Water has perhaps the greatest influence on the growth and yield of a crop. It is needed in much larger quantity than that of any other substance that contributes to growth or yield. Water serves the following functions in relation to plant life.

(i)It is an essential part of plant food. it constitutes nearly 90% of plant tissues.
(ii)It serves as a solvent and carrier of plant nutrients.
(iii)It maintains cell turgidity and regulates temperature.

Water is held in soil in the following forms:
1.HYGROSCOPIC WATER. It occurs as a thin film (4-5 millimicron) and is held tenaciously with a tension of 31 atmospheres or more. It is not available to plants.
2.CAPILLARY WATER. It forms a continuous film around soil particles(outside the film of hygroscopic water) and in the micropore spaces. It is held by surface tension. Capillary water is held at a twnsion ranging from 1/3 to 31 atmospheres.
3.GRAVITATONAL WATER. It is free water held at a tension below 1/3 atmospheres. It saturates the soil and percolates downwards under the influence of gravity.

The maximum amount of capillary water remaining in the soil after the removal of gravitational water is called its field capacity. In most cases it represents the water held at a tension of 1/3 atmospheres. It is generally recognised that capillary wwater held a tension greater than 15 atm is not available to plants. At this point of soil moisture, the plant wilts permanently and hence the percentage moisture at 15 atmospheres is called its wilting point or wilting percentage. It follows then that the moisture held at tensions between 1/3 atm (field capacity) to 15 atm (wilting point) is the water available to plants to the maximum extent. Thus capillary water is the major source of water used by plants.

Capillary water is capable of movment upwards, downwards or laterally, the movment taking place from the thicker part of the film to the thinner part. Similiarly, as the capillary-water zone moves farther from the watertable below the rate of movment becomes less and the suction needed to draw up water increases. Hence in general, a water table lying more than 2-3 metres below the root zone is not of much use to crops.

Crops differ greatly in their water requirement according to growth characteristics, climate and water-supply. Water requirement of a crop includes the evapo-transpiration needs, the water needed for metabolic activities for leaching and other unavoidable losses. It is the water needed for raising a crop in a given period and is expressed as depth of water in millimetres. Generally the water requirement of a crop is related to the potential evaporation during the growth of the crop.

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