Phosphorus is immobile in soil because

[a] Phosphate ions are held by anion exchange

[b] Phosphate ions are held by ligand exchange

[c] Phosphate ions are adsorbed as sparingly soluble phosphates of Fe/Al/Ca

[d] Phosphate ions are leached as completely soluble phosphates of Fe/Al/Ca

Choose the correct answer from the options given below:

Correct option is A

Correct option: (a) — [a], [b] and [c] only)
Explanation of correct statements:
· (a) Anion exchange: Phosphate (H₂PO₄⁻/HPO₄²⁻) can be retained on positively charged sites of variable-charge minerals (e.g., Fe/Al oxides), reducing mobility.
· (b) Ligand exchange: Phosphate forms strong inner-sphere complexes with Fe/Al oxides and hydroxides, a primary reason for strong fixation.
· (c) Sparingly soluble phosphates: Phosphate precipitates as Fe/Al phosphates in acidic soils and Ca phosphates in calcareous soils, limiting solubility and movement.
Why (d) is incorrect:
· (d) is false because phosphate is not leached as completely soluble phosphates of Fe/Al/Ca; these compounds are sparingly soluble.
Key Points
· Phosphorus mobility is low due to strong fixation and precipitation.
· Management often requires placement strategies and pH management.