Soil cations - Soil Essentials: Managing Your Farm's Primary Asset - page 48

essential for healthy plant growth and they are both required in reasonably large
quantities. The potassium requirement is less than for calcium and magnesium.
Sodium and manganese are required in very much smaller amounts and are quite
toxic to plants if available in excessive amounts. Aluminium is not required at all
by plants, but is toxic to plants in acidic soils.
Cations and soil structure
Three of the cations in particular have a profound effect on soil and subsoil
structure. These are calcium, sodium and magnesium.
A soil with these nutrients present in the right ratios will often be granular and
crumbly when moist. In other words it will be well structured as long as it is not
damaged in other ways. This will allow easy movement of water and plant roots
down into the soil profile. If too much magnesium is present in subsoil compared
with calcium, the subsoil will tend to be very dense solid clay that can severely
restrict water and root movement into the soil profile. If too much sodium is
present, it will displace calcium from the clay particles, causing them to lose their
crumb structure and slump together into a dense sticky mass when moist. These
soils are referred to as sodic soils, and are generally inhospitable to plants. Often
when plant roots encounter a sodic layer of soil, they turn and stay above it rather
than penetrate downwards. The problems with sodic soil from a plant's point of
view is that because the clay has slumped together so tightly, there are no air spaces
to encourage plant roots to enter, and roots find the soil too strong to grow into
because of the small pore spaces. Further, because there is a high sodium content
in these soils, the osmotic mechanism plants use to access and transport nutrients
in their sap does not operate well. Thus, a plant in sodic soil can show drought
stress even when the soil is quite moist. Plants are also stressed simply due to the
toxic effect of the salt (Figure 16 on p. 107).
Cation exchange capacity
This is a term often quoted in soil test results, and is a measure of the capacity of
the soil to hold the major soil cations: calcium, potassium, magnesium, sodium
and hydrogen (the exchangable cations).
'Cation exchange capacity' (CEC) is thus a measure of the ability of the soil to
hold plant nutrients, and is an important measure of soil fertility. A high cation
exchange capacity means a good supply of these plant foods are available in the
soil. It is also important that the cations be in the correct ratio in the soil if plant
growth is to be optimised.