5.11 Erosion Potential

Erosion of soil by water and wind reduces its productive potential by: o Removing nutrients o Removing organic matter o Removing soil organisms o Reducing water storage capacity/root zone depth o Cutting plant growth by sandblasting

In dry climates like much of South Australia, it may take 100 years to form 1mm of topsoil, but erosive forces can remove this in a matter of minutes. The loss of 1mm of soil (14 tonnes/ha) can represent the loss of 10kg/ha nitrogen and 2kg/ha phosphorus.

Water Erosion

When rainfall intensity is greater than soil infiltration rate, run-off is likely to occur, risking the removal of the topsoil and the nutrients within it. Three processes are involved in water erosion. The first involves the generation of quantities of run-off water by poor soil infiltration. The second is detachment of soil particles by raindrop impact or running water. The third involves the transport of soil particles by running water. For erosion to occur, individual soil particles must be fine enough to move and there must be water running across the surface to pick up these particles. 

Factors Affecting Water Erosion Potential 

Topography and Management

The slope of the land and the length of the slope both affect the risk of erosion by changing the quantity of water in surface flow and the speed of flow. The layout of paddocks and contour banks can be used to reduce water erosion. Over stocking and trafficking can increase the potential of erosion on a slope. 

Soil Type and Structure

Some soils are more prone to detachment of particles than others, all other things being equal.

Poorly structured surfaces which have little resistance to raindrop impact are easily broken into tiny particles. These seal the surface and force rain to run off. Fine sands, as well as dense sandy loams and loams, fall into this category. The finer the particles, the more easily they are picked up by running water. Any cultivated soils can be susceptible.

Impermeable layers at shallow depth, which allow water to build up, increase the chance of run-off.

Dispersive clays disintegrate into extremely fine particles on contact with water and run-off with the water.

Soils which are well drained because they are deep and sandy, or well structured, have a low risk of water erosion because water rarely runs off. Non-wetting sands would be the exception here.

Soils that have granular structure are able to resist soil transport in low overland flows because the particles are too big and heavy. Unfortunately, in channelled or watercourse flow, these soils are extremely susceptible to erosion, because the coherence between the particles is low and there is enough energy in the water to move the granules. 

Rainfall

The intensity, duration and time of year of rainfall are the key factors. Some districts are more prone to erosive rains at critical times (eg late summer) than others. The use of contour banks and retaining vegetative cover at critical times are significant management techniques in the control of soil loss. The most dangerous raindrops are 6mm diameter, but if broken up, the danger is reduced to around 1/10th. A single 4mm wide straw from stubble will protect not only the 4mm it covers, but also 6mm each side (a total of 16mm) as drops, which strike a glancing blow, will be broken up before they hit the ground. 

Assessing Erosion Potential

Erosion potential is calculated using various combinations of slope and soil erodibility to classify land into soil management classes. This is illustrated below.

Figure 1:
Maximum slope for crop production for soil types with different erosion potential Source: Department of Agriculture, Bulletin 462, 1960

Wind Erosion

Firstly, there is the process that sorts soil particles into fine material containing most of the silt, clay and organic matter. This is carried away as dust leaving behind the coarser material. Secondly, wind erosion can move bulk soil from the surface as seen in sand drifts. The wind velocity must be greater than 20-30km/hr (10-20 knots) for erosion to occur on a bare soil, but if the surface is disturbed by stock or cultivation during a wind, then a wind of only 5km/hr is required to start soil moving and once moving, it will continue to move. 'Feed lotting' stock - confining them in a small paddock- is a good way to reduce wind erosion during droughts. 3mm of topsoil loss will have occurred before fence line drifts will be obvious. 

Factors Affecting Wind Erosion Potential 

Soil Type and Surface Condition

Sandy soils are most at risk. Sand particles have less ability to bind together. This lack of structure allows particles to be moved more readily.

Soils where soil structure has broken down due to lack of organic matter, high levels of cultivation or trampling by stock are very susceptible to wind erosion. Particles of less than 1mm diameter can be blown by wind. 

Vegetative Cover

A soil with an anchored vegetative cover of 30-50% will not blow. The amount, anchorage and type of stubble left on a soil is very important. Lupin stubble is coarse and bulky, cereal stubble is finer and grain legume stubble can decompose rapidly and can be blown away itself. 

Stock Management

The total removal of stock from paddocks in danger of wind erosion is a very effective control measure when surface vegetation becomes too low. By confining stock to an established properly located feedlot during windy periods, erosion can be dramatically reduced. 

Assessing Erosion Potential

Figure 2:

Cover assessment
(Click for a larger image)

15% Surface cover,  0.5 t/ha wheat stubble	35% cover,  1.0 t/ha wheat stubble
60% Surface cover,  2.0 t/ha wheat stubble	85% cover,  4.0 t/ha wheat stubble

Figure 3:
Texture assessment:

5.11 Erosion Potential

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