Question

Olsen method is used for phosphorus estimation in :

• A. Acidic soil
• B. Neutral to alkali soil
• C. Saline soils
• D. Black soil

Hint:

Remember the soil test extractants based on pH:
- Acid soils (\(\text{pH} < 6.5\)): Bray's Method (\(0.03\text{ M } \text{NH}_4\text{F} + 0.025\text{ M } \text{HCl}\)).
- Neutral/Alkaline soils (\(\text{pH} > 7.0\)): Olsen's Method (\(0.5\text{ M } \text{NaHCO}_3\) at \(\text{pH } 8.5\)).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Soil phosphorus testing is critical for predicting the nutrient availability for crop uptake.
Different soils require specific chemical extractants depending on their pH, as phosphorus forms different compounds under acidic and alkaline conditions.

Step 2: Detailed Explanation:
In neutral, alkaline, and calcareous soils, phosphorus is primarily precipitated as calcium phosphates, which have very low solubility.
The Olsen method utilizes a sodium bicarbonate (\(0.5\text{ M } \text{NaHCO}_3\)) solution adjusted to a pH of \(8.5\) as the extracting reagent.
At this high pH, the bicarbonate ions (\(\text{HCO}_3^-\)) decrease the activity of calcium in the soil solution by precipitating calcium as calcium carbonate (\(\text{CaCO}_3\)).
This precipitation reaction shifts the equilibrium, releasing phosphate ions (\(\text{HPO}_4^{2-}\) and \(\text{H}_2\text{PO}_4^-\)) into the solution, which can then be measured colorimetrically.
In contrast, in acidic soils, phosphorus is primarily bound to iron and aluminum oxides, and acid-fluoride reagents like Bray's No. 1 or Bray's No. 2 are more appropriate.
Therefore, the Olsen method is the standard and most reliable method for estimating plant-available phosphorus in neutral to alkali soils.

Step 3: Final Answer:
The Olsen method is used for phosphorus estimation in neutral to alkali soil.