Nitrogen losses: Q&A with Peter Grace
Understanding how nitrogen is lost from soils can help growers plan their nitrogen strategy. Queensland University of Technology’s Professor Peter Grace answers some common questions about the causes of nitrogen loss and strategies to avoid it.
Q: What are the types of nitrogen losses?
A: There are three ways nitrogen is lost from soil: volatilisation, denitrification and leaching.
- Volatilisation is the loss of ammonia from fertilisers that have been broadcast and left on the soil surface. The loss is caused by chemical reactions on the surface of the soil, and only occurs in alkaline soils when ammonium-based fertiliser is broadcast and there is no rain to wash it into the soil. Soils with high clay content normally have a low risk of volatilisation compared with sandy or loam soils.
- Denitrification occurs when a soil is more than 80 per cent saturated with water and microorganisms in the soil convert soluble nitrate into gaseous forms of nitrogen. As moisture content increases beyond the drained upper limit (also known as field capacity), the loss of nitrogen through denitrification increases. Heavier-textured soils, such as clays, are more susceptible to denitrification and large losses as they may stay wetter longer.
- Leaching is the loss of nitrate down through the soil profile. Nitrate is highly soluble and therefore mobile so when water drains through the profile, nitrate travels with it. If nitrate drains below the effective crop root zone, it cannot be recovered. Coarse-textured soils such as sands are more likely to experience nitrogen leaching than clays.
Q: Are soils with sandy topsoils safe from denitrification?
A: A lot of soils in south-east Australia are duplex, so even where a grower has a sandy or loam topsoil, if there is a clay subsoil, this will potentially restrict drainage and can still result in saturation and therefore denitrification. The topsoil may look dry but the subsoil may stay wet for extended periods of time and nitrogen losses from denitrification may continue to be significant.
Q: How much nitrogen should I apply up-front?
A: Unfortunately research shows that there are no fixed rules for optimum fertiliser rates and timing of applications, as it depends heavily on the seasonal forecast.
If the current forecast is for a normal or a dry season I would recommend keeping the up-front additions low to keep costs down and then as the season progresses, consider when and how much to top-dress. If the forecast is for a wet year, there will be higher yield potential and possibly fewer opportunities to top-dress so then growers may want to go heavier up-front.
Q: When is best to top-dress urea?
A: This depends on the soil type and forecast. Rain is required to wash the fertiliser deeper into the soil profile, but not too much that it could cause denitrification or leaching.
The ideal application time to maximise nitrogen use efficiency, based on trials in the Wimmera and Victorian high-rainfall zone, is to apply the majority of the fertiliser at about GS30.
Q: What if forecast rain does not eventuate and fertiliser has been applied?
A: Growers with alkaline soils may lose nitrogen fertiliser through volatilisation if there is not adequate rain to wash-in the fertiliser. These losses occur gradually for one to two weeks after urea application, so any rain within this time will reduce the loss.
In trials in north-west New South Wales, fallowed soils experienced loss of 12 per cent after urea application, down to five per cent in-crop. Previous trials have shown up to 18 per cent urea loss.
In acid soils volatilisation will not occur, so the only risk is that if there is inadequate follow-up rain to give the expected yield potential then the nitrogen application may not be cost-effective.
Q: What is the number-one tip for growers to minimise nitrogen loss?
A: Put the effort into understanding the soils on your farm, and then learn about how best to use seasonal and short-term weather forecasts.
Growers who know how their soils respond to rain, either through experience, soil moisture monitoring and/or soil testing, will know how much rain is needed to get the most bang for the buck (that is, maximum nitrogen use efficiency), and how much rain is too much.
Then, once growers understand the available forecasting information (for example the Bureau of Meteorology free online products – MetEye, Climate Outlooks, POAMA) they can decide when and how much nitrogen fertiliser to apply based on the best available information.
More information:Professor Peter Grace
07 3138 9283
GRDC Project Code DAF00004-15, UNE000012, DAN00144, MCV00037
Region North, South