Question

Rockphosphate is applied as a fertilizer in which type of soils?

• A. Saline Soils
• B. Acidic Soils
• C. Calcareous Soils
• D. Organic Soils

Hint:

For neutral and alkaline soils, highly soluble phosphorus fertilizers like Single Super Phosphate (SSP) or Diammonium Phosphate (DAP) are preferred. Insoluble sources like Rock Phosphate or Basic Slag are reserved for acidic soils.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Rock phosphate is an insoluble, naturally occurring mineral source of phosphorus.
Because its phosphorus is bound in apatite forms, it requires chemical weathering to convert into plant-available orthophosphate ions.

Step 3: Detailed Explanation:
The dissolution of rock phosphate can be represented by the following chemical equation:
\[ \text{Ca}_3(\text{PO}_4)_2 + 4\text{H}^+ \rightleftharpoons 3\text{Ca}^{2+} + 2\text{H}_2\text{PO}_4^- \]
This reaction proceeds to the right in the presence of a high concentration of hydrogen ions (\(\text{H}^+\)), which is characteristic of acidic soils (pH \(<\) 5).
- In acidic soils, the abundant hydrogen ions break down the calcium phosphate structure, releasing plant-available phosphorus (\(\text{H}_2\text{PO}_4^-\)) over time.
- In neutral, saline, or calcareous soils, the lack of acidity prevents the dissolution of rock phosphate, making it ineffective.
Consequently, rock phosphate is used as a slow-release phosphorus fertilizer primarily in acidic soils.

Step 4: Final Answer:
Rock phosphate is applied as a fertilizer in acidic soils.