Soil protection from erosion
Erosion is a natural process, however the clearance and cultivation of land for agriculture has resulted in rates of soil loss many times higher than in undisturbed environments. Soil erosion is the highest priority threat to the agricultural soils in South Australia. Approximately 6.0 million hectares (58% of cleared land) of agricultural land is inherently susceptible to wind erosion, and 3.2 million hectares (31%) is inherently susceptible to water erosion. The magnitude of this threat is recognised in South Australia’s Strategic Plan, target 70 (sustainable land management).
Without intervention, soil erosion can have adverse social, economic and environmental impacts. Soil erosion depletes the productive capacity of land as it removes nutrients, organic matter and clay from soil, which are most important for plant growth. Soil erosion has a wide range of costly off-site impacts including damage to roads, disruption to transport and electricity supply, contamination of wetlands, watercourses and marine environments, and human health impacts caused by raised dust.
Soil erosion has steadily declined in the agricultural areas of South Australia over the past 70 years due to improvements in farming practices, but soil losses still occur with extreme wind or rainfall events, and after severe or prolonged drought.
Soil is predisposed to a risk of erosion by physical disturbance or removal of surface vegetative cover. Very dry seasonal conditions increase the risk of erosion where there is reduced vegetative cover resulting from poor crop and pasture growth.
The critical management practices that affect the risk of soil erosion are:
- the occurrence, intensity and timing of tillage operations
- the quantity and nature of surface cover.
Most of the erosion risk is due to cropping practices such as tillage and stubble burning. Grazing management is also an important factor, especially in dry years and droughts. The highest risks associated with grazing occur in late summer and autumn when feed availability and the cover of annual crop and pasture residues is declining.
Soil protection is expressed as the average number of days per year that agricultural cropping land is adequately protected from erosion. There has been an overall upward trend over the last 10 years despite several years of drought and other challenging management issues.
The adoption of more sustainable land management practices, such as no-till sowing and stubble retention, has improved the protection of soil from erosion. No-till sowing involves sowing the seed in a narrow slot in the soil to minimise soil disturbance and maximise residue protection on the soil surface.
Telephone surveys show that the proportion of crop area sown using no-till methods has increased from 16% in 2000 to 66% in 2011. This trend has occurred in all the major cropping regions. There has also been a corresponding reduction in the use of tillage and stubble burning prior to sowing the crop. The trend in adoption of no-till is levelling off and this may limit further improvement in erosion protection.
The use of clay spreading and delving to manage water repellent soil is becoming a significant factor in the protection of soils from wind erosion. These techniques are widely used in the Southern Mallee and Upper South East areas where there are large areas of severely water repellent soils. Clay spreading and delving increases the clay content of the surface soil, improving soil strength and resistance to erosion. Crop and pasture production is also increased, providing higher levels of plant cover to protect the soil from erosion.
Confinement feeding allows stock to be removed from paddocks before surface cover declines below critical protective levels. It is a very important technique for preventing erosion during droughts and in late summer and autumn when ground cover is declining.
The Future Farming Industries Co-operative Research Centre (FFICRC) is evaluating and developing farming systems based on perennial plants in medium to low rainfall areas. This will provide land managers with more options to protect the soil from erosion.
There are ongoing challenges to further adoption of farm management practices for improved erosion protection, including herbicide resistance in weeds, mice, snails, crop diseases, and a limited range of perennial pasture species.
Climate change is likely to increase soil erosion given that it is expected to deliver a warmer, dryer climate with an increase of severe weather events. This will bring many management challenges and further research into adaptive responses to climate change is necessary.