The total water contents present in soil is called holard. Out of this, the water which can be absorbed by plants is chresard and remaining is called echard. Chresard is also called available water or growth water. Soil water is most important than any other ecological factor in the distribution of plant communities.
Soil is the major source of water for plants. Water in the soil consists of three principal fractions:
(a) Run away water. All the rain water falling on the soil is not retained by it. Run away water does not enter the soil and gets drained away from soil surface. The quantity of run away water is controlled by factors like permeability of soil, moisture content of soil, degree of slope and number of ditches present in that area.
(b) Gravitational water. When the water enters the soil and passes the spaces between the soil particles and reaches the water table, the type of soil water is called gravitational water. In fact gravitational water is surplus to the water retaining capacity of soil and drains from it to reach in deep saturated zone of earth i.e. ground water, upper surface of which is called water table.
(c) Capillary water. In smaller pores of the soil, water is held against the force of gravity by capillary forces and is called capillary water. This form of water is most important to plants and constitutes the only available source of water to plant roots. Very thick forms of capillary water become mobile. This movement is slowed down with thinness of water film in soil particles. Capillary water is actually retained in micropores having the size of 20 mm or less in diameter. Coarse textured soils have macropores, as a result quick percolation of rain water occurs and soil becomes physically dry.
(d) Hygroscopic water. This is the form of water which is held by soil particles of soil surfaces. The water is held tightly around the soil particles due to cohesive and adhesive forces. Hygroscopic water cannot be easily removed by the plants. Cohesive and adhesive forces greatly reduce the water potential and thus this type of water in soil is not available to plants.
(e) Combined water. Some water is present in the form of hydrated oxides of aluminum, iron, silicon etc. inside the soil. This is also non-available form of water for plant roots.