Question

Optimum temperature and relative humidity for microbial degradation of herbicides in soil is

• A. 25-35 \(^\circ\)C and 50-100%
• B. 20-25 \(^\circ\)C and 50-75%
• C. 25-30 \(^\circ\)C and 75-100%
• D. 25-35 \(^\circ\)C and 75-90%

Hint:

Herbicide persistence is longest in cold, dry soils because microbial metabolic rates are extremely depressed under those conditions.
Conversely, warm, moist soils promote rapid degradation.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Microbial degradation is the primary pathway for the breakdown and detoxification of most herbicides in agricultural soils.
Environmental factors that favor general microbial activity also accelerate herbicide degradation.

Step 3: Detailed Explanation:
Soil microorganisms (primarily bacteria and fungi) thrive under warm and moist conditions:
- Temperature: The optimum temperature range for mesophilic soil microbes is 25--35 \(^\circ\)C. Below 10 \(^\circ\)C, activity slows drastically; above 45 \(^\circ\)C, enzyme systems begin to denature.
- Moisture (Relative Humidity/Field Capacity): Soil moisture levels between 50% and 100% of field capacity (high relative humidity in soil pores) provide the ideal balance of water and oxygen for aerobic microbial respiration.

Step 4: Final Answer:
The optimum conditions are 25-35 \(^\circ\)C and 50-100% moisture/humidity, corresponding to option (A).