Trials test soil nitrogen and water-holding efficiency
GroundCover™ Issue: 117 | Author: Brad Collis
Research is being stepped-up into the efficiency of different soils in delivering nitrogen to crops
Research into nitrogen and the water-holding capacity of soils across Western Australia’s wheatbelt is intensifying as part of a concerted effort to lift the resilience of cropping as annual rainfall averages continue to decline.
In particular, the research is seeking to better understand nitrous oxide (N2O) emissions from different soil types, while also providing data to improve the local application of crop-modelling programs such as Yield Prophet®.
N2O makes up most of Australian agriculture’s greenhouse gas emissions and the Australian Government’s Action on the Ground (AotG) project is supporting on-farm trials to better match nitrogen applications to soil type. This has both environmental and economic considerations.
In WA, Esperance-based Precision Agronomics Australia is leading a three-year AotG project looking at how nitrogen applications can be better tailored to soil type and production potential. The research is being undertaken in collaboration with the Department of Agriculture and Food, WA (DAFWA), and CSIRO.
Project leader Frank D’Emden says the research is following on from the earlier GRDC-supported water use efficiency program and Agronomy Jigsaw project.
“This work created a heightened awareness of the need to better understand what is happening at depth in the soil, with the GRDC, DAFWA and CSIRO continuing their support in this area through major projects aimed at mapping subsoil constraints and understanding soil water,” he says.
“There are plenty of stories of people digging into the ground after a harvest affected by a dry finish and finding moisture not reached by the crop roots due to particular soil constraints. The idea now is to map these constraints by soil type.”
Mr D’Emden says previous research and work undertaken for clients has already found strong correlations between subsoil constraints and electromagnetic (EM) and radiometric data: “Different soils have different EM and radiation signatures, which are correlated to a soil’s particular chemical and physical characteristics,” he explains.
“This is allowing us to build an objective picture of what is happening in the soil, across the cropping landscape.”
Mr D’Emden’s three WA trial sites are on grower Deane Aynsley’s property east of Beverley, Bevan and Karyn Tuckett’s property north of Ravensthorpe and at a Lawson Grains property at Munglinup. The trials comprise a geophysical survey of paddocks using EM and radiometrics, ground-truthed with deep soil tests to map the soil types. Soil moisture probes, automatic weather stations, an unmanned aerial vehicle-mounted infra-red camera, yield monitors and gas flux chambers (to measure N2O) round out the arsenal of monitoring equipment for the project.
“The main purpose is to map the different soil types for plant-available water capacity, assess the responsiveness to nitrogen in soils with different water-holding capacity, and monitor N2O fluxes across different nitrogen treatments,” Mr D’Emden says.
The soil tests for estimating water-holding capacity are going as deep as 80 centimetres: “Although for pH and potassium, we are looking more at the topsoil and mid-soils (10 to 30cm), where you often start to find rooting depth constraints.”
The nitrogen surveys are part of a broader assessment of N2O emissions included in the AotG project’s check on agriculture’s contribution to greenhouse gases.
He says this information could lead to a practical guide to more efficient levels of starter and, possibly, top-up nitrogen for different soil types.
“2014, the first year of the three-year project, showed us that N2O emissions vary more by soil type than by the amount of nitrogen applied. However, the variation wasn’t consistent. For example, N2O emissions from the lower-yielding deep sand at the high-rainfall site (Munglinup) were eight times higher than for high-yielding deep clay loam. Yet just 70 kilometres away at the low-rainfall site north of Ravensthorpe we observed N2O emissions up to 25 times higher on the high-yielding calcareous loam compared with the low-yielding sandy duplex soil,” he says.
Each of the trial strips cross different soil types as mapped by EM and radiometrics, with observations in 2014 showing significant differences in soil moisture, temperature, pH and organic carbon between the soil types at each site. “All these factors influence N2O emissions, so unravelling the data to see how they interact at each site over three years is going to be a big job. However, the end game is to find ways to use this data to improve nitrogen management throughout the growing season.”
There were also significant differences in yield, biomass and protein between each soil type, with protein showing the most significant difference between nitrogen treatments.
Mr D’Emden says the overarching objective is to lift productivity by optimising inputs for different soils’ yield potential: “It is not about aiming for that extra tonne because in a drying climate that just increases risk. This is about optimising quality and productivity, and reducing risk.”
More information:Frank D’Emden,
This project is funded by the Australian Government Department of Agriculture’s Carbon Farming Futures Action on the Ground project.