Deep banding of phosphorus and potassium so the nutrients can be picked up by crops in later stages of growth is sound in theory, but in practice, positive grain yield results are proving variable.
Extensive ground-truthing of deep-banding trials across Queensland and New South Wales has shown that banding rarely lessens crop yield, but it doesn’t always mean a profitable increase either.
That’s a puzzle that Queensland Department of Agriculture and Fisheries (DAF) senior research scientist Dr David Lester and a team led by Professor Mike Bell, from the University of Queensland’s Alliance for Agriculture and Food Innovation (QAAFI), are now trying to work through as part of an ongoing project (UQ00078) funded by the Grains Research and Development Corporation (GRDC).
“We are aiming to develop a better understanding of how plant roots acquire deep nutrient in drier environments and how the plant uses that to affect harvestable yield in different crops in different seasons,” Dr Lester said
These complex questions arose from testing of a simple proposition in a long-standing project (UQ00063): if decades of scavenging by crop roots has run down nutrient levels deep in the soil (10-30 centimetres), shouldn’t banding replacement nutrients at the same depths boost crop yields?
Initial work in Central Queensland done with DAF regional research agronomist Doug Sands suggested an unequivocal “yes”.
Chickpea yields jumped when phosphorus (P) and potassium (K) were banded about 20 cm deep. Trial crops regularly added up to 500 kilograms per hectare to their yield in a year, and over three to four years the cumulative yield increase could be more than 1000 kg/ha.
However, results were not consistent, and that inconsistency became even more pronounced when similar trials were conducted in southern Queensland around Goondiwindi.
At Inglestone, north-west of Goondiwindi, a wheat-chickpea rotation only responded to starter applications of P. There was no in-crop response to deep-banded P and K.
“Not applying starter reduced grain yield by nearly 1000 kg/ha in three crops, so we’re confident that it’s worth growers continuing to use their starter application at sowing,” Dr Lester said.
But at Wondalli, east of Goondiwindi, dual applications of starter P and deep-banded P produced a cumulative 1500 kg/ha of extra grain from a wheat-sorghum system over two cropping seasons.
When the trials were extended into northern and central west NSW by Dr Tim Weaver and Rick Graham from NSW Department of Primary Industries (DPI), similarly inconsistent results were mapped. On some farms, the treatment quickly produced a highly profitable yield response. On others, it didn’t.
Research results also suggested that crops sown directly after the acute disturbance of deep tillage could suffer yield loss.
“Our lesson has been to get the deep treatments on as far in-front of sowing the next crop as possible to let the seedbed reconsolidate and not compromise establishment of the next crop,” Dr Lester said.
He said some of the variability is explained by background levels of P and K.
“In Central Queensland, these nutrients are typically in low levels at depth, so crops responded well to their mechanical replacement,” he said.
“Further south, there are slightly higher background levels of P and K, so crop response to supplementation is often not as significant as in central Queensland.
“This highlights the importance of soil nutrient testing before embarking on an expensive deep-banding program.
“That said, where evaluated, responses to starter fertiliser are demonstrable in most of our research sites where Colwell P in the top 10cm is low.
“Growers are encouraged to continue using starter P fertilisers at rates appropriate for the crop row spacing and soil moisture conditions at sowing. Applying small amounts of P in the seed row at sowing is offering excellent utilisation of the nutrient by the emerging crop.”
Dr Lester said yield increases with deep P application were predicated on a crops’ ability to access and utilise the nutrient in the band.
That may be to do with the location, or the nature of the crop itself. For instance, where there is less moisture fertiliser may not be able to diffuse from the band to the nearby roots, and where the crop root system is coarse (e.g. chickpea) the roots have proved less able to consistently use fertiliser banded at 50 cm row spacings than in fibrous-rooted grass crops like wheat.
At the same time, the trial results have raised some interesting discussion among researchers over the nutrient interaction effect – typically with nitrogen (N). This has been particularly evident in trials conducted in later years when the residual benefit of the bands is being assessed and where crops have not responded to P when expected to.
On closer examination, very low grain proteins indicated that the crops were N-limited, highlighting the simple message that crops will only improve yield performance up to the next limiting factor. Put simply, if there is only enough N for a two tonne wheat crop, growers won’t increase yields to 3t with extra P unless there is sufficient available N.
Getting a satisfactory answer to all these questions is the next step in a line of research that began in 2012, and which is still delivering a mixed bag of surprises.
For further information, download a copy of the GRDC Update paper Phosphorus and potassium nutrition from the research and development section of the GRDC website or by following this link.
DAF Manager (Media)
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Sarah Jeffrey, Senior Consultant Cox Inall Communications
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