Ground cover protects soil from erosion, encourages microbial activity and increases the water holding capacity of soils. A minimum of 70% ground cover is the accepted benchmark for high-rainfall zones, except for sloping country where close to 100% is needed. For semi-arid areas (200–300 mm) a minimum target of 40% is desirable.
Soil organic matter levels are mainly influenced by climate and soil type (the main drivers of plant growth) erosion, cultivation and fertility, making it almost impossible to set target levels, but for soil health, more soil organic matter is always better. As a rule, targets for low-rainfall environments are more than 2% and more than 3% in high-rainfall areas. Woodland generally maintains higher organic matter levels than pastures, while pastures contain more soil organic matter than land cultivated for crops.
Soil can take thousands of years to be replaced through the weathering of parent material, highlighting the need to protect the soil asset by maintaining ground cover. Where topsoil has been lost through erosion, the only way to restore some of the productive capacity is by building up organic matter from plant and animal residues.
One millimetre of topsoil lost by erosion equals about 10 tonnes of soil per hectare. Depending on soil type and paddock history, this could represent a loss equivalent to 130 kg/ha of organic carbon, 20 kg/ha nitrogen (i.e. 40 kg/ha urea) and 8 kg/ha phosphorus (i.e. 100 kg/ha single super).
Rainfall erosion is most likely to occur during heavy rainfall events where ground cover is low, as it minimises raindrop impact and water loss through run-off. Litter on the soil surface can also reduce evaporation by up to 1 mm per day.
Ground cover also decreases wind speed across the paddock, reducing wind erosion and nutrient loss. Wind erosion is most likely to occur during drought or dry seasonal conditions.