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IRRIGATION SCHEDULING
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A. REVIEW OF SOIL-WATER-PLANT RELATIONSHIPS
In the previous chapter you were introduced to some key concepts of soil-water-plant relationships.
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Soil will hold water against the pull of gravity, available for crop use.
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There are limits to this ability, field capacity (upper limit) and permanent wilting point (lower limit).
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The water-holding forces generated by the soil will increase as the soil moisture level goes down. Thus as soil moisture decreases, it becomes harder and harder for the plant to extract water from the soil at the rate it wants to.
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We are generally only interested in soil moisture in the effective root zone, which may or may not be the actual extent of active plant roots. The effective root zone is where you want to control soil moisture.
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The combination of soil evaporation and plant transpiration is called evapotranspiration, ETc.
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There are standards of measurement for soil moisture (inches of water held per foot of soil) and ETc (inches of water use per day, or per season.)
Effective Root Zone and the Soil Moisture Reservoir
The available water holding capacity of the soil measured over the depth of the effective root zone results in a soil moisture reservoir . This reservoir is available for the crop to use. It's as if the plant was pulling from a pan of water, with a specific depth of water in the pan.
Some soils, because of their higher available water-holding capacity, will provide a deeper pan than others. And, some plants will provide a deeper pan because of their deeper rooting systems.
A key idea is that we cannot let the plant use up all the water in the soil moisture reservoir. Remember that the bottom limit of available soil moisture is the permanent wilting point. If we let the plant use water up to the permanent wilting point, then it dies. If fact, past a certain level of soil moisture depletion, the plant comes under more and more stress as it becomes harder and harder for it to extract water from the soil. This stress can reduce yields and/or quality.
Last updated September 2000