Soils and Drought
Australian soils are vulnerable to erosion and other forms of degradation, and this vulnerability is exacerbated in drought times.
During extended dry periods the soil surface and even the lower profile, becomes desiccated and protective ground cover is reduced or entirely removed, especially if grazing by stock continues.
Wind removes soil particles from a bare surface which would normally be protected by pasture, litter, crop or crop residues. These particles end up as drift sand along fences, tree lines or other obstacles, or as fine dust in the air, with dust storms travelling up to thousands of kilometers and causing consternation in urban areas.
When it does rain again, bombardment of the soil surface and high water runoff create a real soil loss hazard. Long term research at six former Soil Conservation Service Research Centers shows that 90 % of soil loss from runoff plots comes from only 10% of runoff events, and was almost entirely confined to periods when groundcover was below 50%. Drought (and/or overgrazing) creates the conditions for these levels of erosion to occur. Soil loss by water erosion increases exponentially when ground cover is reduced below 70% in central and eastern NSW, and below about 40% on the western plains. About 600 kg/hectare of standing or flat vegetation or crop stubble is required to protect clay loam soils from wind erosion, and up to 2500 kg/hectare is required to protect very sandy soils.
Of course, some soil types are more erodible than others and therefore show different levels of resistance to erosion and degradation during and following drought. Sandier soils are most prone to wind erosion, while loamy soils are most prone to water erosion. Dry, bare topsoil is pulverised by trampling where stock are concentrated, and thereby further exposed to both wind and water.
When topsoil is lost, it takes with it the best part of the soil for plant growth, as well as nutrients, organic matter, fertiliser and seeds. Often the exposed soil remaining has a much lower water infiltration rate, so subsequent runoff increases and the amount of soil water available for plant growth is reduced. Reduced nutrient levels also decrease subsequent plant growth.
SOIL KNOWLEDGE NETWORK POSITION STATEMENT ON DROUGHT
Soil is a non-renewable resource and drought creates critical vulnerability to loss and degradation. Although stock are valuable, soil is even more so.
The key to soil management in drought is groundcover management.
In an increasingly erratic climatic regime it is important to continually monitor groundcover levels. Stocking rates need to be adjusted so as to retain the recommended minimum levels of groundcover.
In serious drought paddocks should be destocked, or stock confined to yards or to “sacrifice” paddocks on the more stable soil types for feeding. These sacrifice paddocks should be spelled and/or rehabilitated afterwards, if necessary by means of soil conservation earthworks.
Retaining crop residues will best protect cropping soils going into and during drought. If, despite management, soils do become bare, rough ploughing to bring up clods may reduce wind erosion, and contour ripping is useful to reduce runoff and water erosion.
Following drought, attention should be given to –
- Delaying re-stocking sufficiently to allow natural regeneration of native pasture species
- Re-sowing pastures and/or pioneer cover crops
- Fertilising to replace lost plant nutrients
- Rehabilitating eroded areas
Walker, PJ (1992). Managing for Drought. Technical Paper No. 1, NSW Department of Conservation and Land Management, Sydney.