In soil–water–plant relations, “useful moisture” of soil is defined as which of the following within the effective root zone of plants?

Introduction / Context:
“Useful moisture” or “available water” is the portion of soil water that plants can actually extract. This concept directly affects irrigation scheduling, duty, and the design of canal capacities because only a part of the stored soil water supports transpiration without inducing physiological stress.

Given Data / Assumptions:

• Root zone depth is properly identified for the crop.
• Field capacity and permanent wilting point are measured or estimated for the soil.
• No salinity or osmotic constraints are specified.

Concept / Approach:
Field capacity is the upper limit of water retained in soil after free drainage. Permanent wilting point is the lower limit at which plants cannot extract water and do not recover overnight. The difference between these two, integrated over the effective root zone, represents the useful moisture available for plant uptake without stress.

Step-by-Step Solution:
1) Define FC = field capacity (% or depth).2) Define PWP = permanent wilting point.3) Useful moisture (available water) = FC − PWP within root zone.4) Evaluate options: only the option specifying FC − PWP in the root zone is correct.

Verification / Alternative check:
Standard agronomy and irrigation texts consistently define available water as the difference between FC and PWP; irrigation intervals are often set so soil moisture does not fall below an allowable depletion fraction of this range.

Why Other Options Are Wrong:

• Field capacity alone: includes water that may drain and ignores lower limit.
• Saturation capacity: includes gravitational water not held for plant use.
• Permanent wilting point: the lower, not the usable, limit.
• Available water above saturation capacity: not a standard term; surplus drains rapidly.

Common Pitfalls:

• Confusing total soil moisture with available water fraction.
• Ignoring root zone depth; availability depends on depth as well as on FC − PWP.

Final Answer:
Difference between field capacity and permanent wilting point within the root zone.