World’s Soil Resources

15.5 | Threats to soils in the region
15.5.1 | Erosion by wind and water
The rates of soil erosion occurring today in Australia and New Zealand are significantly less than in previous
decades. The situation in the rest of the region is less clear. Very fast rates of erosion are occurring in countries
with uncontrolled land clearing and logging (e.g. Papua New Guinea and the Solomon Islands). Unsustainable
rates of erosion are also likely to be occurring in marginal and hilly lands used for agriculture in some Pacific
countries, for example Fiji (Liedtke, 1989).
Soil erosion by wind is a significant problem in Australia and an account of trends and current status is
presented below. It is not common elsewhere in the region because of the humid climate, although drier areas
in New Zealand (primarily on the South Island) are prone to wind erosion (Eyles, 1983).
Australia
Current rates of soil erosion by water in Australia are much less than the peak periods just after land clearing.
In many parts of the country, widespread gully erosion occurred during this time and the hydrological regime
of many river systems was changed. In southern Australia, gully and river bank erosion are the dominant
sources of sediment supplied to streams. Gully erosion in southern Australia has now been largely stabilised,
but gullies are still actively forming in northern Queensland and in some agricultural regions of Western
Australia (NLWRA, 2001a).
Despite the apparent stabilization, current rates of soil erosion by water across much of Australia now
exceed soil formation rates by a factor of at least several hundred and, in some areas, several thousand. As
a result, the expected half-life of soils (the time for half the soil to be eroded) in some upland areas used for
agriculture ranges from less than a century to several hundred years. The latest assessment concluded that
soil erosion by water in Australia is still at unsustainable rates, but there are large uncertainties about the time
until soil loss will have a critical impact on agricultural productivity (SOE, 2011; Bui et al., 2010; Bui, Hancock
and Wilkinson, 2011). Environmental impacts of excessive sedimentation and nutrient delivery on inland
waters, estuaries and coasts are already occurring.
It is estimated that up to 10 million ha of land have less than 500 years until the soil’s A-horizon (effectively
the more fertile topsoil) will be lost to erosion. Most of this land is in humid subtropical Queensland. Integrated
studies of soil formation and erosion using a variety of techniques will be needed to better understand
the extent, severity and significance of the problem. However, it is clear that a concerted program of soil
conservation is essential to control this chronic form of land degradation across large areas of Australia. The
problem is arguably having its greatest environmental impact on the World Heritage listed Great Barrier Reef.
The latest scientific consensus is that the decline of marine water quality associated with terrestrial runoff
from the adjacent catchments is a major cause of the current poor state of many of the key marine ecosystems
of the Great Barrier Reef. The main source of excess nutrients, fine sediments and pesticides from Great Barrier
Reef catchments is diffuse source pollution from agriculture (Brodie et al., 2013).
Status of the World’s Soil Resources | Main Report Regional Assessment of Soil Changes
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in the Southwest Pacific