Question

Soil pH is measured from the soil water suspension in the ratio of

• A. 1 : 1
• B. 1 : 2.5
• C. 1 : 5
• D. 1 : 10

Hint:

Standard Soil Suspension = 1 : 2.5.
Remember: Diluting the suspension (e.g., going from 1:1 to 1:5) usually increases the measured pH value due to the "Suspension Effect" and dilution of acids.
Always check the ratio before comparing two soil reports!

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This question asks for the standard laboratory protocol regarding the weight-to-volume ratio of soil and water used to prepare a suspension for pH measurement.
Detailed Explanation:

• Standard Methodology:
Soil pH is not measured in "solid" soil but in a solution or suspension.
In India and many international standards (like the USDA), the most widely accepted and practiced ratio for routine soil testing is 1 : 2.5.
This means $10$ grams of air-dried soil is mixed with $25$ ml of distilled water.

• Why this Ratio?
A $1 : 1$ ratio is often used for "saturated paste" extracts in salt-affected soils, but it is often too thick for regular pH electrodes.
A $1 : 5$ ratio is common in Australia but dilutes the ions too much for standard Indian soil calibrations.
The $1 : 2.5$ ratio provides a good balance, ensuring enough liquid to submerge the glass electrode while maintaining a representative concentration of hydrogen ions.

• Equilibrium:
After mixing, the suspension is usually stirred intermittently for $30$ minutes to reach equilibrium before the reading is taken.

Step 2: Final Answer:
The standard soil-to-water ratio for pH measurement is 1 : 2.5.