Sandy soils are common across much of the grain growing regions in Southern and Western Australia. More than 5 million hectares of this land is affected by water repellency.
Water repellent soils prevent normal infiltration of water, which either pools on the surface and evaporates, or moves down 'preferred pathways' leaving large volumes of soil dry. Uneven wetting of soils causes poor germination of crop, pasture and weed plants and increased risks from wind and water erosion.
Water repellency is caused when hydrophobic (water repellent) 'skins', made from plant waxes and other products from the natural process of plant biodegradation, form around individual sand grains. These waxy skins effectively repel water from the soil and limit water availability to the crop.
Estimated losses for crop and pasture production due to water repellent soils exceed $100M.
Bacteria may hold the key
In the 1990s Dr Margaret Roper, a CSIRO soil microbiologist in Perth, identified different types of bacteria that help break down the wax barriers which prevent soils absorbing water.
The bacteria, from the actinobacteria group, are common soil bacteria that decompose organic matter returning important nutrients back to the soil, but they need a moist environment to be effective.
Boosting the effectiveness of the bacteria
Several farmers in Western and Southern Australia had reported that lime improved water infiltration into their soils. Field trials conducted in south-western Australia by Dr Roper revealed a ten-fold increase in the number of wax-degrading bacteria after an application of lime.
This resulted in a significant and substantial reduction in water repellency for at least four years. Lime is relatively cheap and easy to apply and has the benefit of raising soil pH to levels that favour other soil micro-organisms also important in soil health.
Reducing disturbance of the soil increases infiltration
Sandy soils are highly susceptible to erosion which has led to the adoption of farming practices such as zero-tillage and stubble retention. Stubble retention is a double edged sword as the extra biological material will produce more waxes increasing water repellency, however, several farmers in the South Coast region of Western Australia are successfully managing their water repellent sands with zero-tillage and stubble retention to grow excellent crops.
In a three year study investigating water infiltration, water repellency, soil carbon and crop performance in sandy soils, a research team led by Dr Roper and Dr Phil Ward studied different crop management treatments comparing zero-tillage vs. cultivation and stubble retention vs. stubble removal.
The team chose three field sites at which to run their tests. Tillage and stubble management impacted soil carbon, water repellency and soil water content at all sites. Measurements of soil carbon content were highest under zero-tillage and stubble retention which corresponded to an expected high level of water repellency as measured in the laboratory.
Surprisingly zero-tillage and stubble retention also produced the highest measurements in the field of soil water content, contradicting the water repellency findings. This indicates there are mechanisms other than water repellency involved in water infiltration into the soil.
The team hypothesise that bio-pores (holes or tunnels in the soil) formed by roots and small animals provide pathways for water to move through the soil to where it is needed. Zero-tillage preserves these bio-pores increasing the ability of water to infiltrate the soil.
This research highlights the importance of field experiments and measurements rather than relying on laboratory measurements alone.
In a new GRDC funded project, the team will further explore the mechanisms and strategies which increase water infiltration in water repellent soils. They will also focus on investigating water repellency in and around the furrows where the crop is sown.
This project will be conducted in collaboration with Dr Stephen Davies of the Department of Agriculture and Food in Western Australia.