After reading this article you will learn about the relationship between soil and moisture.
1. Field Capacity:
Assume that water is applied to the surface of a soil. With the downward movement of water all macro and micro pores are filed up. The soil is said to be saturated with respect to water and is at maximum water-holding capacity or maximum retentive capacity. It is the amount of water held in the soil when all pores are filled.
Sometime, after application of water in the soil all the gravitational water has drained away, then the wet soil is almost uniformly moist. The amount of water held by the soil at this stage is known as the field capacity or normal moisture capacity of that soil. It is the capacity of the soil to retain water against the downward pull of the force of gravity.
At this stage only micro pores or capillary pores are filled with water and plants absorb water for their use. At field capacity water is held with a force of 1/3 atmosphere. Water at field capacity is readily available to plants and microorganism (Fig. 4.5.).
2. Wilting Coefficient:
As the moisture contents falls, a point is reached when the water is so firmly held by the soil particles that plant roots are unable to draw water. The plant begins to wilt. At this stage even if the plant is kept in a saturated atmosphere it does not regain its turgidity and wilts unless water is applied to the soil.
This stage at which this occurs is termed the Wilting Point and the percentage amount of water held by the soil at this stage is known as the Wilting coefficient. It represents the point at which the soil is unable to supply water to the plant. Water at wilting coefficient is held with a force of 15 atmosphere. Between the wilting percentage (15 atmosphere) and the field capacity (1/3 atmosphere), the water in available to plants.
3. Hygroscopic Coefficient:
The hygroscopic coefficient is the maximum amount of hygroscopic water absorbed by 100 gm. of dry soil under standard conditions of humidity (50% relative humidity) and temperature (25Â°C). The tension is equal to a force of 31 atmospheres. Water at this tension is not available to plant but may be available to certain bacteria.