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

The correct option is Ammonium sulphate.

Regular use of ammonium sulphate increases the acidity of the soil. Here is a step-by-step chemical and biological explanation of why this happens:

1. Chemical Composition:
Ammonium sulphate has the chemical formula ${\left(N{H}_4\right)}_{2}S{O}_4$. When it is applied to the soil, it dissolves in soil moisture to release ammonium ions ($N{H}_4^{+}$) and sulphate ions ($S{O}_4^{2-}$).

2. Nitrification Process:
Soil contains naturally occurring nitrifying bacteria (such as Nitrosomonas and Nitrobacter). These bacteria convert the ammonium ions ($N{H}_4^{+}$) into nitrate ions ($N{O}_3^{-}$), which plants can easily absorb. This biological oxidation process is represented by the following chemical equation:

$N{H}_4^{+}+2O_2\to N{O}_3^{-}+H_{2}O+2H^{+}$

3. Release of Hydrogen Ions (H⁺):
As shown in the nitrification equation, the conversion of each ammonium ion releases hydrogen ions ($H^{+}$) into the soil solution. The accumulation of these free $H^{+}$ ions directly lowers the pH of the soil, making it more acidic.

4. Leaching of Basic Cations:
Additionally, the sulphate ions ($S{O}_4^{2-}$) remaining in the soil can bind with basic cations like calcium ($C{a}^{2+}$) and magnesium ($M{g}^{2+}$). When water drains through the soil, these basic compounds leach away, leaving the soil further depleted of bases and increasing its overall acidity.