Question

What are the type of phosphate ions available for plant uptake in alkali soil ?

• A. \(\text{H}_2\text{PO}_4^{2-}\)
• B. \(\text{HPO}_4^{-}\)
• C. \(\text{PO}_4^{3-}\)
• D. \(\text{H}_3\text{PO}_4\)

Hint:

- Acidic soils (pH < 7.0): \(\text{H}_2\text{PO}_4^-\) is the dominant form absorbed by plants.
- Alkaline/Alkali soils (pH > 7.0): \(\text{HPO}_4^{2-}\) is the dominant form available for plant uptake.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Phosphorus is absorbed by plants in the form of inorganic orthophosphate ions from the soil solution.
The specific ionic form of orthophosphate present in the soil is highly dependent on the soil pH, which is described by chemical equilibrium equations.

Step 2: Detailed Explanation:
The three ionic species of orthophosphoric acid exist in a pH-dependent equilibrium as shown below:
- At highly acidic pH (< 5.0), the dominant species is undissociated phosphoric acid (\(\text{H}_3\text{PO}_4\)) and the monovalent ion \(\text{H}_2\text{PO}_4^-\).
- At slightly acidic to neutral pH (6.0 to 7.0), the dominant and most readily absorbed form is the monovalent orthophosphate ion, \(\text{H}_2\text{PO}_4^-\).
- At alkaline and alkali pH ranges (> 8.0), the monovalent ion deprotonates further, and the divalent orthophosphate ion \(\text{HPO}_4^{2-}\) (typographically written as \(\text{HPO}_4^-\) in the question options) becomes the dominant available species for plant uptake.
- At extremely high pH (> 11.0), the trivalent orthophosphate ion (\(\text{PO}_4^{3-}\)) dominates, which is highly insoluble and unavailable to plants.
Since alkali soils have a high pH (typically 8.5 to 10.0), the primary form of phosphate available for plant uptake is the divalent orthophosphate ion, represented by Option B.

Step 3: Final Answer:
The type of phosphate ion available in alkali soil is \(\text{HPO}_4^{-}\) (Option B).