Erosion: robbing the soil

Erosion, the transport of soil by wind and water, is a process that occurs naturally. It is a vital part of the rock weathering cycle, shaping the solid part of the biosphere. The landscape we see in Western Australia is the result of the processes of weathering and erosion over many millions of years. The rate of erosion can be quite rapid in nature but is mostly very slow. The presence of vegetation is a major constraint on rates of erosion.

The 1992 State of the Environment Report estimated that wind erosion in the agricultural region reduced cereal and pasture productivity at a cost of $21.3 million annually.

Water erosion

In the native bush, vegetation protects soil from excessive erosion. Clearing vegetation allows wind and water to erode the land.

Water erosion in Western Australia occurs mainly in the late summer from isolated thunderstorms or in the autumn-early winter period from the opening rains of the season. In late summer the land is mostly bare and the surface is compacted from grazing, causing water to run off and erode the soil. In autumn-early winter, soil is bare and loose from cultivation, again giving run-off a chance to erode soil.

Water erosion removes nutrient-containing topsoil, lowers soil fertility and, in its worst form, produces gullies that obstruct traffic and make cultivation very difficult.

Silt build-up causes rivers to lose their capacity to carry high water volumes with consequently greater risk of flooding. Silt may completely fill dams and reservoirs, while ports and harbours will require regular dredging.

Wind erosion

The soils of Western Australia are predominantly sandy-surfaced, making them naturally prone to wind erosion where the ground lacks protective vegetation. Normal farming practices such as grazing and cultivation remove this cover. Then a wind velocity of 28 km/hour is enough to move the soil. This wind velocity is common.

Wind erosion removes the nutrient-containing topsoil, lowering soil fertility; it may sandblast plants; and causes damage to roads and fences. One of the farmers major expenses is the addition of fertilisers to his soils. It can prove to be very costly to allow wind erosion to occur on fertilised paddocks.

Solutions

Reducing cultivation to the minimum required for satisfactory crop production is the single most effective way of beating wind and water erosion. Crops can be sown directly without prior cultivation and using a herbicide to control weeds.

Water erosion can be further controlled by contour farming and by placing banks on the contour. Wind erosion can be avoided by retaining a vegetative cover, for example in the form of crop stubble. This slows down the wind. Planting trees in belts across the prevailing wind can permanently protect land against wind erosion. Removal of stock from susceptible areas is often required. Alley farming systems also do much to reduce or eliminate wind erosion.

Davies-Ward, Edwina and Finlayson, Rob, (Ed.)(1997), Environment Western Australia 1997 Draft State of the Environment Report for Western Australia

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