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Soil

As identified in previous state of the environment reports, soil loss  and soil movement are important determinants of land condition because they create negative feed back loops through reducing the organic matter and nutrients available to sustain vegetative growth, and hence ground cover. Nevertheless, soil related trend datasets have not been made available for this commentary.

The data presented in SoE2001 show that annual soil loss is commonly greater than one tonne per hectare across most of Australia. This is approximately twice the rate that Loughran, et al. (2004) cite as the agreed rate (adjusted for location and seasonality) at which soil is replaced by organic decomposition.

Soil loss is principally due to water and wind erosion, both heightened by practices that reduce surface cover. An in-depth description of the process of soil loss can be found in State of the Environment Report Tasmania 2003 (RPDC 2003).

The National Land and Water Resources Audit (NLWRA 2001a) shows that, in the ILZ, erosion on native pastures accounts for 76 per cent of total soil erosion (5.7 t/ha/year), with a further eight per cent (4.7t/ha/year) and six per cent (8.9 t/ha/year) being accounted for by erosion in unmanaged woodlands and national parks respectively. The Audit concludes that ‘the biggest total contribution to soil erosion in Australia is from the vast semi-arid woodlands and grazing lands in northern regions’.

In their nationwide study of rates of soil loss, Loughran et al. (2004) measured annual rates of erosion of 5.5 tonnes per hectare in rangelands sites around Australia. The Audit data show also relatively high rates of erosion on sugar cane (37.2 t/ha/year) and cotton (7.0 t/ha/year) lands but together those lands account for only 1.7 per cent of total erosion in the ILZ.

The overall picture is that over 70 per cent of the ILZ has erosion rates ten times greater than the estimated average natural rate of erosion. The effects of this erosion over time are, at best, cumulative resulting in continued deterioration of the condition of land.

The net flow of soil nutrients reflects the balance between nutrient export through erosion and agricultural production, and nutrient import through soil formation and applied nutrients.

Particulate (sediment-bound) phosphorus and nitrogen flows arise from hillslopes, gullies and river banks with hillslope erosion being the greatest contributor (approximately 65 per cent) of particulate phosphorus and nitrogen flows; with nutrient flows into the Burdekin (nitrogen only), Fitzroy and Murray-Darling river systems being of particular quantitative importance (NLWRA 2001a). Urban point-source discharges can also be a substantial source of nutrient discharges into waterways, representing, for instance, 31 per cent of the total phosphorus load in the Moreton Bay region in Queensland (NLWRA 2001a).

There are both local and distant impacts of these soil related nutrient flows, with great variation between river systems in the proportion of nutrient loads  being discharged at river mouths.  

The environmental and human impacts of discharges vary greatly, depending on the location of the discharge, with particular sensitivity being assigned to nutrient discharges in close proximity to human settlements and to the Great Barrier Reef. It is reasonable to presume a causal relationship between grazing and cropping and soil erosion given that agricultural activities dominate much of the ILZ and the patterns of erosion according to land types and landuses as described above. However the high rates of soil erosion in national parks signal a need for caution in attributing high erosion rates simply to agricultural causes and further analysis of the extent and causes of erosion in national parks appears warranted. Furthermore, the NLWRA (2001a) concluded that an assessment of the impact of landuse practice on erosion rates could not be undertaken due to a lack of spatial information.

The need to address soil erosion has been evident for at least 50 years and agricultural management practices to do this, generally having the intermediate impact of maintaining adequate ground cover, are well known. They include retention of native vegetation, not undertaking agricultural activities on areas particularly susceptible to erosion, adjusting grazing pressure from both domestic and wild species, modifying grazing strategies (as distinct to grazing pressure), minimum tillage, soil conservation works and stubble retention.

In the absence of a long-term land care ethos or interventions to encourage adoption, increasing productivity rather than the prevention of soil erosion is likely to be the primary driver for most of the practices listed above. Perhaps it is not surprising then that there has been no significant reduction in the amount of soil lost from grazing lands (the largest contributor to soil loss) across this time (Loughran et al. 2004). Additionally, erosion rates from cultivated lands remain high notwithstanding the widespread introduction of no till and minimum till cropping practices.  

Over the span of the last 50 years or so, public sector responses to the problems of soil erosion and nutrient loss have included establishing public sector soil conservation and natural resource management agencies with charters related to soil condition. These agencies have generally not been well integrated with agencies promoting strategies to enhance agricultural productivity. In many instances these agricultural productivity strategies have had an adverse impact on the natural resource base, for instance the introduction, particularly into northern Australia, of Bos Indicus breeds of cattle that are highly adapted to exploiting available vegetation, and, more recently, the use of supplementary feeding regimes that enable livestock to remain on native pastures having low levels of retained dry matter.

Greater leverage of private sector inputs, including information, knowledge and wisdom, remains the key to reducing soil erosion. Strategies to achieve this object need to be well integrated with overall property management activities rather than, as is more common, taking the form of prescribed discrete additions to routine practices, such as those covered by favourable taxation arrangements for certain conservation related activities.

Lastly it needs to be recognised that the compositional and functional features of soil biota are fundamentally important to conservation, biodiversity and agricultural production. Hence the crucial issue of soil health requires consideration of the state of the soil biota in addition to considerations of, for instance, soil erosion, compaction, loss of nutrients and pH balance (see the section on salinity and acidity). Nevertheless, no data were provided in relation to soil biota. 

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