Question

Type of soil water available for normal crop growth

• A. Hygroscopic
• B. Gravitational
• C. Capillary
• D. Hygroscopic and Gravitational

Hint:

Ask which form of soil water is neither drained away by gravity nor held too tightly for roots to pull.

Answer 1

The Correct Option is C

Soil holds water in three broad forms based on how tightly it is held, and this question asks which of those forms a growing crop can actually take up and use.

1. Hygroscopic water: this is the thin film of water held very tightly around soil particles by strong surface forces. It resists a pull of many atmospheres, so plant roots cannot pull it away from the particle, and it stays unavailable to the crop.
2. Gravitational water: this is the extra water that fills large pores after rain or irrigation and drains downward under gravity within a day or two. It moves out of the root zone too fast for the crop to use most of it, and it can even push air out of the root zone if it lingers.
3. Capillary water: this is the water held in the fine pores between soil particles by capillary forces, against the pull of gravity but weak enough for roots to draw out. This is the main store of water that meets a crop's day to day needs and it is treated as the plant-available water in soil.
4. Hygroscopic and Gravitational: combining these two does not create availability, because one is held too tightly for roots and the other drains away too fast, so this option is wrong for the same reasons given above.

Because capillary water sits in the moisture range between the tightly held hygroscopic water and the freely draining gravitational water, it is the form that stays in the root zone long enough, and loosely enough held, for roots to absorb it.

Let's summarize:

• Hygroscopic water is too tightly bound for roots to use.
• Gravitational water drains away before it can be fully used.
• Capillary water is held loosely enough and long enough to support normal crop growth.

The correct answer is capillary water, option (3).