Utilising soil biological processes to better manage water repellent soils

Biological degradation of waxes

Research undertaken by soil microbiologist Dr Margaret Roper (CSIRO) has identified soil micro-organisms that are capable of breaking down the waxes and hydrophobic organic materials that cause soil water repellence. Enhancing the breakdown of repellent soil utilising these microbes has focused on two possible approaches:

1. Enhancing the activity of wax degrading organisms already present in the soil by providing an improved soil environment for them.
   • Improving the fertility of the sand medium using slow release fertilisers has led to a reduction in the time required for micro-organisms to break down waxes. The application of slow release fertiliser significantly reduced the severity of water repellence in laboratory studies in the absence of plants. However, in the presence of plants the initial reduction in repellence could not be sustained.
   • Applications of lime have been used to improve the soil pH of acidic soils improving the soil environment for these wax-degrading microbes. In field trials application of lime to acidic topsoil resulted in marginal reductions in the severity of water repellence and increased the population of wax-degrading microbes. Other factors such as soil moisture and competition from other soil microbes still limit the capacity of these micro-organisms to degrade the water repellence.
   • The activity of the wax-degrading microbes is imporved when the soil is already moist. Research has shown that there are more wax-degrading microbes in the previous years crop row than on the inter-row (see below) and this may in aprt be driven by improved water infiltration in these areas.
2. Specific strains of wax-degrading actinomycete bacteria have been isolated and found to significantly reduce the repellence from severe to low levels in laboratory experiments under ideal conditions. However, reducing repellence by inoculating paddocks with these wax degrading organisms is limited by competition from other soil micro-organisms and insufficient soil moisture to maintain effective populations and activity throughout the year.

Utilising plant root systems for improved water infiltration into repellent soils

Intact plant root systems, both living and dead, can act as preferred pathways for water entry into water repellent soils. The root systems are often still attached to the plant crown at the soil surface and extend through the repellent topsoil into the subsoil. The interface of the surface of the root and the soil appears to be a pathway along which water can infiltrate, provided they are intact and there is continuity from the soil surface through the repellent topsoil.

This has several implications for how this mechanism for water entry might be managed and exploited:

1. Maintain intact remnant root systems from the previous years crop or pasture as well as pre-existing biopores and preferred pathways for water entry by using minimum and zero tillage seeding systems.
2. Minimise traffic and driving on stubbles after harvest to maintain upright stubbles and intact plant crowns with attached roots.
3. Include crop and/or pasture species with dense fibrous root systems in the rotation as roots of this type may provide more pathways for water entry into repellent soil.
4. Using minimal disturbance methods seed onto or right alongside the previous years crop row (Figure 1) so that seed is sown into soil that wets up easier as a result of more rapid water infiltration into the previous row. This can require accurate guidance and some growers have fitted steering systems attached to the seeders drawbar while others have achieved some of the benefit by seeding at a slight angle across the previous years crop rows. Coulters may be useful for stubble management. It should be recognised that there will likely be other agronomic consequences both good and bad from seeding on the previous years row, particularly when it comes to soil-borne diseases.

Figure 1 Canola establishment on water repellent gravel improved when seeded in the previous years crop row (left of image) compared to establishment when seeded between the previous years crop row (right of image)

In highly repellent soils it can be difficult to establish enough plants in the first instance to get sufficient root density in the soil to be pathways for water entry. Some other intervention such as use of banded wetting agents, improved furrow sowing or soil amelioration such as rotary spading or soil inversion may need to be undertaken first to improve plant establishment and root density. It is hoped that once root density is increased and sustained over a number of seasons that this may be a relatively sustainable method for improving water infiltration and crop establishment in water repellent soils.

Acknowledgement

Soil water repellence research is supported by DPIRD and the Grains Research and Development Corporation (GRDC) through DAW00244 Delivering enhanced agronomic strategies for improved crop performance on water repellent soils in Western Australia and other initiatives. The input of Margaret Roper and Paul Blackwell is gratefully acknowledged.