Regular use of which of the following fertilizer increases the acidity of soil?

When any ammonium salt is added to soil, the $$NH_4^{+}$$ ion is first retained on the clay particles and then slowly oxidised by soil bacteria to nitrate in the nitrification process:

$$NH_4^{+} + 2O_2\rightarrow [\text{Nitrosomonas/Nitrobacter}]NO_3^{-} + 2\,H^{+} + H_2O$$

This biochemical reaction releases two protons $$H^{+}$$ in the soil solution for every mole of ammonium oxidised, thereby lowering the soil pH (making the soil acidic).

Among the listed fertilisers, only ammonium sulphate, $$(NH_4)_2SO_4$$, supplies nitrogen exclusively in the ammonium form; hence its long-term use markedly increases soil acidity.

Why the other options do not acidify the soil appreciably:

• Potassium nitrate, $$KNO_3$$, provides nitrogen directly as the nitrate ion $$NO_3^{-}$$, so no acid-generating nitrification step occurs.
• Urea, $$CO(NH_2)_2$$, is first hydrolysed to $$NH_3$$ (which is basic) and $$CO_2$$; overall it has a near-neutral effect on soil pH.
• Superphosphate of lime, $$Ca(H_2PO_4)_2\cdot 2H_2O$$, contains calcium which actually tends to counteract acidity.

Therefore, regular application of ammonium sulphate is known to make the soil increasingly acidic.

Option D which is: Ammonium sulphate