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(3) High pH (alkaline) soils
These soils are usually derived from limestone or basalt and are found, for
example, in the Mallee regions of Victoria, South Australia and New South Wales,
and the Darling Downs in Queensland. Alkalinity can be reduced by adding
acidifying fertilisers such as ammonium sulphate, sulphur, or by consistent use of
DAP (di-ammonium phosphate); however, very few soils show an economic
response to these treatments. This is due to excessive amounts of free limestone in
the soil that simply react with the applied fertiliser, leaving the soil pH much the
same as it was before treatment. Soils with a pH(w) of 8.0 or above will require
special management, and the range of suitable crops will be limited. Consistent use
of legume crops may help reduce pH levels over time. It is also essential to attend
to any nutrient deficiencies in these soils (see Figure 4).
What happens if nothing is done?
Again, the most important aspect of soil pH is the trend over time. Soils becoming
progressively more acid will eventually end up quite infertile over time if nothing
is done to reverse the trend. Acidic soils have lower potential production and
therefore adversely affect farm income. The option of doing nothing to treat
slightly acidic soils may be acceptable if management does not lead to increasing
acidity over time. Farm income will be significantly lower than that which could
potentially be achieved if pH was not a limiting factor. The risk with this
management option is that acidity will increase with time.
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