1. P.A.S.S. Pushpakumara | Advanced Technological Institute- Gampaha
AG 2105 Soil and Water Management
Practical No. 09
Calculation of available soil moisture in different soils by using FC and PWP
Terminology
• Available soil moisture - the difference between the amount of water in the soil at field
capacity and the amount at the permanent wilting point.
• Saturation - Occurs when all the voids in the soil are completely filled with water. Although
there is plenty of water available to the crop at saturation, water uptake is seriously curtailed by
the lack of oxygen in the soil at soil water contents greater than field capacity.
• Deep percolation - Water that drains beyond the plant root zone.
• Field capacity - The water content of the soil where all free water has been drained form the
soil through gravity. Sandy soils may drain within a few hours but fine textured soils such as
clay may take a few days to drain. Proper irrigation brings soil moisture up to filed capacity.
• Permanent wilting point (PWP) - The soil moisture content at which the plant will wilt and
die. While there still may be water in the soil, the plant is not able to extract sufficient water
from the soil to meet its needs.
• Maximum soil water deficit (MSWD) - Only a portion of the available water is easily used
by the crop. The maximum soil water deficit is the amount of water stored in the plant’s root
zone that is readily available to the plant. To prevent plant water, stress an allowable depletion
factor is used to calculate the manageable allowable depletion. This factor varies but is usually
around 50%.
Advanced Technological Institute - Gampaha
Department of Agriculture
Year II Semester I
2. P.A.S.S. Pushpakumara | Advanced Technological Institute- Gampaha
Field Capacity
Field capacity is the maximum water which can be held by the soil against gravity. It depends on
porosity and capillarity. Moisture content at field capacity includes hydroscopic water and capillary
water.
𝐹𝐶 =
𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑟𝑒𝑡𝑎𝑖𝑛𝑒𝑑 𝑖𝑛 𝑐𝑒𝑟𝑡𝑎𝑖𝑛 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓𝑠𝑜𝑖𝑙
𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑎𝑚𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑑𝑟𝑦 𝑠𝑜𝑖𝑙
• Soil moisture tension at field capacity ranges between (1/10 atm – 1/3 atm)
Permanent wilting point
PWP depends on nature of soil and it is the lower limit of capillary water and upper limit of
hygroscopic water. At PWP, Soil moisture tension is in the range of (7-32) atm. But for calculation
purpose we take an average value of 15 atm for all the soil.
Available water content (AWC)
𝐴𝑊𝐶 = 𝐹𝐶 − 𝑃𝑊𝑃
Where;
FC = field capacity
PWP = Permanent wilting point
Note: The available water content depends greatly on the soil texture and structure
