A soil mineral nitrogen investigation by Farmers Weekly Group has revealed that growers are having to rely on incomplete information as they struggle to please the Environment Agency and the bank manager.

To test the robustness of current practice, we sent identical soil samples from two arable sites to four laboratories.

The soil mineral nitrogen (SMN) analyses ranged from 63-132kgN/ha.

Working those figures through RB209’s (DEFRA’s official fertiliser guidance) SMN tables gave a soil nitrogen supply index of a three and five, allowing us to apply either as little as 40kgN/ha or as much as 150kgN/ha.

Pressure to comply with Nitrate Vulnerable Zone legislation and cross-compliance measures means growers have to justify nitrogen applications based, in part, on an assessment of available nitrogen in the soil.

“There are clearly areas of concern,” The Arable Group’s Stuart Knight told Farmers Weekly.

“Like any biological process, measuring soil nitrogen is not an exact science.

People’s expectation of the level of accuracy is probably a bit high.”

Variability could come through errors in soils sampling, sample handling and differences in laboratory procedures, he suggested.

“We also have difficulties interpreting the data.”

With RB209 currently being reviewed – a consultation closed at the end of last month – there is recognition that its advice might be outdated, particularly with mounting evidence that modern varieties require considerably more nitrogen to achieve yield and quality than RB209 allows.

“We will be providing a report to DEFRA having taken consultations with the industry to establish if there is a case for RB209 to be revised,” ADAS’s Peter Dampney confirmed.

“It is then for DEFRA to decide.”

In the meantime growers can deviate from RB209 providing they can justify their actions, the Environment Agency’s James Letts said.

“We’ve said all along we’ve gone along with advice given by FACTS agronomists and RB209 and other evidence providing it’s been peer reviewed.

We will accept that – the farmer is adopting best practice.”