Environmental Engineering Reference
In-Depth Information
Figure 18.4
The shapes of the common soil structural units.
and the soil will be described as
saturated
. Under the influence of gravity, water would
drain out of the larger
transmission pores
. This water which quickly drains away is
termed
gravitational water
and it drains out of pores larger than about 0·05 mm
diameter. When it has all drained away the soil is said to be at field capacity, i.e. at the
upper limit of wetness at which a soil can retain water without gravitational loss.
Smaller pores less than 0·05 mm diameter can hold water against gravitational
removal owing to capillary forces. Such water is classed as
capillary water
. Capillary
water forms the bulk of
available water
for plant roots. The plant root expends energy in
absorbing water from the soil, but eventually there comes a point where, as the soil dries,
the forces between solid phase and water exceed the energy available to the root for water
absorption. This limit of wetness, below which plants can no longer extract water, is the
wilting point. Between field capacity and wilting point water is available for plant growth
and is therefore termed
available water
. Capillary water and available water are the same
for many plants, though there is some debate whether all plants are able to utilize all
available water. Some water is retained in the soil under the dryest of natural conditions;
such water is called hygroscopic water.
