Managing Late Nitrogen Topdress in a Wet Year
Many parts of southern and central NSW have received above average rainfall this year, resulting in high yield potential but significant waterlogging. The challenge for growers is to determine how much nitrogen will be required to maximise yield and avoid low protein, and the best time to apply it.
The questions growers need to consider are:
- How efficient was my previous nitrogen strategy and did I leave yield on the table?
- Should I consider later applications to improve grain quality?
- What is the long-term N bank going to look like if minimal rates of nitrogen are applied?
- What losses are occurring from leaching and denitrification and what effects are they having on crop yields and mineral nitrogen levels?
CLINT SHEATHER – TECHNICAL AGRONOMIST
Don't topdress waterlogged crops
Crop type, growth stage and the duration of the waterlogging event will determine how well a crop tolerates a waterlogging event and if nitrogen should be applied. It is important that water has started to recede, and plants are actively growing before applying any nitrogen.
The challenge for growers remains knowing when and how much additional nitrogen is required to maximise yield and protein. Historical crop records are an indicator of likely crop and nitrogen performance. Often low protein wheat is a sign of reduced yield potential and strategic nitrogen applications can help minimise low protein wheat. As a guide, a wheat crop with a protein level of less than 10.5% is nitrogen limited while a crop with protein between 11.5-13% is unlikely to have benefited from additional nitrogen as yield and protein was maximised.
The nitrogen (N) bank is a method of maintaining a base level of mineral-nitrogen in paddocks where the risk of leaching and denitrification losses are low. A target level of soil-nitrogen is set based on local and climatic conditions, so crops are not deficient. When using this strategy, minimal input into the N bank can result in nitrogen deficiency in the current year, but also mining of soil nitrogen reserves and a reduction in the carryover of residual nitrogen for the following crop. Growers can assure that late topdressing can increase the N bank for the following year if the nitrogen is not utilised for the current crop
Dentification losses are accelerated in heavy soil types that have been saturated for period longer than 7 days. Denitrification also accelerates in soil temperatures between 15-30˚C and peak between 23-27˚C. Nitrate leaching is typically a problem in duplex or sandy soils that have high hydraulic conductivity levels. Excessive leaching is not a problem most eastern cropping soils. Knowing that loss pathways are low allows greater confidence can expected this topdress season as yield potential remains high with minimal nitrogen losses occurring.
There’s still time
Growers have time to ensure that nitrogen will not be the limiting factor this season. Later nitrogen applications from stem elongation to flag leaf emergence (GS31-39) predominantly increase grain protein and yield. Protein responses to nitrogen applied around GS39 flag leaf emergence are more reliable when yield is already maximised, and seasonal conditions are above average. In canola, nitrogen and sulphur can be applied into early flowering.
IPF trial work in 2015 also showed canola yields improved with late nitrogen applications. Split nitrogen applications at stem elongation and 20% flowering increased yield. None limiting nitrogen and using multiple application times outperformed all other nitrogen treatments.
Nitrogen is crucial in high rainfall years. For example, in 2016 at IPF long-term trial site at Grenfell above average rainfall (including 120 mm in September) resulted in both waterlogging and increased yield potential.
All nitrogen treatments have a split application timing, half is applied at sowing and the second half is applied at stem elongation (GS31) at increasing rates, figure 1 shows as the nitrogen increases the yield and protein increases showing not only the importance later application timing but also the amount of nitrogen applied at that application moving the protein into APW quality.
Consider Green Urea this Spring
Late urea applications into spring increase the risk of ammonia volatilisation. Rainfall of more than 10-15mm is required to reduce losses and make sure the nitrogen is available for crops. Green Urea contains a urease inhibitor. It can be useful when imminent rainfall cannot be guaranteed, daytime temperatures and windy weather is increasing and where soil pH is neutral to alkaline. Green Urea delays the hydrolysis process, extending the time for incorporating rainfall.
Take home messages
• Soil, tissue and historical records remain important tools in determining nitrogen reserves and can be used for nutrient budgeting.
• Deep soil sampling in increments can be useful to determine where nitrogen is in the soil profile.
• Nitrogen applications late in the season can increase yield and protein in cereals.
• Don’t rely on soil mineralisation to get your crops home. Apply nitrogen throughout the season and utilise the ‘N’ bank.
• Be proactive and manage your nitrogen with all the tools available and ensure nutrients are not overlooked
Angus, J. F., & Grace, P. R. (2017). Nitrogen balance in Australia and nitrogen use efficiency on Australian farms. Soil Research, 55(6), 435-450.
Harris, R. H., Armstrong, R. D., Wallace, A. J., & Belyaeva, O. N. (2016). Effect of nitrogen fertiliser management on soil mineral nitrogen, nitrous oxide losses, yield and nitrogen uptake of wheat growing in waterlogging-prone soils of south-eastern Australia. Soil Research, 54(5), 619-633.
Hunt, J. (2020) Managing nitrogen for high crop yields and sustainable farming systems. Grain Research and Development Cooperation, Resources and publications, GRDC Update Papers
Turner, D. A., Edis, R. E., Chen, D., Freney, J. R., & Denmead, O. T. (2012). Ammonia volatilization from nitrogen fertilizers applied to cereals in two cropping areas of southern Australia. Nutrient Cycling in Agroecosystems, 93(2), 113-126
For more information please contact
Clint Sheather - Technical Agronomist Phone: 0475 439 316 Email: firstname.lastname@example.org
Figure 1: Incitec Pivot Fertilisers’ long-term NxP trial at Grenfell in 2016 showing the value of nitrogen topdressing in a wet year. LSD <0.05, Yield 0.44, Protein 0.81%.