Question

Phosphorous is taken up by the plant from the soil in the form of

• A. $H_2PO_4^-$ only
• B. $HPO_4^{2-}$ only
• C. $PO_4^{3-}$ only
• D. Both $H_2PO_4^-$ & $HPO_4^{2-}$

Hint:

Think of Phosphorus uptake as a "pH Spectrum."
Acidic Soil ($pH < 7$) $\rightarrow$ $H_2PO_4^-$.
Alkaline Soil ($pH > 7$) $\rightarrow$ $HPO_4^{2-}$.
The most available "sweet spot" for Phosphorus is at $pH 6.5$.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
This question focuses on the plant-available forms of phosphorus in the soil solution and how they vary depending on the chemical environment.
Detailed Explanation:

• Phosphate Ions in Soil Solution:
Phosphorus exists in soil water as orthophosphate ions. The specific form depends strictly on the soil pH.
At low pH (acidic, pH $< 7.2$), the primary form is the monovalent dihydrogen phosphate ion ($H_2PO_4^-$).
At high pH (alkaline, pH $> 7.2$), the primary form is the divalent monohydrogen phosphate ion ($HPO_4^{2-}$).

• Plant Uptake:
Plants are capable of absorbing both $H_2PO_4^-$ and $HPO_4^{2-}$.
However, $H_2PO_4^-$ is generally absorbed more rapidly than $HPO_4^{2-}$ because of its smaller charge and higher mobility across root membranes.
At the neutral point (pH $7.0$), both ions are present in roughly equal concentrations.

• Highly Alkaline Conditions:
In very alkaline soils (pH $> 10$), $PO_4^{3-}$ might exist, but this form is generally not preferred by plants and is usually precipitated with calcium.

• Conclusion:
Since soil pH can range from acidic to alkaline, plants have evolved to utilize both major orthophosphate forms.

Step 2: Final Answer:
Plants absorb phosphorus in the forms of $H_2PO_4^-$ and $HPO_4^{2-}$ depending on the soil pH.