Soil–water relations: Field capacity of a soil primarily depends on which factors?

Introduction / Context:
Field capacity is the moisture content held in the soil after excess gravity water has drained away and downward movement becomes negligible. It is central to irrigation scheduling and estimating plant-available water.

Given Data / Assumptions:

• Soils with different textures exhibit different pore-size distributions.
• Surface tension and capillary forces govern water retention post-drainage.

Concept / Approach:
Field capacity is controlled by the soil’s ability to hold water against gravity—largely a function of pore geometry (porosity and pore-size distribution) and capillary tension. Finer soils (clays) retain more water at higher tensions than sands because of smaller pores and larger surface area.

Step-by-Step Solution:
Identify controlling factors: capillary tension and porosity/pore sizes.Relate finer textures to higher field capacity; coarser textures to lower field capacity.Select “Both (a) and (b).”

Verification / Alternative check:
Soil water retention curves directly show dependence on capillary pressure and pore structure, confirming both factors as primary controls.

Why Other Options Are Wrong:

• (a) or (b) alone: incomplete—both act together.
• (d) ignores well-known physics of unsaturated flow.
• (e) Soil color is not a primary determinant.

Common Pitfalls:
Equating total porosity with available water (pore-size distribution matters); neglecting structure and organic matter.

Final Answer:
Both (a) and (b)