WATER RELATIONS OF SOIL
The mineral and organic compounds of soil from a solid (though not rigid) matrix,
the interstices of which consists of irregularly shaped pores with a geometry defined by
the boundaries of the matrix .
The pore space, in general, is filled partly with soil air and liquid vapour and partly with the liquid phase of soil water. Soil moisture is,one of the most important ingredients of the soil. It is also one of its most dynamic properties. Water affects intensely many physical and chemical reactions of the soil as well as plant growth.
The properties of water can be explained by the structure of its molecule. Two
atoms of hydrogen and one atom of oxygen combine to form a mole largely determined
by that of the oxygen ion. The two hydrogen ions take up practically no space. Water
molecules do not exist individually. The hydrogen in the water serves as a connecting
link from one molecule to the other.
Soil serves as the storage reservoir for water. Only the water stored in the root
zone of a crop can be utilized by it for its transpiration and buildup of plant tissues.
When ample water is in the root zone, plants can obtain their daily water requirements for
proper growth and development. As the plants continue to use water, the available supply
diminishes, and unless more water is added, the plants stop growing and finally die.
Before the stage is reached when crop growth is adversely affected, it is necessary to irrigate again. The amount of water to be applied to each irrigation, and the frequency of
irrigation are dependent on the properties of the soil and the crop to be irrigated.
Kinds of soil water: When is added to a dry soil either by rain or irrigation, it is
distributed around the soil particles where it is held by adhesive and cohesive forces; it
displaces air in the pore spaces and eventually fills the pores. When all the pores, large
and small, are filled, the soil is said to be saturated and is at its maximum retentive
capacity. The following re the three main classes of soil water:
(i) Hygroscopic water. Water held tightly to the surface of soil particles by
(ii) Capillary water. Water held by forces of surface tension and continuous films
around soil particles and in the capillary spaces.
(iii) Gravitational water. Water that moves freely in response to gravity and drains
out of the soil.
Adhesion is the attraction of solid surfaces for water molecules. Adhesion is
operative only at the solid-liquid interface and hence the film of water established by it is
very thin. Cohesion is the attraction of water molecules for each other. This force makes possible a marked thickness of the films of water established by hydration until they attain microscopic size. As the film gets thicker and thicker the forces of gravity act and water flows downward through the large pores.
Such water is loosely held. Thus, when a soil is near saturation it is easy to remove an increment of water, but as moisture becomes less and less in the soil, the greater will be the force required to remove a unit amount of moisture.
When a dry soil samples is exposed to water vapour, it will take up moisture. The amount adsorbed depends on the nature and magnitude of the surface exposed the temperature and the degree of humidity. The moisture thus adsorbed is the water of hydration, water of adhesion, or commonly the hygroscopic water. When the air saturation is 100 per cent the maximum amount of such moisture will be acquired.
The capillary water is held between tensions of about 31 atmospheres and onethird atmosphere. Between 31 and 15 atmospheres, capillary adjustment is very sluggish.
Comparatively easy movement does not occur until the water film thickens and pressures near one-third atmosphere are reached. As a result of its energy relations, the capillary water is the only fluid water bearing solutes, that remains in the soil for any length of time, if drainage is satisfactory.
Thus, it functions physically and chemically as the soil solution. The principal factors influencing the amount of capillary water in soils are the structure, texture and organic matter. The finger the texture of the mineral soil particles, the greater is likely its capillary capacity. Granular soil structure produces higher capillary capacity. Presence of organic matter increases the capillary capacity.
Water held in the soil at tensions of one-third atmosphere or less will respond to gravity and move downward, hence the name gravitational water. The water thus affected is that present in the non-capillary (large) pores. Of the three forms of water, only capillary and gravitational water are of interest to the irrigationists since hygroscopic water is not available to plants.
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