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 extractants based on soil 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 to 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 exists in various organic and inorganic forms, and its availability is heavily dependent on soil pH.
Different chemical extractants are used depending on the soil reaction to estimate plant-available phosphorus.
The Olsen method is specifically developed for soils with neutral, alkaline, or calcareous properties.

Step 2: Detailed Explanation:
In neutral to alkaline soils, phosphorus is primarily precipitated as calcium phosphates, which have low solubility.
The Olsen method utilizes a $0.5\text{ M } \text{NaHCO}_3$ solution adjusted to a pH of $8.5$ as the extracting reagent.
The bicarbonate ion ($\text{HCO}_3^{-}$) in the extractant decreases the activity of calcium in the soil solution by precipitating calcium as calcium carbonate ($\text{CaCO}_3$).
This precipitation shifts the equilibrium and releases phosphate ions ($\text{HPO}_4^{2-}$ and $\text{H}_2\text{PO}_4^{-}$) into the solution, which can then be measured colorimetrically.
In contrast, acidic soils are extracted using acid-fluoride reagents like Bray's No. 1 or Bray's No. 2 method, as phosphorus in acid soils is primarily bound to iron and aluminum oxides.
Therefore, the Olsen method is highly effective and widely adopted for neutral to alkali soils.

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