Classification of water
- Physical classification of water
Water is present in the soil as moisture in the following forms:
a) Free water
- It is not bound to the soil but percolates it and drains under the influence of gravity.
b) Capillary water or field capacity
It is held in capillary pores by its own surface tension and adhesive properties. The field capacity of soil is the moisture content after drainage of gravitational water has become very slow and the moisture content has become relatively stable. This situation usually occurs 1-3 days after the soil has become thoroughly wetted by rain or irrigation.
c) Hygroscopic water: Water hold tightly to the surface of soil particles by adsorption forces and is not available to the plants.
d) Gravitational water: Water that moves freely in response to gravity and drains out of the soil.
The water, which is lost and goes below the ground due to the pull of gravity, is called gravitational or super flow water. This water is not available for the plants.
- Biological classification
a) Available water
The amount of water between field capacity (FC) and permanent wilting point (PWP) is considered as available water for plant use. But the availability of water to plants is a function of soil-water plant system rather than soil-water system alone.
b) Unavailable water
Water held at tensions greater than 15 bars is said to be unavailable to plants. Depending upon the reduction in availability of soil water, crop plants are subjected to water stress of varying degrees ranging from mild to severe stress.
