Most water is consumed by transpiration. In addition, water from precipitation or irrigation can be lost by surface run-off, drainage and evaporation from the soil. Efficient use of water determines crop yield. Water demand for crop yield varies depending on the crop and the growing conditions.
How can plant nutrition contribute to make best use of available water for food production?
Table 1 summarizes the water demand for key crops under good and bad conditions. In this context, "good conditions" means sufficient nutrient supply and efficient use of water, whereas "bad conditions" stands for nutrient insufficiency, high evaporation losses, high temperature and low air humidity.
|Water Consumption (mm/T harvest *ha)
|Table 1: Water consumption per ton of harvested product varies according to meteorological and nutritive conditions. Under good conditions, significantly less water is needed to grow the same amount of crop than under bad conditions.
Water leaves the field by four main pathways: transpiration, evaporation, run-off and drainage. Contour cropping and zero-tillage are best agricultural practice to improve water retention. Under good farming practice, evaporation and transpiration are the main causes for water consumption.
Evaporation from the soil is an unproductive loss. Reducing evaporation losses by a well-developed crop canopy makes more water available for biomass growth.
Transpiration is necessary for plant growth. As the plant takesup carbon dioxide (CO2) for photosynthesis through the respiration openings in the leaves (stomata), water is transpiredand leaves the crop tissue. As soon as water is in short supply, stomata close and biomass production declines because of a lack of CO2 inside the plant tissue. Transpiration is also essential to cool the crop tissue and to prevent heat stress. It triggers the uptake and flow of water in the plant and thereby the uptake and movement of nutrients.
The objective of good crop management is to get as much CO2 as possible into the plant tissue per unit of water transpired. The water demand per unit of yield depends on whether crops are grown under good or unfavorable conditions like heat, low humidity or lack of nutrients.