Capillary water is readily available to plant roots in soil.

Capillary water refers to the water that is held in the soil against the force of gravity and is available for plant roots to absorb. It is essential for plant growth as it provides the necessary moisture and nutrients for plants to thrive. Let's understand why capillary water is the correct answer in more detail:

1. Gravitational water:
Gravitational water is the water that drains through the soil due to the force of gravity. It moves quickly through the soil profile and is not readily available to plant roots. This water moves downwards, away from the root zone, and can lead to waterlogging if the drainage is poor.

2. Capillary water:
Capillary water is held in the soil pores by capillary forces. These forces arise from the attraction between water molecules and soil particles. Capillary water moves upwards against gravity and is available for plant roots to extract. It is held in the smaller soil pores, and the roots can access it through capillary action.

3. Hygroscopic water:
Hygroscopic water refers to the water that is held tightly by the soil particles and is not available for plant roots. It is the water that remains bound to the soil surface and is strongly attracted to the soil particles. Plants cannot extract this water as it is held too tightly.

4. Bound water:
Bound water is the water that is chemically bound to the soil particles and is not available for plant uptake. It is strongly held within the crystal lattice structure of soil minerals and cannot be accessed by plant roots.

In conclusion, capillary water is the water that is readily available to plant roots in soil. It is held in the soil pores against the force of gravity and can be accessed by plants for their growth and development. Gravitational water, hygroscopic water, and bound water are not easily available to plants and do not contribute significantly to their water requirements.