Retention of Soil Moisture
The movement of water into and within the soil, moisture storing capacity of soils and the availability of moisture to plants are governed by soil properties. Each of these factors is related directly or indirectly to the size and distribution of soil pores and the affinity of particles to moisture. To understand these aspects more clearly, we shall now discuss a few concepts given below.
Maximum Retentive Capacity: Let us take the example of soil that is well granulated, silt loam, and has a uniform texture and structure. After heavy rain or irrigation, as waterenters the soil, air is displaced and the soil pores whether large or small are filled with water. At this point the soil is said to be saturated and is at its maximum retentive capacity.
Field Capacity: In the same soil as above, if we cut off the supply of water, that is, if there is no further rain, or we shut off imgation water, there would berelatively rapid downwardmovehent of soine of the water. After a day or so, this rapid downward movement could cease. At this point we can find that the water from the larger pores or the macropores has drained out and its place is taken by air. The smaller pores or the micropores are still filled with water, and it is the main source from where plant roots absorb water. So the field capacity refers to the water content in the soil after gravitational water has drained away.
Wilting Coefficient: Plants absorb water from the soil and reduce the quantity of moisture in the soil as they lose water to the atmosphere by transpiration through their leaves. The second avenue of loss is evaporation directly from the soil surface. Both these losses are taking place simultaneously and are responsible for a markedlv rapid rate of water dissipation from soils.