If high levels of nitrogen are applied in a season that is considered favourable but crops subsequently fail due to frost, is there a chance that the nitrogen can carry over to the next season?

This was the question posed by growers in the Geraldton and Kwinana East port zones of Western Australia in 2022 following just that scenario in 2021. GRDC subsequently supported investment with Dr Darren Hughes from Laconik to investigate the situation.

“Research in the eastern states studying carryover or ‘banked’ nitrogen has shown a range of results,” Dr Hughes says. “Ranging from low rates of between one and eight per cent on soils around Wagga Wagga following droughted crops to the potential that nitrogen can be banked in the soils of southern NSW, but there has been little work on this issue in WA.”

Method

Through GRDC’s National Grower Network, 20 growers in the Geraldton and Kwinana East port zones established 40 farm-scale trials.

Using Laconik’s trial design, a ‘swarm’ of replicates was randomly allocated across paddocks (Figure 1). Individual plots were 50 metres long by 36m wide. The number of replicates per trial was governed by the size of the paddock and ranged from 10 at the Yelbeni trial site to 28 at the Bonnie Rock trial site.

Each grower was provided with a custom trial design that was uploaded to the variable rate controller in their seeders. As the seeders traversed a paddock, different rates of nitrogen fertiliser were applied in the form of either urea or liquid ammonium nitrate.

Each replicate contained four treatments arranged in a square as described in Table 1 and Figure 1. In March and July 2022 and January 2023, soil samples were taken from each site at intervals of zero to 10 centimetres, 10 to 30cm and 30 to 60cm. Samples were tested for nitrate and ammonium, and grain yield and economic analysis were assessed at harvest in 2022. Soil nitrogen and grain yield and economic results are presented for four of the 20 sites in Tables 2 and 3.

Dr Hughes says the results from these four trial sites show that the soils were nitrogen deficient prior to the growing season, early in the growing season and after the growing season. “As most of the sites had deficient levels of nitrogen at all depths it indicates the importance of assessing your nitrogen bank thoroughly to a depth of 60 centimetres.

Figure 1: Laconik's trial design at the Yelbeni site in 2022. Source Laconik

“The results show that on average for cereals there was a 620 kilogram per hectare yield penalty and $150/ha economic loss between Treatment 1 – nil nitrogen and Treatment 3 – grower practice nitrogen. Similarly, the yield loss for canola was 140 kg/ha and $52/ha economic loss.

“To maximise grain yield and economic returns in 2022 crops needed nitrogen; relying on nitrogen to carry over from 2021 was insufficient to meet crop demands.”

Dr Hughes says there is a theory within the industry that nitrogen applied to crops that get frosted is stored in the soil, but the soil test results from this project do not support this theory. Soil nitrogen was well below desired levels across all depths sampled.

“Soils in these regions are sandy, which means nitrogen can leach quite readily and organic matter can break down rapidly. Growers and agronomists should be aware of this when making fertiliser decisions for 2023.”

Dr Hughes says that the conclusions from this project need to be considered cautiously as they are based on one year of field trials conducted in 2022. Many growers who hosted the trials experienced very favourable seasonal conditions and recorded grain yields well above their five-year average. The magnitude of the responses recorded in the project may not be as big in a more normal year.

More information: Dr Darren Hughes, 0436 115 462, darren.hughes@laconik.com.au