World’s Soil Resources

Figure 15.3 (a) Trends in winter rainfall in south-western Australia for the period 1900–2012. Source: Australian Bureau of
Meteorology 1 .
The 15-year running average is shown by the black line. (b) Annual mean temperature anomaly time series map for south-western Australia (1910–2012),
using a baseline annual temperature (1961–1990) of 16.3 °C. The 15-year running average is shown by the black line.
Soil acidification
Soil acidity in Western Australia is estimated to cost broad-acre agriculture AUD $498 million per year
(Herbert, 2009) or about 9 percent of the average annual crop. It is one of the few soil constraints (particularly
subsurface constraints) that can be treated with appropriate management.
Between 2005 and 2012 a total of 161 000 samples was collected from over 93 000 sites to determine
soil pH (determined in calcium chloride solution - pHCa) status and trend. Figure 15.4 shows the proportion
of samples below the nominated targets for Western Australia for the surface layer (0–10 cm) of pHCa 5.5
(desired target) and pHCa 5.0 (critical threshold). Soil acidity is widespread and extreme in many areas of the
southwest of Western Australia, particularly in sandy soils. Surface soil pH can be increased to above target
(pHCa 5.5) over significant areas with the application of 1 to 3 tonnes ha -1 of good quality lime. This will cost
from AUD $50 to $150 per ha. If soil surface pH can be raised and maintained above the target this will ensure
that management of subsurface acidity will be achieved over time.
There is general recognition that lime use needs to increase and the trend is positive (Figure 15.5). However,
current agricultural lime sales are still only 40 percent of the estimated annual amount required to reach the
recommended targets over the next 10years (Gazey et al., 2013).
1 http://www.bom.gov.au/
Status of the World’s Soil Resources | Main Report Regional Assessment of Soil Changes
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in the Southwest Pacific