Do I or don’t I, and how much? Growers ponder the nitrogen equation

How much nitrogen fertiliser should I apply to my crops to achieve grain yield potential? Is it too late now to be applying nitrogen?

These are among the burning questions being pondered by grain growers in some regions as the window for top dressing crops begins to close.

The many questions growers have around in-season nitrogen application are raised and discussed in a new Grains Research and Development Corporation (GRDC) podcast, Good guidance on in-crop nitrogen application in 2020, that has been produced to assist growers with their fertiliser strategies.

The podcast draws insights, advice and tips from a leading agricultural consultant who contributed to development of the GRDC’s new A Nitrogen Reference Manual For The Southern Cropping Region, as well as a soil scientist and a grower.

Ag Consulting Co agronomist Bill Long, who had input into the new manual, says in the podcast that good nitrogen decision making, at this point in the season, is essentially about making sure the crop has satisfactory nitrogen to reach yield potential, which is set by soil water availability and the expected rainfall for the remainder of the season.

“A lot of growers are sitting on a fair bit of soil water and some of the seasonal forecasts are looking promising – these are two crucial factors that underpin a fairly aggressive nitrogen program,” says Mr Long, who is also a South Australian grower.

He suggests growers interrogate the new manual to help them understand what their yield potential might be and therefore how much nitrogen is needed to achieve that potential.

“Crops currently range in growth stage from mid tillering through to flag leaf emergence across the southern region of Australia. Growers will be asking: is it too late to apply nitrogen; how late can I put it on; how effective will it be if I don’t have a decent follow-up rain; how much mineralisation might I expect to get for the remainder of the season; how much nitrogen am I already sitting on; how much nitrogen did that bean crop fix last year that I’ve sown this wheat crop on?” Mr Long says.

“There are specific questions that growers need to be thinking about in that complex nitrogen decision matrix which is dealt with in the manual.”

Mr Long says the economics of nitrogen decision making can be challenging, especially where variability between paddocks can be significant: “When you’re considering your nitrogen inputs, should you target that maximum yield or should you consider targeting a yield that is a little less risky, such as 80 per cent of yield potential,” he says.

“There’s a component of risk that’s being considered in that decision you have to make in the next couple of weeks that reflects the fact that you may not get the favourable outcome you’re looking for. And there is a risk associated with applying too much nitrogen and then having a ‘haying off’ event, which can be potentially detrimental to the return.”

The new reference manual, available for viewing and downloading, is a comprehensive guide to understanding, managing and estimating nitrogen requirements from paddock to paddock and season to season, and includes information about the various nitrogen decision support tools available to growers and advisers.

Mr Long says it is important that growers explore these tools rather than relying on someone else’s rules of thumb which may not be applicable to their particular situation.

“It’s important to recognise that these tools are there to use, and from there growers and advisers can develop their own knowledge and sophistication in their thinking. The manual provides useful equations to help you understand some of the vital aspects around nitrogen decision making,” he says.

North east Victoria soil scientist Cassie Schefe, of AgriSci, says in the podcast that opportunities do exist for growers to top up nitrogen in favourable circumstances.

“If there is any doubt about whether growers have enough nitrogen in the system to meet yield potential, now is basically the last chance to have a rethink of nutrition programs and to decide if it’s worth putting on some more,” Dr Schefe says.

Dr Schefe says nitrogen profiles in cropped paddocks vary according to soil types and rainfall, as well as crop rotation history (ie cereals following nitrogen-fixing pulses).

“Under a higher rainfall system, we’re going to get more nitrogen movement through the soil, but on the flip side you’re more likely to be putting more nitrogen on because you’re going to get a bigger yield response. It’s that interaction of rain with soil type.

“In zones with higher rainfall but a lighter soil type, I wouldn’t be expecting or assuming that there’s still a lot of nitrogen in the system. Whereas, in a high rainfall area with a clay soil, there’s a higher expectation that the nitrogen would still be there.

“And in our low rainfall systems, we’re likely to get less movement of nitrogen to depth.”

Rutherglen (Victoria) grower Andrew Russell says in the podcast that matching nutrients to potential is vital in maximising his production this year, to take advantage of forecast spring rainfall and positive commodity prices.

“It’s all about making the best educated decision we can at the time,” says Mr Russell, who is also a GRDC Southern Region Panel member.

“We’ll keep going back and re-evaluating every fortnight until we get to a point where any nitrogen that we put on, after certain growth stage, is not going to realise a return on investment.

“The other important factor is the potential for a premium for higher protein wheats – that could also be a part of that decision-making process as to whether we do some late applications of nitrogen.”

A Nitrogen Reference Manual For The Southern Cropping Region was collated by a team from the University of Adelaide, University of New England, the University of Melbourne and advisers as part of a GRDC-invested project.

Contact details

For interviews

Bill Long, Ag Consulting
0417 803034

Cassie Schefe, AgriSci
0419 238798

Andrew Russell, Lilliput Ag
0417 401 004

Contact

GRDC Adelaide office Communications Manager – South
0409 674 100
media@grdc.com.au