1. Identify what is being asked

We need the fertilizer whose regular use increases the acidity of soil.

So we must check which fertilizer leaves the soil more acidic after its chemical transformations.

2. Key concept

Fertilizers containing the ammonium ion $\left(\mathrm{NH_4^+}\right)$ often acidify soil.

This happens because in soil, ammonium gets oxidized to nitrate by nitrifying bacteria:

$\mathrm{NH_4^+ + 2O_2 \rightarrow NO_3^- + 2H^+ + H_2O}$

The formation of $\mathrm{H^+}$ ions increases soil acidity.

3. Check each option

Option A: Potassium nitrate $\left(\mathrm{KNO_3}\right)$

• This is a nitrate fertilizer.
• It does not generate $\mathrm{H^+}$ through nitrification like ammonium fertilizers do.
• Hence, it is not the one that increases soil acidity significantly.

Option B: Urea $\left(\mathrm{NH_2CONH_2}\right)$

• Urea converts in soil to ammonium compounds.
• Although it can have some acidifying effect after nitrification, it is not the standard fertilizer most specifically known for strongly increasing soil acidity in such questions.
• So this is not the best answer.

Option C: Superphosphate of lime

• This is mainly a phosphatic fertilizer.
• It is not known for causing marked soil acidity on regular use in the way ammonium salts do.
• So this is incorrect.

Option D: Ammonium sulphate $\left(\mathrm{(NH_4)_2SO_4}\right)$

• This is an ammonium fertilizer.
• In soil, $\mathrm{NH_4^+}$ undergoes nitrification, producing $\mathrm{H^+}$ ions.
• Therefore, regular use increases soil acidity.
• This is the correct option.

4. Conclusion

The fertilizer that increases the acidity of soil on regular use is:

$\boxed{\text{Ammonium sulphate}}$

So the correct option is D.

5. Comparison with stored correct answer

Stored correct answer: D
Derived answer: D

They match.