Water movement in soil - Soil Essentials: Managing Your Farm's Primary Asset - page 134

How is water movement measured?
The amount of water a soil can absorb is often tested in the field by pushing a
metal or plastic ring into the soil to about 10 centimetres depth, taking care that
the top of the ring is level. The ring is then filled with water, and the rate at which
the water moves into the soil is measured over time using a clock and a ruler.
When water is being put inside the ring, it is necessary to prevent damage to the
soil surface by pouring the water onto a f lat hard material such as a piece of plastic,
or a shovel blade, instead of directly onto the soil itself. The protective material is
then removed. The diameter of the ring is not critical, but is usually of the order of
30 centimetres. The measurement is usually taken in millimetres, and the rate of
water infiltration expressed in millimetres per hour. (See Figure 42 on p. 120.)
The amount of water a soil can hold will influence how much crop or
pasture can be grown on that soil.
Topsoil and subsoil differences
In a soil type with a deep porous topsoil (the A horizon), water will probably move
downwards fairly quickly, unless there are hardpans present that would restrict the
flow of water.
However, in a soil with a porous, light textured topsoil over a heavy clay subsoil
(the B horizon), the water will move quite rapidly through the topsoil, and much
slower through the subsoil below once the topsoil is saturated. In this case it may
be a good idea to dig the topsoil away to the depth of the clay subsoil, and place the
ring at this depth. This will give a more realistic idea of how well this soil will
accept water, and will identify the likelihood of waterlogging occurring in the
topsoil.
Dry or wet soil?
This test can be done in dry or wet soil, but the measurements obtained will be
different. When testing a dry soil, the initial reading will probably be higher than
readings repeated after the water from the first reading has soaked away. It is usual
practice to repeat readings until results from subsequent readings are the same,
and this can be quite time consuming.
Why do this test?
Recharge mapping in salinity work
If salinity is a problem in the area, then by doing infiltration tests at a number of
sites along slopes and on hilltops, areas of high infiltration can be mapped. This