Soil erosion

Smaller soil particles (clay and silt) are the ones most vulnerable to removal by wind or water erosion. For example, particles up to silt size can be moved large distances by aerial dispersion. Water erosion largely removes clay and silt particles, with smaller soil particles being carried farthest by runoff water, especially when this runoff is generated by high intensity rain that exceeds the rate of infiltration into the soil. Soil conditions that predispose to surface runoff of excess rain are surface-sealing, water-repellency, inherently low infiltration capacities (e.g. clay soils, rock outcrops), and surface-compaction (caused by stock and vehicle traffic). Saturated and sodic surface soils are prone to runoff even under low intensity rainfall.

If rainfall intensity exceeds water infiltration capacity over time, or if rain falls on soil already saturated either from past falls or because of low evapotranspiration, erosion risk increases greatly.

Vegetation slows runoff and increases the time available for infiltration. Ground cover reduces water erosion by absorbing the impact of raindrops that can cause sealing of the soil surface and contribute to excessive runoff. Ground cover also reduces the impact of wind on the soil surface. Wind erosion does not occur where the soil is protected by adequate ground cover (trees, grass or stubble) or where the surface is rough due to large soil aggregates, clods and rocks which reduce wind speed at the surface. On cropping soils, 30% standing stubble cover will minimise erosion, while 50% cover is recommended for prostrate stubbles. Salinity predisposes soil to erosion primarily due to a loss of vegetation cover.

Sodic soils are associated with a high erosion risk due to poor soil stability.

Excessive cultivation (both intensity and frequency) increases the risk of both wind and water erosion. For example, a fine seed bed tilth predisposes to wind erosion when the soil is dry and winds are prevalent. Such fine seed beds are also susceptible to water erosion under high intensity storms and on slopes.

Overgrazing reduces ground cover and pasture species composition, increases erosion and nutrient loss, and therefore causes a subsequent loss in animal performance and a longer recovery period for pastures.

Soil forms at a very slow rate, typically about one millimetre (or 14 t/ha) every 100 years. Only a small amount of erosion is required to exceed this rate of soil formation and in Australian agriculture typical losses can be up to 50 t/ha per year from bare fallow, 8 t/ha per year under a crop and 0.24 t/ha under pasture.

Soil erosion results in the loss of topsoil, potentially exposing problematic subsoils (e.g. sodic clays) which require careful management.

Long term declines in productivity result from loss of fine clay and organic material. The removal of small amounts of topsoil results in the loss of organic nitrogen and other nutrients from the soil profile and decreased crop yields. Yields are most affected when soil is removed from pastures and zero-till soils, as organic nutrients become concentrated in the top few millimetres of the soil profile in uncultivated soils. The loss of topsoil leads to structural instability leading to further degradation, and a reduction in the functional capacity of soil due to a loss of soil biota.

The loss of organic matter from the top 2 millimetres of an uncultivated soil is estimated to cause an estimated 5% yield loss in a subsequent wheat crop.

The loss of topsoil results in the loss of small-seeded pasture plants resulting in lower plant density, whilst loosened soil can result in sandblasting damage from wind erosion in young crops.

Soil loss can result in less depth of soil, decreasing the capacity of soil to store water and nutrients and limiting the soil available for root exploration, resulting in reduced root and plant vigour. Deposited soil can bury plants and reduce yields.

Soil erosion increases with runoff.

Management of the soil surface to improve infiltration and soil water storage will improve water availability and crop yields at the same time as reducing erosion.

Minimising soil erosion and runoff has important implications for water quality since runoff usually contains sediment, nutrients, agricultural chemicals or other contaminants.

Soil erosion by water is the major process resulting in sedimentation, potential eutrophication of dams and waterways, and in extreme cases the death of fish and other organisms.

The formation of gullies as a result of soil erosion reduces trafficability of paddocks and affects the timeliness of operations, whilst deposition of soil can bury fences or knock them down resulting in the decline of farm infrastructure.

Ridges and upper slopes will be less likely to erode than lower slopes subject to run-off waters from above. Erosion risk increases as the slope gradient increases – gradients of less than 10% have a low risk of erosion, whilst slopes with gradients of 15-20% are considered a very high erosion risk, and if more than 20% should not be cropped. Consider the location of headlands and the direction of planting to reduce erosion risk.

Contour banks can be used to reduce the length of potential pathways for water erosion, a valuable tool as surface flow speeds up as it runs downhill and erosive force increases exponentially. Flat-topped banks can be used in cropping systems to allow machinery to pass over them for efficient sowing and spraying.

The risk of wind erosion can be decreased by retaining stubble. Standing stubble (anchored by its roots; minimum height of 10 centimetres) is more effective in reducing wind erosion than prostrate stubble and is particularly important for preventing soil erosion in field pea, lentil, chickpea and faba bean crops. Stubble also reduces the risk of water erosion by decreasing the effect of raindrop impact, maintaining infiltration and reducing runoff.

For control of erosion, surface cover is essential.

Ground cover may comprise components of living or dead plants, pebbles and rocks, lichens or fungi that cover all or part of the ground surface and intercept rainfall. The amount of ground cover is influenced by plant habit and growth rate, as well as plant and land management.

Grazing, crop and fire management have a major impact on ground cover in agricultural systems. Establishment of perennial vegetative cover, maximising productivity of annual crops, retention of stubble and trash in broadacre crops, and the use of cover crops in orchards and vineyards can contribute to maintaining ground cover.

Manage grazing pressure so that stubble remains erect and anchored. Paddocks with low stubble loads or those prone to erosion (e.g. field pea, lentil, chickpea and faba bean stubbles) should remain ungrazed.

Pastures should be managed so that adequate ground cover levels are maintained and excessive grazing is avoided during periods of drought or high risk (when cover levels are low and rainfall intensities high).

In intensive cropping systems, straw and other organic residues can be used to cover the soil surface, reducing exposure to raindrop impact and wind erosion.

Additional benefits such as moisture conservation, increased organic matter and enhanced biological processes are also likely to be observed.

Strip cropping can be used by growing alternate strips of winter and summer crops across the slope to provide better ground protection and reduce runoff. Strip cropping is also useful in controlling wind erosion on any class of land because the strips reduce the ‘fetch’ of the wind across bare soil.

Excessive cultivation breaks down soil structure and removes protective ground cover. Fine seedbed tilths or compacted surface soils predispose to erosion. Zero tillage maintains soil stability through high levels of anchored ground cover and reduced soil disturbance.

Minimise stock concentration in vulnerable areas to reduce soil degradation and ensure sufficient ground cover is maintained to protect the soil surface.

Realign fences, tracks, stock watering points, gateways and laneways to avoid channelling run-off, and isolate areas of high erosion risk.

Construct contour sills, grade banks, broad-based banks and interceptor drains to intercept run-off and effectively reduce slope length. Don’t disturb natural watercourses and where possible keep fully vegetated. Repair gullies where feasible and fence to exclude grazing.

Slow water flow and wind velocity to reduce erosion.

Watercourse management

Keep watercourses in a natural vegetated condition and exclude from all tillage operations. Grazing of riparian areas should be managed carefully.

Crop management

For paddocks subject to overland flow, standing stubble is the best way to slow local surface flow and retain (catch) soil transported in runoff. It provides a barrier to air movement at the soil surface and thus reduces the risk of wind erosion as well.

Contour management

Contour banks or contour mulching can be used to intercept and divert water run-off coming from up-slope paddocks, road surfaces and culverts away from cropped areas (crop along the contour or at a slight gradient to slow run-off). Grassed waterways can be installed to collect the surplus water flowing from behind the contour banks. These grassed waterways should be kept mown to allow the water to flow down them quickly.

Photo: CSIRO Science Image Library

Vegetation management

Maintain and improve drainage line vegetation including shallow grassed waterways to improve water use and interception, particularly on upper slopes and along waterways. Tree shelter belts at suitable spaced intervals have been used to reduce wind erosion risk.