In irrigation agronomy, which form of soil water is actually available for plant uptake during the crop growth period? Choose the most useful moisture form for plant growth.

Introduction / Context:
Understanding soil–water states is fundamental in irrigation engineering and agronomy. Plants can only use water that is held in the soil at tensions their roots can overcome. This question distinguishes between capillary water, gravity water, hygroscopic water, and chemically bound water, and asks which one is truly useful for plant growth.

Given Data / Assumptions:

• Soil contains water in multiple forms depending on pore size and suction.
• Root systems can exert limited suction to extract water.
• Field conditions: after rainfall or irrigation, soil water redistributes by gravity and capillarity.

Concept / Approach:

Water in soils exists as: (1) gravity water that drains rapidly from macropores; (2) capillary water held in mesopores by surface tension and capillary forces; (3) hygroscopic water adsorbed as very thin films on particle surfaces; and (4) chemically combined water inside minerals. Plant roots mainly access water stored at suctions between field capacity and permanent wilting point, which corresponds to capillary water.

Step-by-Step Solution:

Verification / Alternative check:

Crop water requirement models and soil–water characteristic curves consistently show that available water capacity is linked to capillary storage rather than gravitational drainage or hygroscopic adsorption.

Why Other Options Are Wrong:

Gravity water: drains rapidly, low residence time in root zone; not reliably available.
Hygroscopic water: held at very high suction; roots cannot extract it.
Chemical water: part of mineral structure; biologically unavailable.
All equally: incorrect because availability differs greatly by form.

Common Pitfalls:

Confusing temporary presence of gravity water after irrigation with plant-available storage; ignoring suction thresholds for root extraction.

Final Answer:

Capillary water