Question

In acid soils at pH < 5, the element that becomes available in toxic amounts is:

• A. Nitrogen
• B. Phosphorus
• C. Aluminium
• D. Molybdenum

Hint:

Liming is the common agricultural practice used to raise the pH of acid soils, which precipitates Aluminium out of the soil solution, rendering it non-toxic.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Soil pH significantly affects the solubility and availability of various elements. In strongly acidic soils (pH \(<\) 5.0), the solubility of certain metallic elements increases drastically to levels that can inhibit root growth and metabolic processes.

Step 2: Detailed Explanation:
As soil pH drops below 5.5, the minerals in the soil (like aluminosilicates) begin to dissolve.
1. Aluminium (Al): In these acidic conditions, aluminium is released from soil minerals in the form of \(\text{Al}^{3+}\) ions. \(\text{Al}^{3+}\) is highly toxic to plants. It binds to DNA, interferes with cell wall expansion, and inhibits root elongation, leading to stunted, stubby roots that cannot take up water or nutrients effectively. This is the primary limitation for crop production in acid soils globally.
2. Manganese (Mn): Manganese also becomes highly available and potentially toxic in acid soils, but Aluminium toxicity is generally considered the more dominant and devastating factor at pH \(<\) 5.
3. Nutrient Availability: Conversely, elements like Phosphorus and Molybdenum become less available in acid soils because they form insoluble complexes with iron and aluminium.

Step 3: Final Answer:
Aluminium is the element that becomes available in toxic amounts in highly acidic soils.