Forms of Soil Water
Gravitational Water or Ground Water: After a heavy rain or irrigation, much of the water drains or sinks downwards. This is called gravitational water. This is of little significance in most soils because it drains downwards rather quickly under the influence of gravity.
Capillary Water: Some amount of water is retained aganst the pull of gravity in the minute spaces between the fine soil particles as thin films surrounding the particles and thicker ones where the particles touch each other, and a part of water is absorbed by the soil colloids. The downwards pull of gravity reduces these films to a minimum thickness. This form of soil moisture is in such a state that its movement as a liquid in any direction is so limited as to be practically negligible.
Hygroscopic Water: Even air-dried soil contains appreciable amount of water, If we heat apparently dry soil in a closed container, we can see drops of water deposited on the lid. This relatively small amount of moisture is termed as hygroscopic water. It is held so tenaciously by the soil colloids that it is not available to plants.