Soil chemical constraints
Some land types in North Queensland have soil
chemical constraints that may affect soil health.
These soil constraints include acidity, salinity and
high levels of sodium (sodic soils). See “Further
information and resources about soil health and
sustainable agriculture” on page 27 of this
manual.
Sodic soils
Sodic soils result when sodium is dominant in
the minerals that formed the soil. They can
often be part of the subsoil in some land types.
Sodic soils disperse and are prone to severe
erosion when exposed to water. This can lead to
scalds, rill erosion and then severe gully erosion.
Grazing management that maintains maximum
ground cover and productive plant coverage is
recommended to prevent loss of top soil and
sodic erosion.
Acid soils
Many native grasses and legumes in North
Queensland grazing landscapes are adapted to
naturally occurring acid soils, however, historical
land use and some pasture practices such as
dominance of legumes can increase soil acidity
on light textured soils. Grazing management
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practices that build soil carbon and humus are
important, as soil organic matter can provide a
buffer against extremes in soil acidity. Fire can be
used to reduce legume dominance and encourage
3P grasses.
Saline soils
Salts naturally occur in some land types. Salts
dissolved in groundwater can also find their way
to the topsoil. This process can take many years
and be revealed as surface scalds with white
crusts. It is generally a response to the removal of
perennial vegetation and the consequent rising
of the water table. Grazing management that
maintains maximum ground cover and productive
plant coverage, and achieves a good balance of
trees and grasses, is recommended to prevent soil
salinity.
When considering sowing new pasture species,
it is important to identify if any of these
chemical constraints are present. The best
approach is to choose pasture species adapted
to match the soil conditions. Some improved
pasture species are intolerant of extreme soil
constraints, and addressing these with inputs is
often uneconomical due to the scale of grazing
properties in North Queensland.