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Soil Erosion
The topsoil is the most productive and fertile part of the soil. It contains nutrients, organic matter, and microorganisms. In bushland areas it hosts most of the seed bank from which regeneration will occur. The top soil has taken thousands of years to form and can be lost to erosion within a short period of time if care is not taken. The loss of soil takes with it soil treatments such as lime and fertiliser and possibly seed. This has economic impacts such as loss of soil fertility and reduced yields. Eroded soils add to the problems for our waterways by causing pollution, sedimentation and restricting the flow of water.
Soil Erosion can be caused by water and/or wind, or by burrowing animals (such as rabbits).
Water Erosion
Water affects the soil in two ways:
1. rain drops impact the soil like tiny hammer blows, damaging the soil structure and then dislodging particles; and
2. run off carries away the soil particles and nutrients.
When rainfall is greater than infiltration (absorption by the soil), runoff will occur. The amount of erosion will be effected by slope, management (vegetation cover, contour banks) and the timing and intensity of the rainfall. Economic viability is not only effected by the loss of soil but by the reduced amount of water infiltration. If water is lost to runoff rather than infiltrating the soil, the potential to increase yields is lost. The force of rain drops can compact the soil breaking down aggregates into small particles and preventing further water infiltration. This breakdown is exacerbated by repeated cultivation.
However, when the ground is covered by vegetation or stubble, the force of the raindrops is broken by leaves and plant litter, reducing the runoff and increasing infiltration.
Types of water erosion.
Splash erosion - soil will splash due to the impact of rain drops, washing down hill if this area is on a slope.
Sheet erosion - is the even removal of a thin layer of topsoil through runoff from sloping land. It may occur over large areas and may be gradual which makes it harder to see. Exposed tree roots and very stony soils are good indicators.
Rill erosion - is the formation of shallow channels up to 30 cm deep due to the channelling of run off. If left unchecked it will develop into gully erosion.
Gully erosion - advanced rill erosion, often on slopes greater than 2%. Can form from animal or vehicle tracks where there is no plant cover. Gullies may expand to head wall erosion where the gully extends upwards against the direction of flow by the action of a water fall undercutting and slumping at the head or lateral erosion where the gully is widened by flow undercutting the sides.
Stream bank erosion - the removal of soil from the side of streams during period of high flow.
When water erosion is likely to occur
Water erosion is most likely to occur during the late summer, autumn and early winter period from the opening rains of the season. At this time, isolated but heavy thunderstorms can occur while the ground has reduced plant cover; is bare; is compacted by grazing; or has been cultivated ready for cropping.
Management
Maintain plant cover;
Reduce heavy grazing;
Change farming operations to include minimum or zero tillage
Firebreak Erosion
Firebreaks and drive ways can be common sites of water erosion if not planned and constructed carefully. Locate fire breaks across the slope, avoid waterways and creeks. Incorporate curves and existing farm roads into your fire break system where possible, and deviate over natural bare areas. Fire break variations are available on application to the Shire. If you are worried about erosion use chemical control to maintain the cleared area to comply with Shire fire break regulations rather than by cultivation. A chemical fire break will retain plant roots and small amounts of residue that will hold the soil together to reduce erosion. Chemical fire breaks are firmer and make better access tracks. Cultivation of fire breaks will expose the area to erosion from summer storms and provide a perfect area for weeds to grow the following season. If you have to cultivate, then use a flat blade rather than a scarifier or plough to avoid creating gullies that channel run off. If you have a sloping drive way or firebreak, incorporate side culverts along the length to take away water. You can reduce the cumulative effect and speed of flowing water by placing small barriers (or riffles) across at intervals along steep culverts.
Further references:
Farm Monitoring Handbook: Natalie Hunt and Bob Gilkes, UWA Press
Farmnote: How to prevent firebreak erosion, No 27/93
W.A. Dept Agriculture and Food
Toodyay Landcare - A Strategy for the Future, 2.2 (Page 12)
Wind Erosion
Wind erosion affects up to 20% of Western Australian soils. The most fertile part of our soils can be picked up by the wind and dumped in the Indian ocean. Besides removing topsoil wind can sand-blast crops, increase evaporation, kill new born lambs and recently shorn sheep. Also topsoil can be deposited as sand drifts on roads or up against fences. Erosion will only occur if the soil is bare and poorly structured, as many sandy soils are.Winds can be reduced by the erection of shelterbelts or windbreaks. Soils can be protected by maintenance of adequate plant cover.
Soils in their natural state have adequate plant cover and stable soil and do not suffer wind erosion. Once clearing has taken place, cultivation and stock will degrade soil structure. Wind erosion sorts the particles into fine materials that are carried away as dust, leaving the coarse particles behind. Loss of this soil will include nutrients and organic matter, reducing the soil productivity considerably.
For wind erosion to occur, the wind velocity must be greater than 20-30 km per/hr. A doubling of the wind will increase soil loss eight fold. If you can't control the wind speed then it is your soil management that will determine if the soil will erode. A lack of plant cover with a dry, loose soil will see your soil (and $$) blow away.
Wind Erosion Processes
Management
Reduce cultivation and where possible convert to direct drilling or zero tillage;Retain stubble or ground cover of at least 50%;
Incorporate wind breaks into the farm plan.
Further references: Back to the Soils Page
Farm Monitoring Handbook: Natalie Hunt and Bob Gilkes, UWA Press
Farmnote: Preventing wind erosion, No 35/96 W.A. Dept. of Agriculture and Food
Managing for stubble retention, W. A. Dept. of Agriculture and Food (GRDC) Bulletin 4271
Toodyay Landcare - A Strategy for the Future, 2.1 (Page 11)
