When water is added to dry soil either by rain or irrigation, it is distributed around the soil particles, where it is held by adhesion and cohesive forces. It displaces air in the pore spaces and eventually fills the pores. When all the pores, large and small are filled, soil is said to be saturated and it is at its maximum retentive capacity.
Although the soil water cannot be sharply demarcated, yet for sake of understanding and as per utility of water to plant it is mainly classified into following categories.
- Hygroscopic water
- Capillary water
- Inner capillary
- outer capillary
- Gravitation water
- Water vapour
(1) HYGROSCOPIC WATER:-
It is that part of soil water which is very tightly held on the surface of soil particles in very thin film by adsorption forces such as adhesion and cohesion. It is mostly in vapour form and forces with which it is held on surface of soil particles is estimated about 10,000 atmosphere towards the inner side and about 31 atmosphere at the outer side of hygroscopic water film. (One atmosphere at sea level is about 15 pounds per square inch, which means the force holding the water at one atmosphere is equal to about 15 pounds pre square inch or 1023 centimeters of water column height). This water is not any use to the plants.
(2) CAPILLARY WATER:
It is the water held by the forces of surface tension and continuous film around soil particles and in the capillary spaces. When soil particles absorb water even after the hygroscopic coefficient is reached, additional water is also held around the particles in the form thin film. This retension of water film continues until the film becomes quite thick and micro pores inside the soil mass get filled with water. A stage is then reached when the force of gravity becomes stronger and any further addition of water is pulled down by gravity and flows down as free water. The capillary water is that water, which is held in the soil in excess of hygroscopic water but is up to the point where the gravity pull begins to move the water down wards, when free drainage conditions exist in the soil. Capillary water is rather loosely held water (from 31 atmosphere to 1/3 atmosphere tension) and is capable of movement within the soil. The plant food nutrients are dissolved in it and therefore, it is most useful water for plants. The capillary water itself can be divided into two parts though there is no clear-cut line division.
- INNER CAPILLARY WATER:
It is that part of capillary water, which is nearest to the hygroscopic water and is in the form of a thinner film, held more tightly and moves rather very slowly than outer capillary water.
- OUTER CAPILLARY WATER:
It is that part of capillary water which is not very tightly held in the soil and there after moves readily from place to place. It is the most useful water for plants as it is very quick available. A soil which has a finer texture and granular structure indicating larger proportion of micro pores than macro pores holds more amount of capillary water than a single grained sandy soil having more percentage of macro pores. Soil rich organic matter content also holds much grater quantity of capillary water.
(3) GRAVITATIONAL WATER:
It is that part of soil water, which moves freely in response to gravity and drains out of the soil. When the maximum capillary capacity of a soil gets satisfied and further addition of water comes under the force of gravity. This water starts moving as free water through the macropores and it is called gravitational water. It is superfluous and as such, it is of no use to the plants. Gravitational water is held at zero atmosphere tension. When down ward movement of gravitational water is more, some plant nutrients are leached out and when it is slow, it will adversely affect the aeration of soil.
(4) VAPOUR FORM:
In this category, the water is present in gaseous form in the soil atmosphere but it is not directly used by plants and is therefore, not important unlike the first three kinds.