Repeated use of which one of the following fertilizers would increase the acidity of the soil?

Options

A. Urea
B. Superphosphate of lime
C. Ammonium sulphate
D. Potassium nitrate

Explanation

To determine which fertilizer increases soil acidity, we need to analyze the properties of each option provided. Here’s a step-by-step solution:

### Step 1: Identify the fertilizers
The question refers to several fertilizers, including ammonium sulfate, potassium nitrate, super phosphate of lime, and urea.

### Step 2: Analyze ammonium sulfate
Ammonium sulfate is a salt formed from a strong acid (sulfuric acid) and a weak base (ammonium hydroxide). When ammonium sulfate is added to the soil, it undergoes hydrolysis:
\[ \text{(NH}_4)_2\text{SO}_4 + \text{H}_2\text{O} \rightarrow 2 \text{NH}_4^+ + \text{SO}_4^{2-} \]
The ammonium ions (NH₄⁺) can react with water to form ammonium hydroxide (NH₄OH), which is utilized by plants. This process releases hydrogen ions (H⁺), contributing to increased acidity in the soil:
\[ \text{NH}_4^+ + \text{H}_2\text{O} \rightarrow \text{NH}_4\text{OH} + \text{H}^+ \]

### Step 3: Analyze potassium nitrate
Potassium nitrate is a salt of a strong acid (nitric acid) and a strong base (potassium hydroxide). It does not undergo hydrolysis and therefore does not affect the pH of the soil:
\[ \text{KNO}_3 \rightarrow \text{K}^+ + \text{NO}_3^- \]
Since it does not produce H⁺ ions, it does not increase soil acidity.

### Step 4: Analyze super phosphate of lime
Super phosphate of lime is formed from a weak acid (phosphoric acid) and a strong base (calcium hydroxide). It also does not significantly affect soil pH because the basic component neutralizes any acidic ions produced:
\[ \text{Ca(OH)}_2 + \text{H}_3\text{PO}_4 \rightarrow \text{Ca}_3(\text{PO}_4)_2 + \text{H}_2\text{O} \]
Thus, it does not contribute to increased acidity.

### Step 5: Analyze urea
Urea is an organic compound that is converted to ammonia in the soil. It does not produce H⁺ ions directly and is primarily used by plants for nitrogen:
\[ \text{(NH}_2)_2\text{CO} \rightarrow 2 \text{NH}_3 + \text{CO}_2 \]
Since it does not affect the pH significantly, it does not increase soil acidity.

### Conclusion
Among the fertilizers analyzed, ammonium sulfate is the one that increases the acidity of the soil due to the release of hydrogen ions during hydrolysis.

### Final Answer
**Ammonium sulfate** is the fertilizer that would increase the acidity of the soil.

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