It is evident that all the water present in the soil profile is not available for the use of plants. Even the capillary water which is considered to be loosely held by the soil particles is not utilized by plants.
Three tentative divisions of the soil water may be made on the basis of availability
i) Unavailable water
ii) Desirably available water
iii) Superfluous or excess of water not needed by plants.
Available and unavailable water
|Type of water||Atmospheric Pressure||Status|
Unavailable soil water:
Types of water are not available to the plants are
a) Hygroscopic water
b) Fraction of inner capillary
c) Water vapour
Water below the hygroscopic co-efficient is held so tenaciously above 31 atmosphere that is unavailable to plants. The water held between the hygroscopic co- efficient (31 atmosphere) and the wilting point (15 atmospheres) is inner capillary water. Its movement is extremely sluggish and is only difficultly available to plants. Only certain type of plants under arid conditions make its use. So also some bacteria and fungi use the inner capillary water. It includes whole of the hygroscopic water plus a part of inner capillary water being below the wilting point.
Available or Desirably available water:
The range of water between the limits of field capacity and wilting point (co- efficient) is considered as the desirably or available water. The soil moisture between field capacity (1/3 atmosphere) and wilting point (15 atmosphere) is readily available moisture.
It includes gravitational water (excess of field capacity). This water is also unavailable to the use of plants because it is lost due to deep percolation. The preference of superfluous water in soil for longer period is harmful to plant growth.