Previously, soil water was classified as gravitational, capillary, and hygroscopic water. Under specific circumstances, the hygroscopic and capillary fluids are in balance with the soil. When the soil has reached its maximum concentration of hygroscopic and capillary fluids, respectively, the hygroscopic coefficient and the maximum capillary capacity are the two equilibrium points. The soil moisture constant is the quantity of water that a soil has at each of these equilibrium points.
Therefore, the soil moisture constant reflects a clear link between soil moisture and the retention of soil moisture in the field. The interactions between soil and water that exist under field settings are not adequately represented by the three kinds of water (gravitational, capillary, and hygroscopic), which are quite wide.
Although a soil’s maximal capillary capacity indicates how much capillary water it can contain, plants cannot consume the whole capillary water supply. Because it approaches the hygroscopic coefficient at its lowest point, the plants are unable to use any of it. Similar to this, a portion of the water in the capillaries at their maximum limit is similarly unavailable to plants. In order to explain the soil-plant-water interactions as they were discovered to occur under field settings, two additional soil constants—field capacity and wilting coefficient—have been proposed.
1. Field capacity: It is the soil’s ability to hold onto water in the face of gravity’s pull downward. The only water-filled holes at this point are capillary pores or micropores, which plants employ to absorb water. Water is held with a force of 1/3 atmosphere at field capacity. Plants and microorganisms have easy access to water at field capacity.
2. Wilting coefficient: The Wilting Point is the point at which plants begin to wilt due to a lack of water, and the Wilting Coefficient is the percentage of water retained by the soil at this point. It symbolizes the point at which the plant can no longer get water from the soil. A force of 15 atmospheres is used to hold water at its wilting coefficient.
3. Hygroscopic coefficient: The greatest quantity of hygroscopic water that 100 g of dry soil can absorb under typical humidity circumstances (50% relative humidity) and temperature (15°C) is known as the hygroscopic coefficient. This stress is equivalent to 31 atmospheres of force. Plants cannot access water at this pressure, although microbes may.
4. Available water capacity: The “available” water is the volume of water that must be applied to a soil at the point of wilting in order to fill the field to capacity. The capacity of a soil to store water is connected to its ability to deliver water. The difference between the quantity of water at Field Capacity (0.3 bar) and the amount of water at the Permanent Wilting Point indicates the amount of water that is now accessible (15 bars).
5. Maximum water holding capacity: It is sometimes referred to as maximal retention power. When a soil’s pores, including its micro- and macro-capillaries, are entirely filled with water, this is how much moisture is present in the soil. It is an approximate estimate of the total soil pore space. In the range of 1/100th to 1/1000th of an atmosphere, or pF 1 to 0, the soil moisture tension is very low.
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