Question

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 sparing 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 :

• A. (A), (B) and (C) only.
• B. (B), (C) and (D) only.
• C. (A), (B), (C) and (D).
• D. (A), (C) and (D) only.

Hint:

Because phosphorus is highly immobile, plants can only absorb it from the soil within a few millimeters of their root surfaces. Consequently, plants rely on extensive root growth and mycorrhizal fungi networks to access soil phosphorus.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Phosphorus (P) is a major essential plant nutrient, but it is highly immobile in most soils.
Unlike nitrate anions, which leach easily, phosphate anions interact strongly with soil mineral surfaces, keeping their concentration in the soil solution extremely low.

Step 3: Detailed Explanation:
Let us evaluate each statement explaining phosphorus immobility:
- (A) Phosphate ions are held by anion exchange: Phosphate anions ($\text{H}_2\text{PO}_4^-$ and $\text{HPO}_4^{2-}$) can be electrostatically attracted to positive charges on clay edges and iron/aluminum hydrous oxides, retaining them via anion exchange. (True)
- (B) Phosphate ions are held by ligand exchange: This is a stronger, chemisorption mechanism where phosphate oxygen atoms replace hydroxyl ($\text{OH}$) groups on the surfaces of iron and aluminum oxides, forming stable inner-sphere complexes. (True)
- (C) Phosphate ions are adsorbed as sparingly soluble phosphates:
In acidic soils, phosphates react with dissolved iron and aluminum to precipitate as highly insoluble iron/aluminum phosphates (like variscite and strengthite).
In alkaline soils, they react with calcium to precipitate as insoluble calcium phosphates (like apatite). These precipitates are sparingly soluble, keeping phosphorus locked in the solid phase. (True)
- (D) Phosphate ions are leached as completely soluble phosphates: This is incorrect. Because phosphorus is held strongly by adsorption and precipitation, it does not leach through the soil profile.
Therefore, explanations (A), (B), and (C) are correct.

Step 4: Final Answer:
The correct option is (A), (B), and (C) only, corresponding to option (A).