Economic optimum N likely to fall as fertiliser prices rise

Next season’s economic optimum fertiliser application rate is likely to be about 40kg/ha less than RB209 recommendations, due to continuing increases in ammonium nitrate (AN) prices, a respected soil scientist suggested at a recent GrowHow Arable Advice seminar.

Roger Sylvester-Bradley of ADAS Boxworth, predicted many growers would be paying about £350/t for AN next season, compared with a grain price of nearer £130-140/t. This effectively meant the ratio between the cost per unit of fertiliser and value of extra gain produced had jumped from 3:1 when the recommendations were produced in 2000, to 7:1 now, he said.

“There’s a diminishing return from increasing fertiliser use and at a 7:1 ratio, the economic optimum is about 40kg/ha of nitrogen less than at 3:1. In 2007-08 the ratio was 4:1 or 5:1, and the adjustment was only 10-20kg/ha,” he said.

But he cautioned against adopting a blanket rate – whatever that might be – across all wheat fields on the farm. “The stakes are that much higher now. You’ve got to be vigilant and be prepared to spend more on N if grain N, or soil mineral N is falling, or crops are not growing properly.”

It was, therefore, vital to know what was in the soil already and get fields tested, he advised.

To illustrate the importance of measuring soil nitrogen, Prof Sylvester-Bradley highlighted results from three years of trials from 15 sites across eastern England and Scotland, which found no statistically significant response from applied fertiliser on 20% of sites.

“We’re not entirely sure why there were so many unresponsive sites we’d previously seen only 5% of fields showing nil responses.” High soil mineral nitrogen levels could provide some explanation, as in a few cases SMN levels were up to 200kg/ha, he said. “When selecting the sites, it may be that we weren’t aware of something vital about their previous history or cropping. But clearly if there’s that much residual N, you won’t see much of a response to fertiliser.”

The average optimum fertiliser rate across all trial sites was about 150kg/ha, which, although less than many farmers typically apply, masks huge variability, he added.

Old versus new

While the trials revealed some interesting findings around nitrogen response, the main purpose of the three-year project was to compare the nitrogen requirements of old and new varieties, Prof Sylvester-Bradley continued. “For wheat it is clear that newer varieties do need more fertiliser N. But our work shows the increase is smaller than many people might think at less than 20kg/ha. In other words, if one variety yields 1t/ha more than another, you will need something like 17kg/ha more N.”

This information on how old and new varieties differ in their N requirements would be incorporated into DEFRA’s new fertiliser recommendations (revised RB209), which was due out in time for fertilising the 2009 crop, he said.