Soil structural decline - Soil Essentials: Managing Your Farm's Primary Asset - page 130

The soil is dense and hard to penetrate with hand tools or machinery when
dry, and boggy when wet. Sodic soils can also develop strong surface crusting.
Hardpans
These are hard layers, usually at the depth of maximum cultivation, but can occur
almost at the surface, or even deeper in the soil profile. They reduce water, air and
plant roots penetrating through them, and can cause serious loss of yield and
waterlogging of the surface soil by inhibiting drainage.
The occurrence of hardpans can be determined by pushing a 6 mm diameter
steel rod up to one metre in length into the soil when it is moist, and noting
increases in effort needed to push the rod to depth. If a hardpan is present, it will
be difficult to push the rod through this layer. This rod can be attached to a device
that measures force to make an instrument known as a penetrometer.
Loss of subsoil structure
If the subsoil has become compacted, water penetration down the profile will be
poor, and will usually show up as surface waterlogging, or excessive water runoff
after rain.
Plant roots will have great difficulty penetrating to depth, and will have less
drought resistance and poorer growth.
To check subsoil structure, it will be necessary to dig a hole, preferably a metre
or more deep, and carefully inspect the face of the soil profile. Dispersion can be
seen as very dense sticky clay, often due to too much sodium compared to calcium.
Clays high in magnesium are very hard and dense, with a large columnar
structure.
The extent to which water and plant roots are able to penetrate soil profiles can
also be determined by looking for roots at depth. Roots may be seen to reach a
point where they turn and go sideways instead of continuing down the soil profile
if they hit a hardpan. This effect may also be due to acidity, sodicity or salinity.
(See Figure 39 on p. 118.)
Detecting sodic soil
Too much sodium in the subsoil will cause the clay particles to disperse and slump
together in a dense mass.
This can be checked by taking two or three pea-sized samples of the clay and
placing them in a shallow container of rainwater. If the clay is sodic, a cloudy
appearance will develop in the water around the clay. The quicker and more
completely this happens the more sodic the soil.
Note . Dispersion should not be mistaken for slaking, which occurs when the
clay sample crumbles down to a slurry with little or no cloudiness in the water.