Question

Soil extractants used for available nutrients:
(A) 2 M KCl extract is used for determination of mineral N (NH₄ and NO₃) using soil:solution ratio of 1:10.
(B) DTPA extractant (pH 7.5) is used for determination of micronutrients using soil:solution ratio of 1:20.
(C) Ammonium acetate (1 N) solution is used for determination of potassium using soil:solution ratio of 1:5.
(D) Olsen reagent (0.5 M NaHCO₃, pH 8.5) is used for determination of available P in soil using soil:solution ratio of 1:20.
Choose the correct answer from the options given below:

• A. (A), (B) and (C) only
• B. (A), (B) and (D) only
• C. (B), (C) and (D) only
• D. (A), (C) and (D) only

Hint:

Check the DTPA method's pH and soil:solution ratio carefully against the other three statements.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
We need to check four statements about the standard chemical extractants used to test soils for available nitrogen, micronutrients, potassium and phosphorus, and pick the set that is fully correct.

Step 2: Key Formula or Approach:
Each nutrient test method has a fixed reagent, a fixed strength and pH, and a fixed soil to solution ratio, laid down so results from different laboratories can be compared. Getting any one of these details wrong makes the statement false.

Step 3: Detailed Explanation:
Statement (A): Mineral nitrogen (ammonium and nitrate) is taken out of soil using 2 M KCl solution at a soil to solution ratio of 1:10. This matches the standard method, so (A) is correct.
Statement (B): The DTPA method for micronutrients like zinc, iron, copper and manganese is done at pH 7.3, not pH 7.5, and the soil to solution ratio used is 1:2 (10 g soil shaken with 20 mL DTPA solution), not 1:20. Since both the pH and the ratio given here are wrong, statement (B) is false.
Statement (C): Available potassium is taken out using neutral normal ammonium acetate (1 N, pH 7) at a soil to solution ratio of 1:5. This is the standard method, so (C) is correct.
Statement (D): Available phosphorus in normal to alkaline soils is taken out using Olsen's reagent, 0.5 M sodium bicarbonate at pH 8.5, at a soil to solution ratio of 1:20. This matches the standard method, so (D) is correct.
Since (A), (C) and (D) are correct and (B) is wrong, the right combination is (A), (C) and (D) only.

Step 4: Final Answer:
The correct set of statements is (A), (C) and (D) only, so option 4 is correct.