Page 35 - soil-plant-water relationship and water requirement

SOIL-PLANT-WATER RELATIONSHIP AND WATER REQUIREMENT

Permanent wilting point:

This is the point at which crops can no longer get enough water from the soil to meet
their evapotranspiration needs. If there isn't enough water, the plant will wilt and
eventually die. At PWP, the water tension of the soil is usually taken to be 15 bars.

For field estimation, a crop is planted and when it wilts, the moisture content is the
PWP. This technique requires personal judgment and prone to mistakes.

Two stages of wilting points are recognized and they are:
• Temporary wilting point: this denotes the soil water content at which the plant
wilts at day time, but recovers during right or when water is added to the soil.
• Ultimate wilting point: at such a soil water content, the plant wilts and fails
to regain life even after addition of water to soil.

It must be noted that the above water contents are expressed as percentage of water
held in the soil pores, compared to a fully saturated soil.

Figure 3.2-3 : Soil Water Parameters and Water Classes

Available Water (AW):

This is the water available to crops. It is the water content at field capacity minus
that at permanent wilting point.

Readily Available Water (RAW):

This is the level to which the available water in the soil can be used up without
causing stress in the crop. For most crops, 50 to 60% available water is taken as
readily available.

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