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In this section you will find detailed information, including national overviews and state and regional level assessments, from the 2000-2002 National Land and Water Resources Audit theme assessments.
Australian Agriculture Assessment 2001
Australian agriculture assessment 2001
National Land and Water Resources Audit, 2001
Appendix 2. Australian Soil Resources Information System
Erodibility
Erodibility is a soil’s inherent tendency to be transported by water or wind. One measure of erodibility (the resistance of a soil to sheet and rill erosion) is the K-factor (used in the Universal Soil Loss Equation - USLE) and is a function of:
- texture of the soil (specifically, soil fraction with grain size less than 0.125 mm);
- amount of organic matter in the soil; and
- permeability (how well water drains).
Only the topsoil is subject to erosion-except where gullying is extreme-so a map of estimated erodibility has been produced only for the topsoil.
Soil erosion-threatening both farmland productivity and water quality-is a consequence of erodibility.
Loss of topsoil implies a diminishing volume of soil for crop production. The transported soil particles carry with them adsorbed forms of calcium, nitrogen and phosphorus. With water-eroded topsoil these nutrients are lost for crop plants but can stimulate algal growth in surface water bodies. When the transported particles are deposited, they can build up behind dams or choke waterways.
How does it vary and what is it related to?
Erodibility depends on the structural stability of the soil, and its capacity to transmit water downward. Structural stability is a function of soil particle size distribution (texture), mineralogy and organic matter content.
- Fine grained soils are more erodible than sands.
- Soils with high organic matter content are less erodible than those with low organic matter content.
The K factor has been estimated from field plot experiments, mainly in South Australia, because data for Australian soils are available only for a few sites. Typical values (from Rosewell 1997) are:
| Erodibility | K factor |
|---|---|
| Very low | < 0.02 |
| Low | 0.04 |
| Moderate | 0.06 |
| High | 0.08 |
| Very high | > 0.08 |
How and why does it vary across Australia?
Comparing erodibility with the soil map of Australia shows that heavy clay soils (Vertosols) are highly erodible as are structurally unstable, chemically dispersible sodic soils (Sodosols). Kandosols and Calcarosols with sandy topsoil are slightly less erodible. Rocky soils (Rudosols) and weakly developed soils (Tenosols) are least erodible.
The digital map of erodibility (Figure A24) shows that there is an obvious State boundary between South Australia and Victoria, partly due to land management differences and to original map sources of soil data.
How can this map be applied?
Erosion can be modelled and thereby predicted by applying the Universal Soil Loss Equation relating erosion on agricultural land to rainfall intensity, soil, hillslope length and gradient, land cover, and management practices. Erodibility is only one input into this model.
For more information on soil erosion in Australia, see the Water-borne erosion section.
Table A20 Soil loss-the K factor-by percent of land use type across Australia.
| very low | low | moderate | High | very high | Total land use class area |
|
|---|---|---|---|---|---|---|
| < 0.02 | 0.04 | 0.06 | 0.08 | > 0.08 | (ha) | |
| Conservation and natural environments | 3 | 50 | 4 | 16 | 27 | 264 040 900 |
| Production from native environments | 2 | 16 | 3 | 10 | 69 | 443 296 900 |
| Cropping | 5 | 21 | 11 | 13 | 50 | 22 522 000 |
| Grazing modified pasture | 8 | 21 | 11 | 19 | 41 | 19 231 800 |
| Horticulture | 8 | 21 | 12 | 21 | 38 | 350 700 |
| Irrigated cropping | 1 | 2 | 1 | 6 | 90 | 949 100 |
| Irrigated modified pasture | 4 | 4 | 3 | 9 | 80 | 1 078 200 |
| Total area | 751 469 600 |
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