The soil provides a structural base to the plants. It allows the root zones of the plants to get firmly embeded in it. Thus the soil act as a foundation for the plants. Further, the soil stores water and nutrients required by the plants for their growth.

The soil water plant relationship include the capacities of the soils to retain water, use of this water by the plants and the movement of the excess water through the soils. The water retained by the soil is used by the plants through their roots. 

Water is required by the plants in a small quantity for the metabolism and for the transportation of the nutrients. However, much larger quantity of water is required for the physiological process of transpiration which protects the plants from the injurious effects of high temperature. Some quantity of water is also required for evaporation. Thus,

Total quantity of water required by plants

Consumtive use of water

= evaporation + transpiration

= evapotranspiration

Water is required for the life of plants and the germination of seed. Moreover, the chemical processes within the plants take place in an aqueous solution for which water is needed. Water is also required for photosynthesis process in the presence of sunlight.

The nutrients required by the plants are normally present in the soil in the form of salts. If the soil is deficient in any of those nutrients, it may be supplimented by use of fertilizers. Water in the soil forms a salt solution by dissolving the salts. The roots of the plants absorb this salt solution by the osmotic action which induces a pressure from a weaker (dilute) salt solution towards a stronger (concentrated salt) salt solution. 

Within the plants the salt solution is relatively concentrated due to the loss of water from the leaves by transpiration. It is therefore, essential that sufficient water is retained by the soil to make the salt solution dilute enough to enable the plants to readily use it. 

Because soil is capable of retaining a limited amount of water and that too goes on depleting due to evaporation and transpiration, it needs to be replenished by irrigation at some interval.

In addition to water, air must also be present in the root zone for the respiration of the micro-organisms. It also provides a favourable environment for the development of roots and the absorption of nutrients. For proper circulation of air, the soil should not remain saturated. Therefore, excess water must be drained out from the soil to enable free air circulation in the root zone.

For proper growth of the plants requisite quantities of water, air and nutrients must be available in the root zone. Therefore while practicing irrigation it must be ensured that these quantities are available in root zone in requisite proportions.

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