Soil Moisture Constants
Plants require adequate moisture constantly within the root zone from germination to maturity. A proper moisture balance has to be maintained for unretarded growth and development of plants at various stages. Hence, certain soil moisture contents are of particular significance in agriculture and are often called 'soil moisture constants'.
These soil moisture constants are explained below.
Saturation capacity: When all the pores of the soil are filled with water, the soil is said to be under saturation capacity or maximum water holding capacity. The tension of water at saturation capacity is almost zero.
Field capacity (FC): The field capacity of the soil is the moisture content after all the gravitational water is drained and the moisture content has become relatively stable. This situation usually exists two to three days after the soil has been thoroughly wetted by rainfall or irrigation. The field capacity is the upper limit of available moisture range in soil moisture and plant relations. The soil moisture tension at field capacity varies from soil to soil, but usually it is considered around 1/3 atmosphere.
Permanent wilting point (PWP): It is the soil moisture content at which plants can no longer obtain enough moisture to meet transpiration requirements, remain wilted and their growth ceases. At the permanent wilting point the films of water around the soil particles are held so tightly that roots in contact with the soil cannot remove water at a sufficient rate to meet the demand. Hence, the plant is wilted. The permanent wilting point is the lower limit of available moisture range in soil moisture and plant relations. The soil moisture tension at permanent wilting point is about 15 atmospheres.
Available water: Soil moisture between field capacity and permanent-wilting point is referred to as readily available water. It is, the moisture available for plant use. It is generally reported that the water depletion within this broadly accepted available range need not be beyond 50% for most of the crops for obtaining water at fairly sufficient rate. In general, fine-textured soils have a wide range of water between field capacity and permanent wilting point than coarse textured soils.
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