The useful moisture of soil, is equal to its A. Field capacity B. Saturation capacity C. Moisture content at permanent wilting point D. Difference between filed capacity and permanent wilting point within the root zone of plants

The correct answer is D. Difference between field capacity and permanent wilting point within the root zone of plants.

Field capacity is the amount of water that a soil can hold after it has been saturated and then allowed to drain freely for a period of time. Permanent wilting point is the amount of water that a soil can hold after it has been drained for a long period of time, and the plants have begun to wilt. The useful moisture of soil is the difference between field capacity and permanent wilting point, because this is the amount of water that is available for plants to use.

A. Field capacity is the amount of water that a soil can hold after it has been saturated and then allowed to drain freely for a period of time. This is not the useful moisture of soil, because some of this water will be held too tightly by the soil particles for plants to be able to use it.

B. Saturation capacity is the amount of water that a soil can hold when it is completely saturated. This is not the useful moisture of soil, because some of this water will be held too tightly by the soil particles for plants to be able to use it.

C. Moisture content at permanent wilting point is the amount of water that a soil can hold after it has been drained for a long period of time, and the plants have begun to wilt. This is not the useful moisture of soil, because the plants are no longer able to use this water.