Ammonium sulphate, $\text{(NH}_4)_2\text{SO}_4$, is a salt of a strong acid (sulphuric acid, $\text{H}_2\text{SO}_4$) and a weak base (ammonium hydroxide, $\text{NH}_4\text{OH}$). When added to the soil, it dissolves in water and the ammonium ions ($\text{NH}_4^+$) undergo cationic hydrolysis to form $\text{NH}_4\text{OH}$ and $\text{H}^+$ ions: $\text{NH}_4^+ (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{NH}_4\text{OH} (aq) + \text{H}^+ (aq)$ This process results in an increase in the concentration of $\text{H}^+$ ions, making the soil solution acidic . Conversely, $\text{CH}_3\text{COONa}$ is a salt of a weak acid and a strong base, so it is basic . $\text{Ca(NO}_3)_2$ is a salt of a strong acid and a strong base, so it is neutral .