Moisture Contents of Soil

Field Capacity:

• It is the amount of water that remains in the soil after all the excess water at saturation has been drained out.
• when sandy soils are allowed to drain for approximately 24 hours after saturation, field capacity is reached.

Saturation Capacity:

• It is the soil water content where all soil pores are filled with water and water readily percolates or drains out from the root zone by gravitational force.
• The metric suction at this condition is almost zero and it is equal to the free water surface.

Permanent Wilting Point:

• Permanent wilting point is considered as the lower limit of available soil moisture.
• At this stage, water is held tightly by the soil particles that the plant roots can no longer obtain enough water to satisfy the transpiration requirements; and remain wilted unless the moisture is replenished.

Readily available moisture: 

• Readily available moisture is that portion of available moisture that can be most readily extracted by plants.

• In general, readily available moisture is approx 75 % of the available moisture.

Available moisture:

• The difference in water content of soil between field capacity and the permanent wilting point is called available moisture.

SC = Saturation capacity, FC = Field capacity, OMC = Optimum moisture content, PWP = Permanent welting point and UWP = Ultimate welting