In higher plants, the main and only source of water is soil water. The rain water is soil water. The rain water is the only source of soil water. The soil water may be of four types.
- Gravitational water: The major part of the soil water gained from rain or irrigation goes deep in to the soil. This water is drained away by gravity. Thus plants cannot absorb this water.
- Hygroscopic water: This water remains around the soil particle as vapour form. This water is not fit for absorption.
- Capillary water: The structure and organic matter of the soil enables the soil to hold water against the force of gravity. This water is called capillary water.
- Inbibitional water: Soil panicles can hold some water due to their coloidal properties. This water is also absorbable.
Absorbable water: Out of the four types of soil water stated above, plants only can abort capillary water. Besides capillary water, plants can absorb imbibitional water in small amount.