Question

Organic mulches are useful to

• A. Increase evaporation
• B. Decrease evaporation
• C. Increase evapotranspiration
• D. Increase potential evapotranspiration

Hint:

Mulch = Blanket for the soil.
Just as a blanket keeps you from losing heat, mulch keeps the soil from losing water.
Always associate mulching with "Decrease" in evaporation.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the primary environmental or physical effect of applying organic mulches (like leaves, bark, or straw) to soil.
Detailed Explanation:

• Function of Organic Mulch:
Organic mulches act as a physical insulator. They block direct sunlight from reaching the soil surface.
By doing so, they keep the soil temperature lower during the day, which reduces the energy available for water molecules to escape as vapor.

• Mechanism of Evaporation Control:
Evaporation occurs at the interface of soil and air.
The mulch layer breaks the wind speed directly at the soil surface and increases the humidity in the tiny air pockets between the mulch and the soil.
This creates a "boundary layer" that significantly slows down the rate of evaporation.

• Biological Benefits:
As organic mulches decompose, they add humus to the soil, improving soil structure and water-holding capacity.
This further aids in moisture retention, although the immediate effect on the surface is the reduction of evaporation.

• Analyzing Other Options:
Increase evaporation: This would be detrimental and is the opposite of why mulches are used.
Increase evapotranspiration: Evapotranspiration is the sum of soil evaporation and plant transpiration. Since mulch reduces evaporation, it decreases total ET.
Potential ET: This is a climatic demand factor and is not directly altered by soil-level mulching.

Step 2: Final Answer:
The core objective of using organic mulch is to create a barrier that prevents soil water from being lost to the atmosphere, thus decreasing evaporation.