Question

Which parameter is not a soil moisture constant?

• A. Saturation capacity
• B. Field capacity
• C. Permanent wilting percentage
• D. Evapo-transpiration

Hint:

Think about which of these describe a fixed soil property versus an ongoing process.

Answer 1

The Correct Option is D

Step 1: Soil moisture constants are fixed points on the soil moisture characteristic curve that describe how much water a given soil can hold under specific conditions. Saturation capacity, field capacity and the permanent wilting percentage all fit this description, each one being a definite moisture content value for a soil.

Step 2: Saturation capacity is the moisture content when all pore spaces are filled with water. Field capacity is the moisture left after gravity drainage stops. Permanent wilting percentage is the moisture content at which plants can no longer extract water and wilt permanently. All three are properties of the soil itself.

Step 3: Evapo-transpiration is not a fixed soil property. It is a rate of water loss from soil and plant surfaces into the atmosphere, and it changes with weather, crop stage and management. It is a process, not a constant.

Step 4: Since saturation capacity, field capacity and permanent wilting percentage are all soil moisture constants while evapo-transpiration is not, option 4 is correct.