Soil acidification, as influenced by N fertiliser, stubble management, and
crop rotations, was investigated using soil samples from a long-term rotation
trial at Tarlee, South Australia. With the effects of combination of treatment
inputs (wheat–lupin, stubble retention and N-fertiliser application),
the pHCa (0–10 cm depth) declined from the
starting value of 6.12 to 4.50 after a 14-year period. All of the treatments
caused the soil pH to decrease. The mean acidification rates for the period
1978–1992 varied from 0.5 to 2.22 kmol
H+/ha.year for different treatments. Although
the rainfall at Tarlee (483 mm) is not sufficient to cause regular drainage
events, it is likely that downward movement of
NO3– in the soil profile has
caused acidification in the surface soil. Also the retention of stubble caused
more acidification than where there was regular stubble burning or removal.
The acidification resulted in an increase in soil exchangeable Al and Mn and
extractable Al and Mn, and the decrease in soil exchangeable base cations. The
values for soluble Al (extracted in CaCl2) in 1992 were
1.58 and 2.45 mg/kg (0–10 cm depth) for the wheat–bean and
wheat–lupin rotations, but the percentage of Al in the ECEC was low. It
is not known whether this acidity has any impact on yields of field crops at
this stage. But with soluble Al and the percentage of Al in ECEC increasing,
it is possible that Al toxicity may occur in the high input-output cropping
soils in South Australia.
acidity, pH, aluminium, manganese.
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