Definition of Available Moisture:
Available moisture refers to the amount of water in the soil that is readily available for plant uptake. It is the difference in water content between the field capacity and the permanent wilting point.

Field Capacity:
Field capacity is the maximum amount of water that a soil can retain against the force of gravity after excess water has drained away. It is the point at which the soil is saturated and the downward movement of water has slowed down. At this point, the soil is at its maximum water holding capacity.

Permanent Wilting Point:
Permanent wilting point is the moisture content at which plants are no longer able to extract water from the soil. It is the point at which the soil becomes so dry that the plant roots cannot extract water and the plants wilt permanently. The soil is at its minimum water holding capacity at this point.

Difference between Field Capacity and Permanent Wilting Point:
The difference between the field capacity and the permanent wilting point represents the available moisture in the soil. This available moisture is crucial for plant growth and survival.

Explanation:
The correct answer is option 'A' because available moisture is defined as the difference in water content of the soil between field capacity (maximum water holding capacity) and permanent wilting point (minimum water holding capacity). This difference represents the amount of water that plants can easily access and utilize for growth.

When the soil is at field capacity, it is at its maximum water holding capacity, meaning it contains the maximum amount of water that it can retain against the force of gravity. This excess water drains away, leaving behind the available moisture for plant uptake.

On the other hand, at the permanent wilting point, the soil is at its minimum water holding capacity, meaning it contains the minimum amount of water that plants require for survival. At this point, the water is so limited that plants are unable to extract it from the soil, resulting in permanent wilting.

Therefore, the difference between the field capacity and the permanent wilting point represents the available moisture in the soil, which is the water that plants can effectively extract and use for their growth and survival.

To summarize, available moisture is the difference in water content between the field capacity and the permanent wilting point. It is the water that is readily available for plant uptake and is essential for plant growth and survival.