In the shadow of sandy hills and ridges, large bare swathes of sodden and scalded soil are increasingly appearing across the cropping landscape in the Mallee region.
This complex blight to grains productivity and profitability, known as 'Mallee seeps' land degradation, is predicted to increase 10-fold in the next five to 10 years to affect 20,500 hectares of Mallee cropping country on South Australian and Victorian farms.
The potential for lost productivity on 20,500ha due to seep degradation is estimated to cost growers $95 million over a 10-year period, based on an average crop yield of 1.7 tonnes/ha and an average value of $250/t.
These are the findings of a 2017 Mallee Sustainable Farming (MSF) survey in collaboration with a four-year research project funded by Natural Resources SA Murray-Darling Basin (SAMDB).
The project from 2015 to 2018 also explored the dynamics of the degradation issue at four seep-monitoring sites established on growers' properties between Karoonda and Mannum, in SA's Murray Mallee region, in 2015.
One of these ongoing research sites is hosted by the Bond family, who run a 2960ha grains-focused operation, about 12 kilometres east of Mannum, SA.
Leading the project, Chris McDonough, from Insight Extension for Agriculture, says monitoring at the sites showed three main factors contribute to seep development.
These are rainfall, landscape (topography and soils) and changed on-farm cultural practices.
More specifically, high rainfall events, particularly outside the growing season, quickly move down through the deep, often water-repellent, sand profile of sandy hills and rises until it hits an impermeable layer of clay in the soil's subsurface layers, Mr McDonough says.
This water moves laterally to lower parts of the landscape, where it forms a 'perched water table', causing the degradation issue to surface in mid-slope areas and at the bottom of swales (where low permeability clay occurs within one to three metres of the soil surface).
Mr McDonough says the topsoil consequently becomes waterlogged, sometimes with water ponding on the soil surface, leading to bare patches of soil. And over time, evaporation and capillary rise also result in surface salt accumulation that manifests as dry scalds and permanent land degradation.
Another major factor contributing to seep formation is more effective chemical control of summer weeds as part of modern intensive cropping systems, he says. In other words, there are fewer summer weeds to intercept and absorb rainfall - an effect that sees more water contribute to recharge, which in turn, spills into seeps.
Mr McDonough says three high-rainfall years in 2010, 2011 and 2016 were the main triggers for the geographically widespread, increasing emergence of seeps on Mallee farms, largely reported by growers in the past 10 years.
He says that following large rainfall events of 25 to 30 millimetres or more, water collected in large on-farm catchment areas tends to moves down through the soil profile, inundating the perched water table that flows into seeps.
But Mr McDonough adds that on non-wetting sands with low water-holding capacity, even small rainfall events as low as 10 to 12mm can cause the perched water table to rise in lower parts of the landscape.
This knowledge of the factors contributing to seep formation was used to inform the development of management strategies. These were studied over a three-year period at the on-farm research sites in SA.
The four main management strategies examined were:
- introducing high water-use crop enterprises, such as growing lucerne for hay and livestock grazing;
- intercepting the lateral movement of the perched water table through the soil profile into seep areas, such as growing strategic strips of lucerne and establishing belts of native trees;
- sandy soil amelioration, such as spading chicken manure, to help improve the soil's water-holding capacity and fertility, plus promote deeper root penetration; and
- increasing use of excess soil moisture, as saturated seep areas develop, by establishing saltbush, pasture species and summer crops (sorghum and millet) which helps maintain ground cover on soils and reduce surface salt accumulation.
Now, building on the earlier research findings, and continuing the work developing these and other management strategies is a new $1.4 million collaborative research effort to better understand and help tackle the problem facing Mallee growers.
Led by Mallee Sustainable Farming, this four-year project from 2019 to 2022 has expanded the scope of the seeps research to include Victorian farms, in addition to the ongoing study of SA farms, including the Bonds' property.
For example, four new monitoring and trial sites were established on seep-affected farms in the Victorian Mallee this year.
A development that is supported under the new project with co-investment from the National Landcare Program's Smart Farming Initiative, GRDC and the South Australian Murray-Darling Basin Natural Resources Management Board.
Mr McDonough says the project aims to model seep formation; identify high-risk areas and seasonal indicators; develop approaches for early detection; trial plant species and strategies suited to seep prevention and management; and determine best practice for remediating old seeps.
"The project is working with farmers to develop the most practical and effective methods within their farming systems to overcome this growing problem," Mr McDonough says.
To achieve these objectives, the project draws on research partnerships with CSIRO, Mallee Catchment Management Authority, Insight Extension for Agriculture, AGRIvision, Coorong Tatiara Local Action Planning Association and the University of Adelaide.