Nitrogen inputs put growers in a spin

Ammonium nitrate or urea, and how much? Growers and advisers have been facing that conundrum this week as forward crops demanded nutrient, but forecasts fluctuated between snow, heavy rain and continuing drought.
Fears of urea volatilisation from bone-dry soils had prompted some growers to seek ammonium nitrate supplies. “I’ve had calls about it all morning,” said Agrovista agronomist Tim Bullock. Others were looking to switch to ammonium nitrate for now and use urea in May, in the hope of wetter conditions then, added GrowHow’s Ross Leadbeater.
“If you only have urea in the shed it is one heck of a conundrum,” accepted Frontier’s Mike Slater. Crops needed nitrogen, and waiting for rain was a gamble, so his preference was to apply urea sooner than later, even if soils were dry.
DEFRA’s three-year NT26 study showed volatilisation losses from urea can exceed 50%. “We saw anything from 2% to over 50%, with an average of 20%,” noted ADAS nutrient expert Brian Chambers. “The trouble is, you can’t say what volatilisation you might experience. If it rains soon after application, you might only get 2%. But in the worst circumstances, with a period of warm days and heavy night dews, you can lose much more.”
Soil type had less influence than previously thought, he added. “The worst loss, of over 50%, was on a heavy clay soil in March. The farming mantra that sandy and chalky soils suffer most was not supported.” A high pH zone created by chemical processes when granules degrade overrode any soil pH effect.
But with urea up to 33% cheaper than ammonium nitrate per unit of nitrogen, its economic case remained strong. If granules were seen to be breaking down slowly in the absence of rain after application, volatilisation losses could be redressed with the final application, Prof Chambers advised.
Nitrogen stabilisers could help too. “Koch Fertiliser’s Agrotain cut ammonia emissions by 70% compared with straight urea in our trials,” he notes. “Agronomically, it was no different to ammonium nitrate.”
Despite dry conditions, growers should not deviate from planned total application rates, he said. Yara’s Mark Tucker agreed. “It was a big lesson last year. Economic optimum rates of 150kgN/ha on a fertile site in Bedfordshire and 250kgN/ha on a low fertility site in Lincolnshire still gave responses as good as in a wet season.”
Combining second and third splits in feed crops now made good sense. “If it stays dry we know 60-70% of yield comes from crop biomass built up early in the season. Last year one early hit out-yielded three splits by 1t/ha in our trials. If it turns wet there may be growth regulator implications, but you’ve avoided operational issues if application windows tighten.”
With crops looking good despite the drought, Mr Leadbeater agreed rates needed keeping up. “After last year’s experience don’t be too quick to cut rates in dry conditions.” But growers should keep individual applications below 100kgN/ha in current conditions, he said.
Urea use prompts health concerns
Fresh pressure on urea comes from the finding that the ammonia it releases can cause fine particulates in the atmosphere, affecting asthmatics. “It is a human health issue, so has significant costs attached to it,” Prof Chambers notes. EU ammonia emission limits have been met in the UK, mainly due to lower livestock numbers. But if all fertiliser nitrogen were supplied as urea, those limits would be broken. “It is something DEFRA is keeping an eye on, and there’s little doubt that urea would come under pressure if usage increased too much,” adds Mr Tucker.