Geoscience Reference
In-Depth Information
only sustain activity when the pF is less than about 2.7 (about 0.5 atmospheres). This
occurs when there is about 17 per cent water in the sandy soil shown in the figure but
when there is as much as 50 per cent in an organic-rich soil. The availability of water
in both cases is the same.
F
IG. 6
Water retention characteristics for 10 different soil texture classes. Water is released at different rates
when suction is increased (artificially or by the action of plant roots) from field capacity (FC) to perman-
ent wilting point (15 atmospheres). This is shown by the horizontal bars on each column. (Adapted from
P.J. Salter & J.B. Williams 1965.)
Agriculturalists like to relate the total available water content (AWC) to the root-
ing depth of a given crop. The top 900mm is conventionally taken as being potentially
exploitable by the roots of seasonal crops. A typical AWC for a well drained sandy
loam would be about 17% by volume or 150mm. Moving to the left along the bottom
of the triangular diagram of soil textures, a loamy sand would have a low AWC of
less than 110mm while a silt loam, at the bottom right hand corner of the diagram,
would have more than 190mm per 900mm depth. With shallow soils overlying hard
