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

Question

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 :

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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.  (b) 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.  (d) '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.

Concept: 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.

State PCS 2024 Q66 — Science & Technology PYQ with Answer

State PCS 2024 Q66 — Science & Technology PYQ with Answer