The following points highlight the four types of water available in the soil.
They are: (1) Gravitational Water or Ground Water (2) Capillary Water (3) Hygroscopic Water and (4) Chemically Combined Water.
Type # 1. Gravitational Water or Ground Water:
After a heavy rain or on application of irrigation water, the surface layer of a soil is temporarily saturated. This water obeys the laws of gravity and thus descends rapidly through the dry layers, leaving a moist zone in its path.
If there is sufficient water this wet layer penetrates deeply. The rate of this downward percolation of this gravitational water is determined by the number, size and continuity of non-capillary or larger pores.
The gravitational water is available to plants only when rain showers follow one another in rapid succession; otherwise it percolates below the reach of the roots within a few days and thus remains unavailable.
Type # 2. Capillary Water:
As ground water drains out of the upper layer of the soil, it leaves behind considerable moisture in the form of films, coating smaller soil particles and as droplets suspended in the angles of larger pores or completely filling the smaller pores.
This water is retained by the forces of surface tension and this capillary water does not respond to the gravitational pull. The forces are, however, small—seldom more than a fraction of an atmosphere depending on the diameter of the capillaries.
This is capillary water. It is primarily this capillary water which is readily available to the plant and this is the source of practically all the water a plant extracts from the soil.
Type # 3. Hygroscopic Water:
Owing to evaporation from the soil surface and absorption by roots, the capillary water held by the soil is gradually depleted. As depletion progresses, the forces of molecular attraction or adsorption between the soil particles and the thin films of water held against their surfaces increase until finally the remaining water passes into a state where it is no longer in the liquid condition and thus ceases- to be chemically or biologically active.
The forces of molecular attraction increase rapidly as the water film surrounding soil particle grows thinner until a point is reached when the films are only a few molecules thick and the forces of attraction may reach values as high as 1000 atm.
Evidently plants can absorb only a relatively small amount of this hygroscopic water. The hygroscopic water cannot be entirely evaporated from a soil under ordinary atmospheric conditions, but it can be done by heating soil to a constant weight in an oven at approximately 150°C.
The above three types of soil are not sharply defined but form a continuous series from water which is not retained by the soil, to water which is held with great force.
Type # 4. Chemically Combined Water:
The water chemically combined in the structure of soil minerals is known as combined water. After the elimination of hygroscopic water by heating soil to about 150°C., the only water that remains is in the hydrated oxides of aluminium, iron, silicon, etc.
This water is absolutely unavailable to the plants and can only be driven off from the soil by resorting to very high temperature but not before bringing about irreversible changes in the physical and chemical composition of the soil itself.