Types of soil moisture constant

i. Saturation Capacity: When all the pores of the soil are completely filled with water, the soil is said to be under saturation capacity or it is the upper limit of the soil moisture capacity. In this condition, soil moisture tension is almost equal to zero (0 bar).

ii. Field Capacity: After the drainage of gravitational water i.e. after 2-3 days after the soil has been thoroughly wetted by rain water or irrigation, the moisture content at that stage is called field capacity. So, it can be considered as the maximum amount of moisture held by soil against gravity. 10 to 30 kPa (0.1 to 0.3 bar).

iii. Permanent wilting Point: It is the soil moisture content at which plant can no longer obtain enough moisture to meet the transpiration requirements and remains wilted unless water is added to the soil.

For most plants this condition develops when the soil water potential (Ψ) has a value of about -1500 kPa (-15 bar). A few plants especially xerophytes (desert type plants) can continue to remove water at even -18 bar or -20 bar.

Approximate PWP= Field capacity/factor

Factor varies from 2 to 2.4 depending upon texture/type of soil.

iv. Ultimate wilting point: It is the moisture content at which the plants are completely wilted and plants die i.e. they can’t recover from wilting even after the addition of water to the soil. At ultimate wilting point, soil moisture is held at -3100 kPa (-31 bar).

Notes;

• The water content of the soil at permanent wilting point is called wilting coefficient or permanent wilting percentage.
• Whereas the water content at ultimate wilting point is called hygroscopic coefficient.
• On very hot windy days, wilting may occur due to excessive transpiration which is called temporary wilting but it recovers soon without addition of water on cooler portion of the day or during night.