New South Wales Department of Primary Industries plant nutritionist Dr Graeme Schwenke.
PHOTO: Rebecca Thyer
Top-dressing nitrogen onto crops, fallows and pastures may not be as risky as is generally thought if rain does not fall soon after application.
This is the conclusion from research on neutral to alkaline cracking clay soils over the past two years by Tamworth-based NSW Department of Primary Industries researcher Dr Graeme Schwenke and colleagues Bruce Haigh, Adam Perfrement, Bill Manning and Dr Guy McMullen.
Nitrogen volatilisation losses were assessed in 15 paddocks covering four scenarios: top-dressing wheat; autumn fallow broadcasting ahead of winter cropping; spring fallow broadcasting ahead of summer cropping; and autumn and spring fertiliser application to a grass-based pasture.
In general, Dr Schwenke and colleagues’ results so far seem to fit well with other Australian and international research where typically less than 20 per cent of applied nitrogen is lost from urea added to arable systems.
GRDC Northern Panel chair James Clark believes these results challenge long-held Australian views and may be a ‘game-changer’ for northern growers. “Growers usually try to time nitrogen applications with rainfall to avoid losing nitrogen into the atmosphere due to expected volatilisation losses,” he says.
The ammonia detection masts are designed to pivot and face into the wind so that they are constantly positioned to measure nitrogen volatisation in all weather conditions.
Dr Schwenke explains that fertiliser nitrogen can be lost from the soil surface as ammonia gas through volatilisation. How much is lost depends on a range of factors including rainfall amount, soil moisture, temperature, pH, naturally occurring lime in the soil, ground cover, wind, soil-clay percentage and fertiliser type.
In-crop nitrogen losses
In 2011 four experiments on medium-to-heavy cracking clay soils from Edgeroi to Mullaley, in north-west NSW, found nitrogen volatilisation losses from fertiliser top-dressed onto mid-tillering wheat crops were minor. There was a less than 10 per cent loss over a month despite little rain after application. Fertiliser was applied to dry soil and temperatures were low.
Nitrogen fertiliser types tested included urea, urease-inhibitor-coated urea, and liquid products such as dissolved urea ammonium nitrate and urea ammonium nitrate.
There were only small differences between the losses from the different products. Liquid urea tended to lose a little more than solid urea. Urea ammonium nitrate and ammonium nitrate liquids showed slightly smaller losses because a portion of these products is in the nitrate form, which does not volatilise. Practically though, these differences are considered insignificant.
Urea with a urease inhibitor (Green Urea™) further reduced volatilisation compared with the standard urea.
Spring fallow losses
To measure nitrogen losses through volatisation into ammonia, a number of 50-metre-diameter circles were inscribed across paddocks of wheat and pasture. The circles were divided into lanes and within each lane a weighed amount of urea was spread. In the centre of each circle was a mast housing the ammonia sensors. This picture shows researchers spreading urea inside the circle lanes.
Nitrogen volatilisation losses from applying nitrogen in the spring fallow prior to sowing summer crops can be higher as temperatures are warmer, small rainfall events can be more frequent and there is no crop to slow wind blowing across the soil surface. Crops can also recapture volatilised ammonia within the leaf canopy.
One experiment on a paddock with carbonate nodules in the top soil layer (relatively uncommon in NSW soils) resulted in the largest losses of nitrogen from top-dressed fertiliser. Carbonates in the soil chemically react with ammonium sulfate resulting in higher nitrogen losses. Losses in these conditions are immediate, whereas those from urea generally take several days to reach their peak rate of loss because the urea must first be hydrolysed to form ammonium in the soil.
In the carbonate-containing soil, volatilisation loss from ammonium sulfate was high (greater than 30 per cent of applied nitrogen). Urea losses were also higher from this paddock, probably because soon after fertiliser application enough rain fell to just dissolve it, but there was no follow-up rain for almost a month.
In the second spring fallow trial, losses from urea and ammonium sulfate (crystalline by-product) were similar and both below 10 per cent. The surface soil in this case had no carbonates.
Pasture phase losses
Dr Schwenke’s third set of trials at Tamworth and Caroona, in NSW, examined top-dressed nitrogen volatilisation losses when applied to grass pastures.
Nitrogen loss at the Tamworth site was about 30 per cent from urea but less than five per cent from ammonium sulfate. In the Caroona trial, nitrogen loss was also higher from urea (23 per cent) compared with 10 per cent from ammonium sulfate.
In both trials, Dr Schwenke believes results were a consequence of little rain in the first two weeks following application, but urea granules were dissolved by humidity, dew and light showers. Without significant rainfall, the dissolved urea would have been trapped in leaf litter.
Graeme Schwenke, NSW DPI
02 6763 1137
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