Key points

• Crop nutrient requirement ‘rules of thumb’ are unlikely to be as effective this winter (2023)
• Potentially high and varied nutrient losses could be found across flood-affected and waterlogged paddocks
• Soil testing before rebuilding nitrogen levels is a good idea
• Agronomists and other grains specialists say the impact of the recent La Niña cycle is unprecedented, and growers should not be afraid to reach out to them about the next steps

With a prolonged La Niña cycle creating waterlogging and flooding across much of the northern region, nitrogen losses are to be expected across many farms and paddocks, affecting the 2023 winter season.

The challenge in rebuilding the soil nitrogen bank – after recent above-average crop removal, potential leaching and denitrification – is knowing the baseline. Soil testing offers a way to establish that for 2023 crop nutrition decisions.

GRDC grower relations manager (north) Graeme Sandral says soil tests are one of the best ways to assess how much plant-available nitrogen (in the form of nitrate and ammonium) remains in the soil. Testing becomes even more valuable if there is a paddock history of mineral nitrogen. “This helps reference the new results and provides additional confidence in subsequent nitrogen management decisions.”

Mr Sandral says soil sampling to at least 60 centimetres is preferred and could be particularly important in areas that have been fallowed because nitrate could be deeper in the profile for lighter soils.

Experts in soil nutrition came together in late 2022 to talk about nitrogen budgeting after the extraordinarily wet 2022 season at a GRDC Research Update. Speakers included the University of Queensland’s Professor Mike Bell, Incitec Pivot’s Bede O’Mara, Dr David Lester from the Queensland Department of Agriculture and Fisheries (DAF) and Back Paddock’s Dr Chris Dowling.

Dr Dowling says that timing will be important for logistics. “I would say we’d need to start testing by March or April. We might not be able to go earlier because some paddocks may still be waterlogged. And in those paddocks, we won’t know what the soil organic matter can contribute – because that will not happen until the soils dry a bit and there is oxygen in the soil.”

Dr Dowling says that to formulate an effective soil testing strategy, knowing where to sample will depend on factors that create significant differences in soil nitrogen.

“The way I think about it is that a previous crop will have an influence, as would whether it was harvested or not. So, it’s good to test a percentage of paddocks fallowed from cereals, oilseeds and pulses. Then you need to factor in major changes in soil type.

“The severity of waterlogging also needs to be considered. Were some paddocks badly inundated and some not? You want samples that can give you a cross-section of the effect of these influences on soil-based crop nitrogen supply.”

Mr Sandral agrees. “There could be great variability in nitrogen across farms and in paddocks. Waterlogging and denitrification can be extremely variable, leading to significant differences in plant-available nitrogen availability.”

He suggests grain yield and/or protein maps combined with deep soil nitrogen testing will help match the 2023 crop demand for nitrogen. “There will be a greater reliance on bagged fertiliser nitrogen for the coming season because of large crop nitrogen off-take in the previous two seasons and denitrification. This will be challenging for grain growers in our high-price environment.”

Having a farming system that has a nitrogen-fixing phase will often reduce input costs, he says. “In these unusual circumstances of excess water and high nitrogen prices, areas south of Dubbo could consider giving up some stored water to grow vetch as an early summer-sown brown manure crop, terminated at four tonnes a hectare or 80 units of fixed nitrogen.

At $3 per kilogram, its potential value is $240/ha and approximately 40 units of this nitrogen will be taken up by the 2023 crop. It will be a matter of doing the maths with your agronomist to inform your decision, weighing up costs and the water use.

Dr Dowling says that if bagged fertiliser is used, the minor differences in the agronomic efficiency of fertiliser products will be swamped by the need to get an effective nitrogen rate on paddocks.

“The product type will make little difference where we have such a screaming deficiency, other than where application logistics favour a particular product.”

He says that soil mineral nitrogen levels will also be low following high-yield crops. “We had big crops for the past two to three years post-drought, so we really are starting at rock bottom.”

Toowoomba-based DAF researcher Dr David Lester says that soil coring work, done as part of a research project in late 2022, had found ‘skinny’ profiles.

"Soil coring work up and down the country shows no one has any mineral nitrogen. That is going to be the challenge this coming season."

Dr Lester says another key to nitrogen budgeting is understanding realistic yield potential. “This year there is good potential with high moisture profiles. We want to convert as much of that water into yield as we can. The challenge is nitrogen – getting it from a bag, into the soil and into the plant. Buy a spreader, book the plane, and chuck it on. Once the plant has it, it will run hard on that.”

Professor Bell says nitrogen needs to be in the soil, not sitting on top of it. “Try to time applications so that there is some dry soil to wash the applied nitrogen into.”

Above all, Dr Dowling says not to be afraid of seeking agronomic or other help. “There will be lots of questions. Don’t be afraid to ask them. I’ve been doing this for more than 40 years and this is unprecedented. The 2010-11 floods are a fairly close comparison but are nothing close to this extent. We are all having to work through this together.”

What is leaching?

Nitrogen leaching is a physical process that occurs as nitrate, which is negatively charged, does not bond to most soil particles and so moves with water as it drains through the profile.

Professor Bell says acidic soils can be an exception. They have an anion exchange capacity – an ability to hold negatively charged anions such as nitrate and sulfate – that will slow leaching.

He says the key factor determining the amount of leaching is how rapidly water drains through the soil profile.

“Whether leaching leads to losses is dependent on whether the leaching process moves nitrogen deeper than the crop root system can retrieve it. Or, from a yield perspective, whether the nitrogen is so deep that by the time crops start accessing water and nitrogen from those deep layers, yield potential is well and truly established.”

Mr Sandral says that in many low-pH surface soils, nitrate typically moves at about half the speed of the wetting front. “Where growers need to retrieve deep mineral nitrogen, canola could be the best option. It will root to three to four metres from an early April sowing in non-constrained soil.”

What is denitrification?

Denitrification is a naturally occurring process in which bacteria in soils with low oxygen availability are forced to consume the oxygen molecules from soil nitrate. This allows nitrogen gases to form and escape from the soil.

Losses from denitrification are normally low in dryland winter crop systems but can be higher during long periods of wet or waterlogged conditions, particularly as soils warm in spring. The denitrified soil nitrate comes from the mineralisation of soil organic matter and nitrogen fertiliser.

Professor Bell says it is important to remember that denitrification requires three things: “You need an elevated soil nitrate concentration, wet and warm soil conditions, active microbes that feed off carbon and other nutrients found in decaying crop residues and roots, and low soil oxygen. If you only get some of these, you will not get a significant denitrification loss.”

More information: Graeme Sandral, 0409 226 235, graeme.sandral@GRDC.com.au; Chris Dowling, 0407 692 251, cdowling@backpaddock.com.au; GRDC Grains Research Update, online - Nitrogen decision-making for summer crop; Strategies for long-term soil nutrient management