Land: Land condition
Beeton RJS (Bob), Buckley Kristal I, Jones Gary J, Morgan Denise, Reichelt Russell E, Trewin Dennis
(2006 Australian State of the Environment Committee), 2006
One legacy of changes in native vegetation is often a decline in soil quality. Changes to ecosystem structure and complexity, species composition, groundwater levels, soil salinity, rates of soil loss, soil acidity and levels of carbon and other nutrients in the soil are all significant problems in varying ways across Australia. Some of these are discussed below (but see also ‘Biodiversity’).
There are some areas, especially those with high-quality soils, where farming practices have improved fertility through the use of fertilisers or lime. In addition, minimal tillage techniques have improved soil organic matter levels, which in turn has improved the functionality of soils. Regardless, the issue of ‘leaky landscapes’ remains and much needs to be done to further improve the condition of Australian soils.
Soil loss and dust storms
Prior to European settlement, there was little bare soil in Australia apart from the sparsely vegetated arid regions. Australia’s agricultural land uses and practices have caused an increase in erosion through vegetation clearing and total grazing pressure. The situation has been exacerbated by low rainfall and changes to fire regimes. The rate of erosion in pasture lands has doubled from the rate under natural conditions, and there has been a fivefold increase for improved pastures. On average, sheet wash erosion from hillslopes has accelerated to three times the natural rate (NLWRA 2001a).There has been no attempt to assess gully erosion at the national scale since 2001. This gap in environmental reporting is surprising because gully erosion is recognised to be one of the most significant sources of sediment to streams and can severely impact on river health.
Overall, the number and intensity of dust storms suggest that wind erosion in Australia has varied considerably since 2000 (Figure 36).Measurements of airborne dust (measured as a Dust Storm Index) show that 2002 and 2003 were the worst years since the 9 year run of high dust storm activity during the dry years of the 1960s. Interestingly, dust storm activity during the 1982–83 drought, which is widely remembered as ‘the worst drought’, was not as nationally significant as would be expected. Indeed, there were big dust storms in the 1880s and the 1940s before monitoring even began.
Note: Bars are DSI; lines are rainfall. DSI – Dust Storm Index
Source: McTainsh et al (2006)
Soil organic matter levels, measured as the amount of soil carbon, are an index of soil health. Although it is likely that large amounts of soil carbon are exported each year—as crops and livestock products—the best long-term, nationally consistent estimates available are from modelling by the Australian Greenhouse Office. Soil carbon levels vary from one year to the next, but the models suggest that, as a result of clearing for agriculture, soil carbon has declined from slightly more than 675 million tonnes in 1990 to about 643 million tonnes in 2004.
The area of land affected by salinity across Australia is difficult to estimate. The states and territories all have different definitions of ‘salinity’ and have mapped it at different times, and in some areas the data have been reviewed in the intervening period.
The most recent estimate from a survey of farmers is that about 2 million hectares on 20 000 farms across Australia showed some signs of salinity (ABS 2002). The NLWRA in 2001 estimated that a total of 2.4 million hectares were saline, of which 1.6 million hectares were in Western Australia, with a total of 5.4 million hectares predicted by models to be at risk of salinisation (NLWRA 2001d), although some interests challenge the predicted increases (Keogh 2005). Approximately 30 per cent of Western Australia’s south-west is considered to be at risk of becoming saline by 2050. It has been suggested that the value of environmental assets should be considered alongside the area affected, so that large areas of low-value grazing land would not receive more attention than horticultural land, an environmentally significant wetland, or a town building (Pannell 2005).
In 2003–04, the NAP and the regional component of the NHT invested $33 million in actions that have a major focus on land salinity . The majority of these funds were disbursed in New South Wales, Victoria and South Australia, to be used primarily for on-ground activities. Their effectiveness is yet to be evaluated.
Soil acidity is a serious concern across large areas of rural Australia, largely because of agricultural practices. After many years of acidification under legume-based pastures, such as clovers, and related cropping rotations, approximately 50 million hectares of Australia’s agricultural land (around half the total area) have a surface soil pH value less than 5.5. At this level most commercial agricultural plants suffer reduced yields. The problem is easily remedied by applying lime (Hamblin 2001); however, while it may be good agricultural practice to do this, it is not financially viable for pasture-based industries. In contrast, lime application in cropping systems is a necessary and increasingly routine operation.
The NLWRA estimated in 2001 that, if current practices were to continue, the area of land with surface pH below 5.5 would double to over 90 million hectares in the next 10 years with a concomitant need for the use of lime. This raises one of many environmental conundrums: the manufacture of the amount of lime needed would produce CO2 in significant amounts. In addition, the use of lime improves soil performance and reduces soil nutrient loading, but this leads to leaky landscapes and, possibly in the long term, soil carbon accumulation.