Question

Given below are two statements:
Statement (I): In strongly acidic soils, \(\text{H}_2\text{PO}_4^-\) form dominates, while in alkaline soils, P is largely present as \(\text{HPO}_4^{2-}\) form.
Statement (II): Plant takes both the form of \(\text{H}_2\text{PO}_4^-\) and \(\text{HPO}_4^{2-}\) in equal amounts for their growth and development.
In light of the above statements, choose the most appropriate answer from the options given below:

• A. Both Statement (I) and Statement (II) are correct.
• B. Both Statement (I) and Statement (II) are incorrect.
• C. Statement (I) is correct but Statement (II) is incorrect.
• D. Statement (I) is incorrect but Statement (II) is correct.

Hint:

Phosphorus availability is highest in the neutral pH range of 6.5 to 7.5.
In this range, both orthophosphate forms are present, and fixation by iron/aluminum (in acid soils) or calcium (in alkaline soils) is minimized.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Phosphorus (P) is an essential macronutrient absorbed by plant roots from the soil solution.
The ionic form of orthophosphate present in the soil solution depends heavily on the soil pH.

Step 2: Detailed Explanation:
Let us evaluate Statement (I):
Orthophosphate exists in soil solution in three main pH-dependent ionic forms: \( \text{H}_2\text{PO}_4^- \), \( \text{HPO}_4^{2-} \), and \( \text{PO}_4^{3-} \).
The chemical equilibrium is governed by the soil pH:
- At acidic pH values (below 7.2), the primary form is the monovalent orthophosphate ion, \( \text{H}_2\text{PO}_4^- \).
- At alkaline pH values (above 7.2), the divalent orthophosphate ion, \( \text{HPO}_4^{2-} \), dominates.
Thus, Statement (I) is correct.
Let us evaluate Statement (II):
Although plants can absorb both \( \text{H}_2\text{PO}_4^- \) and \( \text{HPO}_4^{2-} \), they do not absorb them in equal amounts.
The monovalent form, \( \text{H}_2\text{PO}_4^- \), is absorbed much faster (up to 10 times more rapidly) than the divalent form, \( \text{HPO}_4^{2-} \), due to the lower negative charge density, which allows it to pass through root cell membranes more easily.
Therefore, plants do not absorb both forms in equal amounts.
Thus, Statement (II) is incorrect.

Step 2: Final Answer:
Statement (I) is correct, but Statement (II) is incorrect.