A bold new direction for research into gravel soils could unlock the secret to this unforgiving and often underperforming soil type.
Scientists at The University of Western Australia (UWA) are breaking with traditional soil analysis protocol and including large gravel particles in their soil analyses in an attempt to better understand why these soil types are not producing higher-yielding crops.
Gravel soils, particularly ironstone gravels or forest gravels, are found across southern Australia from Western Australia through to South Australia and Victoria.
Across Western Australia's southern wheatbelt and Great Southern regions, these gravels are generally located in rainfall zones with annual averages of 500 millimetres or more.
However, these regions have traditionally been livestock-dominated because of the difficulty of growing crops on the soil type.
Unlocking soil potential
The co-director of UWA's Soils West, Professor Daniel Murphy, says unlocking the potential of these soils is driving this new research direction.
"When analysing soils, scientists and commercial laboratories sieve the soils to remove any particles greater than two millimetres in diameter," Professor Murphy says.
"In all our previous analysis we have ignored the stones and the gravel because they have not been classified as soils."
Researchers now believe these stones and gravel are not inert, or inactive, as previously thought, and can have a significant impact on the availability of nutrients to a plant.
Previous GRDC-invested research into these soils, led by WA's Department of Primary Industries and Regional Development (DPIRD) senior research officer, Dr Craig Scanlan, concluded a new direction was required to analyse this particular soil type.
Dr Scanlan's research found that ironstone gravel particles change plant available water and can act as a buffer for phosphorus supply.
Understanding large gravel particles
Under the guidance of Professor Murphy and the team of researchers at UWA, different ways to analyse these challenging soils will be developed in a GRDC-invested research project, to better understand large gravel particles - which had previously been discarded in soil analyses.
Professor Murphy says the team is focused on three different gravel soil types across southern Australia - gravels dominated by either iron, silica or aluminium.
The chemical and mineralogical differences in these gravel types are thought to influence water and nutrient interactions with the gravels. For example, silica-dominated gravels tend to be more porous.
The research team will work with growers and corporate groups to collect and collate soil samples, sifting through thousands of individual pieces of gravel, which will all be catalogued and sent for detailed mineralogical and chemical analysis.
Soil samples will be examined at the Australian Synchrotron in Melbourne, which uses a powder diffraction beamline, or high frequency x-ray, to study the crystallographic structure of materials.
Some gravel particles are hard and non-porous, but there are others that have a rind, like an orange.
"Gravels are not just gravels," Professor Murphy says.
"Some gravel particles are hard and non-porous, but there are others that have a rind, like an orange - with clay particles and minerals that coat the surface making the particle quite porous."
Professor Murphy says it is these porous gravels that require greater investigation.
"We want to understand the composition of the gravel - how the mineralogy and the chemistry interact with nitrogen, phosphorus and water," he says.
Following that, the team is hoping to develop a measurement tool that allows growers to have a better understanding of the nutrient and moisture-carrying capacity of the gravel particles.
"Each paddock is going to have different fertiliser histories, so we need some way to measure the saturation points, or the levels to which the gravels in each paddock have already bound up the fertiliser," Professor Murphy says.
"We suspect those paddocks with a long fertiliser history will have gravel particles that have less of an ability to bind up fertiliser than those paddocks that have received less fertiliser applications over time.
"In a sense, we are looking to see how full the bucket is, in terms of phosphorus, nitrogen and other nutrients."
For Dr Scanlan at DPIRD, this new approach to gravel soil analysis is also influencing current research projects.
"Our review identified that we need a consistent protocol for collecting gravelly soils for analysis, preparing them for analysis and measuring plant available nutrients, soil chemical and physical properties," he says.
"We have now adopted a consistent protocol for collecting and analysing samples from gravelly soils in all our current projects and we are keeping the whole soil sample and measuring gravel content and sizes using international standards."
Over time, researchers will build enough information on the physical properties of gravelly soils to assess how they are influencing yield response to applied treatments.
Professor Murphy says further research will investigate the movement of fertilisers and water through these gravel soils.
More Information: Professor Daniel Murphy, daniel.murphy@uwa.edu.au