Soil mineral nitrogen levels are variable this season, according to initial GrowHow N-min sample results.

Just over 40% of the 520 samples analysed are at Index 1 or below, around a third are Index 2 or 3, while just over a quarter are Index 4 or above.

All of the samples have come from sites where organic manures have not been applied, says the firm’s arable agronomist Alison Grundy.

“The difficult growing conditions last year combined with the mild weather through to the end of January are the most likely cause of the variation,” she says.

“Lack of moisture last spring meant a lot of crops failed to take up all the N supplied. Add to this a mild autumn, which encouraged mineralization, together with a dry winter, reducing leaching and limiting crop uptake and the result is some fields are full of N.

“In other parts of the country where moisture hasn’t been a limitation, the crops themselves are now full of N, as they have taken advantage of the N left behind from the 2011 season. Both the N in the soil and the crop must be taken into account when calculating how much N to apply this spring.”

Peter Dampney, principal scientist at ADAS, agrees that last year’s crop performance and the winter rainfall are critical factors to look at this spring.

“Growers, more than ever this season perhaps, need to look at their individual farm performance and weather; look at how your crops performed last season and whether yields were lower than expected, and what winter rainfall you’ve had, and whether drains have been running, which is when nitrogen will have started leaching.”

When assessing how much nitrogen to apply it is essential to know what index the field is in, warns Ms Grundy. “Get it wrong and you will end up over or under fertilising.”

Find out more in Crops

HGCA has recently issued definitive advice about where to use soil mineral nitrogen tests, and how to do them. The latest issue of a revamped Crops magazine, out on Saturday 11 February, has all the details