Nitrogen limitations hurting yield in high rainfall cropping zones

However, for farmers in high rainfall zones (HRZs), such as the Southern Highlands in NSW or Victoria’s Western District independent research has found there are indeed significant portions of cropland where yield is constrained by nutrient limitations, not water, as is the case in most parts.

A Grains Research and Development Corporation (GRDC) research investment has found that under-fertilising in the HRZ appears to be a major cause of yield gaps – the difference between the actual yield achieved by a grower and the water-limited yield potential.

The research, led by Malcolm McCaskill, a soil scientist with Agriculture Victoria, has established that by providing sufficient nutrients, wheat and canola yields could be equal to or even exceed the water-limited potential, except in cases of severe waterlogging or drought.

Rob Norton, a fertiliser research consultant with Norton Agronomic, said HRZ areas occupied a unique place in the Australian cropping industry as it was nutrition, rather than water, that was the primary constraint on yield.

“This gives growers in those areas an important advantage – you can’t do anything about a lack of moisture but you can do something about a lack of nutrient,” Dr Norton said.

He said farmers should concentrate on managing nitrogen (N).

“The yield loss is particularly correlated to low nitrogen levels,” he said.

“Often, not always, paddocks in traditional grazing areas have a good P (phosphorus) history but N levels are lower.”

Dr McCaskill said the reason for the widespread under-application of fertiliser could be due to the bulk of Australia’s agronomic guidelines being developed for low and medium rainfall areas.

“These guidelines may not be appropriate for the HRZ, given its higher yield potential,” he said.

He said the fear of poor seasonal conditions was a big player in fertiliser decision making.

“Some growers are reluctant to use the economic optimum fertiliser rate, because seasonal conditions and variable prices make returns far from certain,” he said.

“This conservative approach limits the downside but also restricts the potential to exploit favourable conditions.”

Dr Norton agreed.

“It does seem farmers can be reluctant to put out the amounts of N in particular to target their true yield potential,” he said.

He said farmers who were still not convinced of the benefits of higher N applications should conduct their own trials.

“Run a test N strip in the paddock and see how it goes, then you can believe what you see with your own eyes.”

Dr McCaskill has also come up with tools to help growers calculate their nutrient needs.

Together with his research team, Dr McCaskill has produced three Excel-based decision-support tools to determine the economic optimum application rate of N, P,  potassium (K) and sulphur (S) under a range of conditions.

“These tools could help growers estimate the most economic rates of fertiliser,” he said.

In terms of ways of assessing whether N has been historically low, Dr Norton said lower protein levels were a sign that the crop had been limited by a lack of N.

“If the protein levels are below 8.5, 9 per cent in wheat then with most varieties I would say you are losing yield.

“When the protein levels are below 8pc you could be losing as much as 25pc of your yield as well having lower quality grain,” he said.

Dr Norton said there had been a lot of talk about the impact of micro nutrient deficiencies on crops in HRZ systems.

He said as a rule he would focus on getting the N, P, K package right before looking for answers at a micro-nutrient level.

“We haven’t seen too many cases of damaging cases of micro-nutrient deficiencies, I would recommend checking the major players first before investigating micro-nutrients as a cause of lost yield.”