Soil water is of paramount importance in the physiology of plants. It occurs in various forms, such as gravitational, capillary, hygroscopic and combined water. Rain is the principal source of water for the soil.
Water which flows down due to the force of gravity is known as gravitational water. The gravitational water is not available to the plants.
However, it is a big soil water reservoir and is trapped out through tube wells. A certain amount of rain water is retained within the intercellular spaces of the soil particles in the form of a capillary network.
It is called capillary water and is used by the plants. Some water molecules form a thin sheet of water around soil particles. It is called hygroscopic water (water of imbibition). The hygroscopic water is also not absorbed by the plants.
The water, which is bound up in chemicals is called combined water or crystalline water. (e.g. MgSo4.7H2O). It is not available to plants. The total water present in the soil is called as field capacity.
Addition of water beyond field capacity causes water logging. It excludes soil air and thus inhibits plant growth. The soils that have poor water holding capacity, cannot afford luxuriant vegetation. In such soils, the plants generally show wilting of their leaves.