The wind change on Friday the 22nd February provided a reminder of how critical the level of vegetative cover has on the protection of the soil surface. The gusty wind change whipped up dust from grazed legume stubbles, bare fallows and stock camps with a considerable amount coming off district roads and bare laneways.

Dust from gravel roads is largely uncontrollable whereas soil movement from paddocks is under the management of landholders. Grazed grain legume stubbles have always provided a challenge for landholders to balance the level of soil protection and the feed value of the residual grain after harvest. Last year the minimal carryover of the 2006 cereal stubble into the grain legume combined with the lack of summer rain to germinate summer weeds and to crust the soil surface, now have grazed grain legume stubbles vulnerable to soil loss.

The image of fences buried by moving sand fortunately is becoming less each year as better management practices are adopted. The amount of soil moved off paddocks with heavier soil types is often much less than that of the lighter soil types. It’s the finer clay fraction of both soil types that is often forgotten about.

Analysis of the fine dust or clay particles blown onto window sills of a house downwind of heavily grazed grain legume stubble in the mid north had an available phosphorous reading of 400 parts per million. The upwind paddock had a standard soil test to ten centimetres of 30 parts per million available phosphorous. The test on the dust sample highlighted the value of the clay fraction of the upper topsoil as a nutrient bank for crop and pasture production.

The current high price of fertiliser and reduced availability further emphasises the value of the nutrient bank in the topsoil.

Currently in most districts the cereal paddocks have cover levels which would be considered adequate to protect the soil surface. Good management and the lack of summer rainfall causing microbial breakdown of cereal stubble has contributed to the current cover level.

Some rules of thumb are that a minimum of 50% vegetative cover of the soil is required on sands prone to wind erosion with at least 1.5 tonnes per hectare of stubble. In sloping land prone to water erosion, the minimal cover required is 75% soil cover with 3 tonnes per hectare stubble.

These levels of cover should be maintained right through to the early crop establishment, which normally occurs sometime in May or June on average. In regenerating pastures the interval may be shorter depending on the break of the season and grazing pressure.

AUTHOR: David Woodard, Soil & Land Management Consultant, Rural Solutions SA
CONTACT: David Woodard, Rural Solutions SA Nuriootpa Office, Telephone: 08 8568 6412 Mobile: 0417 803 525 Email: woodard.david@saugov.sa.gov.au