Force Causing Water Movement:
The forces which cause the water movement in soil are:
(1) Gravitational force or gravity tension: The flow of water due to gravity is very marked when the soil is in saturated condition and generally, the direction of such flow is downward although a little lateral flow takes place. The large pores i.e. macro-pores serve as the main channels for gravitational flow.
(2) Capillary force or capillary tension: In the soil, water is held by the forces of surface in the capillary spaces and around the soil particles. The movement of water under unsaturated soil conditions is due to force of surface tension. Once the flow due to gravitational force has ceased the water moves in the form of thin or capillary film from a wet region to a dry region through finer or micro-pores. The surface or capillary tension is responsible for the capillary movement of water to all directions from low tension to high tension.
(3) vapour tension: If the soil is not water logged, the movement of water vapour may take place to a very little extent from soil layers which gets more heated towards the cooler soil layers particularly when difference between their temperatures are very wide.
(4) Osmotic pressure: The movement of water takes place due to difference in osmotic pressure of the soil solution. the situation is only observed in only saline soil which has excessive salts.
In all these four forces, the gravitational and capillary forces are important because their significance in the movement of water in the soil is more. However, vapour transfer and osmotic pressure are less important because of their negligible significance in case of normal soils.
Retention of water in soil:
Water that enters in the soil is retained by means of the following three forces:
i) Force of adhesion: It is the attraction of solid surface of water molecules (It is the attraction of unlike materials to each other). Due to the force of adhesion, the water molecules are attached to the surface of soil particles and thus a thin film of water is tightly held around the soil particles. Finer the soil particles, greater the surface area and consequently, the water film is held or retained more tightly.
ii) Force of cohesion: It is attraction between similar molecules of like characteristics. Cohesion is attraction of water molecules for each other. When more water is added to the moist soil, the cohesive force comes into action and the freshly added molecules get attracted towards already existing water molecules. This results in thickening of water film around the soil particles.
iii) Soil colloids: (Clay or humus particles): The water is also retained in the soil due to soil colloids like clay or humus particles. The water thus retained in the soil is called imbibitional moisture.
Such retention of moisture is different in different soils. Fine textured soils having greater aggregation and more organic matter or humus retain much more quantity of water than those coarse textured single grained soils which are poor in organic matter.