H⁺ and Al³⁺ ions present in non-exchangeable form with organic matter and silicate clays and aluminium hydroxyl ions account for

Correct option is D

✅ (d) reserve acidity
· Reserve acidity refers to H⁺ and Al³⁺ ions adsorbed on soil colloids (organic matter, silicate clays) that are not in soil solution.
· It represents a large buffer against pH changes and is released slowly.
· Also known as potential acidity.
❌ (a) exchangeable acidity
· Exchangeable acidity includes H⁺ and Al³⁺ ions loosely held on exchange sites and can be replaced by other cations.
· Reserve acidity is more tightly bound.
❌ (b) non-exchangeable acidity
· Non-exchangeable acidity is a broader term but reserve acidity is the precise classification in this context.
❌ (c) active acidity
· Active acidity refers to the concentration of H⁺ ions in soil solution.
· It is a small fraction compared to reserve acidity.