A PROJECT looking at optimising high rainfall zone cropping for profit has found that while canola was highly responsive to nitrogen, the majority of other macro and micro nutrients did not have an impact on yield.
The results of the trial, which ran over 2020 and 2021 in Esperance, were shared at the recent Department of Primary Industries and Regional Development's (DPIRD) Grain Industry Day in Perth.
The project, which was a collaboration between DPIRD, CSIRO and FAR Australia, made possible by investment from the Grains Research and Development Corporation (GRDC), started in 2019 with a series of workshops at Dandaragan, Esperance and Albany.
One of the key questions that came out of those workshops was the idea of nutrition - if growers have addressed subsoil constraints, are using hybrid cultivars and sowing early, then does the nutritional package have to change as they start to achieve yields of 3-4 tonnes a hectare?
If nitrogen is pushed to achieve great yields, then do other macro and micro nutrients start to become limiting?
In order to answer that question, a trial was set up last year with plots of different nitrogen rates - 15 units, 100, 200 and 300 - with a range of treatments of other macro and micronutrients, including phosphorus, potassium and sulphur, applied within each nitrogen rate.
DPIRD research scientist Jeremy Curry said they were hoping to see, particularly as they pushed yields higher, if those nutrients would become limiting or if nitrogen was still the main driver.
"In the 2020 trial at only 15 units of nitrogen we were really surprised to see how good the yield was at 2.3t/ha and when we looked at how much nitrogen we were getting from the soil, it was probably double what we were expecting," Mr Curry said.
"There was a great yield response to increasing our nitrogen up to 100 units with that going just about 3t/ha, while at 200 units we were around 3.5t/ha and plateaued after that."
The standard grower practice is typical at around 150 units in a lot of the high rainfall zone, with the data from 2020 suggesting that is about the right mark.
However, the trial was also looking at whether other nutritional factors would become limiting and the simple answer to that in 2020, was no.
"When we applied a reasonable rate of all of these nutrients to our base treatment, there was nil difference between them, nor was there any difference in any of the other combinations of macro nutrients that we applied," Mr Curry said.
"Even when we were applying ludicrous rates of those macronutrients, there was a negligible yield response and the key thing we actually found with that was the impact on plant establishment.
In the 2021 trial, overall yields were a bit lower due to the water logging pressure and this year the nitrogen rates were set at 100 units, 150, 200 and 250.
Once again there was a relatively high yield response of going from 100 units of nitrogen (about 1.9t/ha) up to the 150 units mark (2.5t/ha), with the response again plateauing after that.
However, what was evident in 2021, after the onset of flowering around July, was that after three months of wet weather, some deficiencies in the crop did start to appear.
Mr Curry said the key difference between healthy plots and those that had discolouration of flowers was the sulphur that they applied.
"We expected that we might see a deficiency and we were seeing these symptoms and come October the plots with a lack of sulphur hadn't set much yield and were still flowering, trying to seek more yield," he said.
"In the three plots where we didn't apply any sulphur at the 100 units rate, there was a negative yield of 300 kilograms per hectare that was lost by not applying it and as the yield potential was pushed higher with higher rates of nitrogen, we cost ourselves close to 1t/ha of yield by not applying sulphur.
"However in terms of the other macronutrients, even when we applied nil phosphorus we weren't seeing a yield response."
Overall in both years, the sites were highly responsive to nitrogen, however in 2020 there was minimal response to other nutrients and in 2021 there was a large sulphur response.
In light of high fertiliser prices, soil test results remain the most useful way to indicate areas of best return.
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