Phosphorus behaviour in deep bands
- Deep-banding of phosphorus has a variable impact on crop yields.
- Research at the synchrotron in Thailand has enabled researchers to better understand the fate of phosphorus in deep bands in Australian soils.
Australian researchers are a step closer to understanding the fate of deep-banded phosphorus (P) after analysis using state-of-the-art synchrotron technology in Thailand.
In the bid to close the yield gap, growers on the clay soils of north eastern Australia are turning to deep-banding of fertilisers - such as phosphorus and potassium (K) - to address declining soil nutrient reserves in subsoil layers.
However, results in the field have been variable, ranging from substantial yield increases to no observable response, depending on soil types and fertiliser sources.
Although combining phosphorus with potassium can benefit potassium recovery in deficient soils, it may also alter the efficiency of phosphorus recovery from deep bands.
This variability may potentially result from unpredictable behaviour of the abnormally high concentrations of phosphorus typically applied in the deep bands.
Higher rates of fertiliser are used because banding occurs periodically, rather than each year.
These variable responses make the profitability of deep-banding unclear.
To better inform recommendations, GRDC has invested in research at the University of Queensland into the behaviour of phosphorus in bands and how this impacts on plant availability.
The unique analysis available at the synchrotron in Thailand has brought us a step closer to understanding why the impact of deep-banded phosphorus on crop yield varies.
These analyses are not simple. The potentially plant-available phosphorus can be estimated using traditional laboratory tests based on chemical solubility, or measured using isotope methods based on exchangeable phosphorus.
Both approaches tell us how fertiliser application strategies affect plant availability, but they do not help to understand the underlying chemical reactions that cause these differences.
The most advanced non-destructive technique available for measuring chemical structures in agriculture is X-ray absorption spectroscopy (XAS), using a synchrotron facility.
A synchrotron accelerates electrons to almost the speed of light, and then uses these electrons to produce X-rays with special properties.
Each chemical element in the sample absorbs these X-rays differently and emits a unique fluorescence photon, allowing researchers to identify the chemical form of the element of interest in the sample.
This unique technology allows us to assess the chemical behaviour of phosphorus directly within the soil in and around the fertiliser band, as well as identify the fertiliser reaction products that are formed in different soils and with different fertiliser sources.
However, not every synchrotron can be used to study every element.
Importance of international collaboration
The Thailand facility is one of only a few internationally that have the specific combinations of spectra and energy range suitable for studying phosphorus.
GRDC's investment allowed us to analyse Australian soil samples in Thailand, in collaboration with Australia's Nuclear Science and Technology Organisation (ANSTO) and the Synchrotron Light Research Institute (SLRI), in Thailand.
We compared three different sources of phosphorus, with and without the co-application of potassium, in different soil types.
Samples were analysed one and four months after application to evaluate the fate of phosphorus across time.
Our work at the synchrotron has given us an insight into the mineral phases of phosphorus that are formed after deep-banding and will guide our future isotopic analyses aimed at understanding the effect on plant availability.
Access to the synchrotron facility in Thailand has taken our research forward in a way that would not have been possible if we had to rely on the traditional analysis methods only.
GRDC Research Code: 9175500
More information: Gregor Meyer, University of Queensland, 07 3346 7081, firstname.lastname@example.org