Classification of Soil Water
Although the soil water can not be demarcated, yet for the sake of under standing, and as per utility of that water to plants, it is mainly classified into three categories:
a. Hygroscopic water
b. Capillary water
c. Gravitational water
a. Hygroscopic Water:
It is the part of soil water which is very tightly held on the surface of soil particle invery thin film by the forces of adhesion and cohesion. It is mostly in vapour from and the force with which it is held on the surface of soil particle is estimated at about 10,000 atmosphere towards the inner side and about 31 atmosphere at the outer side.
Hygroscopic water is not available to the plant and agriculturally it has no importance.
b. Capillary Water:
The capillary water is that water which held in the soil in excess of hygroscopic water, but is upto the point where the gravity pull begins to move the water downwards when fee drainage conditions exist in the soil.
Capillary water is rather lossely held water (from 31 atm, to 1/3 atm tension) and it moves more slowly than free water; it can move in any direction, but always in the direction greatest tension.
A soil, which has a finer texture and granular indicating larger proportion of micro pores than macro pores, holds more amount of capillary water than course textured single grained sandy soil having more percentage of macro pores. Similarly, soils rich in organic matter content also hold a much greater quantity of capillary water.
c) Gravitational Water:
When all the pores, large and small are filled, the soil is to be saturated that is at its maximum retentive capacity.
Within 2 to 3 days after heavy irrigation, the gravitational water or free water gets drained away and the moisture content at this stage in the surface soil is said to be at field capacity the moisture content at this stage is about one third atmosphere.