomach.
- Acidic soil is treated with lime — quicklime (calcium oxide), slaked lime (calcium hydroxide) or chalk and limestone (calcium carbonate); lime water is a solution of slaked lime.
- Heavy use of ammonium-based nitrogenous fertilisers acidifies soil, because the nitrification of ammonium to nitrate by soil bacteria releases hydrogen ions.
- Milk of magnesia is a suspension of magnesium hydroxide used as an antacid in medicine, not as a soil amendment.
- Potash in agricultural usage means potassic fertiliser, a plant nutrient rather than a corrective for soil pH.
- The opposite problem, alkaline or saline-alkali soil, is treated with gypsum (calcium sulphate) rather than with lime.
That last line is also the cause of the problem: ammonium-form nitrogenous fertiliser, nitrified season after season, is what drives soil pH down, which is why extension advice is framed as a 'lime requirement' — the quantity of lime needed to lift a given soil to the wanted pH. The opposite complaint, alkaline or saline-alkali soil, is corrected with gypsum rather than lime.
- Choosing milk of magnesia because it is the neutralisation example most often taught; its use is medicinal, not agricultural
- Treating any base as a valid answer without asking whether it is actually used in the field
- Overlooking that an ammonium source acidifies soil after nitrification, so it aggravates rather than corrects the problem
- Taking 'all the above' without positively confirming every statement in the list
Applied chemistry of this kind recurs in MPSC papers in the form 'which substance is used for X', with X drawn from agriculture, medicine or daily life — treating acid soil, relieving acidity of the stomach, curing a bee sting, whitewashing a wall, softening water. The reliable preparation is a short table of substance, chemical name and use, since the Commission tests the pairing rather than the chemistry. Statement-list packaging, as here, adds only the requirement that each printed statement be judged separately before the options are read.
No directly related past PYQ was found.
- practice — not a real PYQ
Excessive and long-continued use of ammonium-based nitrogenous fertilisers tends to make the soil acidic mainly because :
- (a)Ammonium hydroxide is a strong acid
- (b)Nitrification of ammonium to nitrate by soil bacteria releases hydrogen ions
- (c)The fertiliser dissolves the calcium carbonate already present in the soil
- (d)Ammonium ions displace potassium from the soil
Answer(b) Nitrification of ammonium to nitrate by soil bacteria releases hydrogen ions — the accumulation of these ions lowers the soil pH over successive seasons, which is why acid soils under intensive cultivation need periodic liming.
- practice — not a real PYQ
Which amendment is applied to reclaim an alkaline or saline-alkali soil, as opposed to an acidic one ?
- (a)Slaked lime
- (b)Quicklime
- (c)Gypsum
- (d)Chalk
Answer(c) Gypsum — calcium sulphate is the standard amendment for alkaline and saline-alkali soils, supplying calcium to displace sodium from the soil complex. Slaked lime, quicklime and chalk are all calcium bases used for the opposite problem, the correction of soil acidity.