Geoscience Reference
In-Depth Information
Plate 19.1
Silty soils are frequently structurally unstable. These reclaimed estuarine silts near Goole, Yorkshire, have had their
surface structures destroyed by raindrop impact. When the dispersed 'fines' dehydrate, an impermeable crust is formed which
prevents seedlings' emergence.
Photo: Ken Atkinson
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 driest of natural conditions; such water is called
hygroscopic water
.
Soil water is thus best regarded in terms of the energy
with which it is held by the solid phase in the soil. The
smaller the water content the more tightly the water is held
by solid particles. This force is measured in units of
suction
, i.e. the force required to remove a certain
proportion of the soil water. Suction is measured in
pascals or bars or atmospheres (10
5
pascals = one
atmosphere = one bar = 1,000 mbar).
Table 19.2
shows
the suction with which different classes of water are held
in soil. Wilting point is fifteen bars' suction, the limit
Water properties
After a very wet period most of the porosity will become
filled with water, 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
in soil for plants.
Plant roots expend 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
