Geoscience Reference
In-Depth Information
in the finer pores, the composition of the soil atmosphere may vary markedly around
local microbial concentrations or other sites of biological activity such as plant roots.
While the overall pore space obviously sets a limit to the total volume of soil wa-
ter and soil air, the balance between water and air is all important. Both are needed for
the healthy growth of roots and for animal life. The optimum state, therefore, is when
there is a sponge-like combination of fine pores to hold water, with larger pores to
allow free drainage of excess water and access of air. This state can best be achieved
when there is a good crumb structure, but soil texture is also very important as it de-
termines the natural packing of mineral particles.
When all pore space is filled with water, the soil is saturated or waterlogged.
Seasonal or permanent waterlogging is common in natural soils because of impeded
plants. When there is free drainage, some of this water drains away quickly by gravity,
transporting gases, nutrients and other chemicals in solution down the soil column.
The remaining water fully occupies the finer pore spaces of the soil, and at this stage
its moisture status is said to be at 'field capacity'. This condition would be reached,
for instance, after two or three dry days in winter following heavy rain; it could persist
indefinitely in the absence of losses to the atmosphere.
This capillary-held water is the main source of water that is available to organ-
isms. Protozoa, earthworms and plant root hairs alike depend on the uptake of water
by osmosis through thin membranes. Plants, however, need a continuous supply to
balance the water lost by transpiration from the leaves, and can quickly deplete the
store of available water in the soil during the growing season; up to 25 tonnes a hec-
tare (10 tons an acre) can be sucked up by a crop on a hot summer's day. When all the
capillary water is used up, the soil reaches a 'permanent wilting point' when annual
plants die, and microorganisms generally cease functioning. The root hairs and cell
walls simply cannot withdraw any more water from the soil.
There is still some 'unavailable' water held hygroscopically as thin films around
clay and humus particles. This water is invisible but can be measured by drying soil
in an oven at 105°C, and measuring the loss in weight. Cotton and wool, for example,
are also hygroscopic and can retain up to 8 per cent and 15 per cent respectively
without appearing wet.
The concept of suction force provides a way of calibrating the water-holding
properties of a soil. The actual amounts of water held by a soil at field capacity and
at wilting point, and the rate at which the available water is released under increasing
suctions, vary greatly with the soil texture. It is easy to show that a light sandy soil
holds only about 3% of its volume of water at the wilting point, whereas a heavy clay
