Explanation:

- Option 1: Oxygen

- Oxygen alone doesn't determine soil acidity. It's a key element in soil, affecting chemical reactions and breathing processes.

- Option 2: Phosphorous

- Phosphorous is essential for plant growth, not related to soil acidity. It's mostly present in soil as phosphate.

- Option 3: Hydrogen

- Correct Answer: The concentration of hydrogen ions (H+) directly influences the acidic nature of soil. Higher hydrogen ion concentration lowers soil pH.

- Option 4: Nitrogen

- Nitrogen is crucial for plants but doesn't directly indicate soil acidity. It can exist in different forms like nitrate, which might slightly affect pH.