21
Physical characteristic
Definition and how to determine
Plant available soil water
The amount of water between FC and PWP in volume percentage. This
value should be used with caution. First, plants will start wilting with
subsequent yield losses well before the permanent wilting point.
Secondly, plant available soil water is replenished by capillary rise,
rainfall and irrigation water.
Eg: A fine sandy soil, rich in loam has a rooting depth of about 40 cm.
• The A horizon has a depth of 20 cm.
• The B horizon has a depth of 30 cm.
Calculation of the amount of plant available soil water:
At field capacity, pF 2.0, the A horizon will contain 35 volume % of
water. At the permanent wilting point, pF 4.2, the A horizon will contain 8
volume % of water. As 1 volume % corresponds to 1 mm water per 10 cm
of soil, the amount of available soil water in the A horizon is calculated
as the volume % of water multiplied with the rooted depth of the soil
horizon:
A horizon: 35 - 8 = 27 volume % water x 20 cm soil depth 27 x 2 dm soil
depth = 54 mm
For the B horizon the calculation is similar. Notice that rooting depth is
40 cm, so roots will be present only in the upper 20 cm of the B horizon.
At field capacity 27% of water will be available, at the permanent wilting
point only 6%.
B horizon: 27 - 6 = 21 volume % of water * 20 cm rooted soil depth º 21
* 2 = 42 mm water
In total, 54 + 42 = 96 mm of water is available to plant growth in this
particular soil.