With difficult growing conditions last season many crops were unlikely to have used all the nitrogen fertiliser applied so soil reserves were generally higher than you might expect at the end of August. But how much of that residual N will still be available come spring 2013?
Allison Grundy, Arable Agronomist at GrowHow UK has been looking at rainfall data. Weve had an awful lot of rain since then, she says. Looking at the figures up until the middle of December, rainfall across England and Wales has been on average 150% of the norm, with some regions having over 180% more rain than is usual over this period. Even the driest region, the East has had 125% of its usual rainfall and in the run-up to Christmas we saw even more rain.
Leaching is bound to have been a problem on lighter soils, but where soils have been water logged denitrification is likely to be the main problem. On all soil types the cold and wet will have meant very little mineralisation every which way you look, nitrogen is being lost, she warns.
So will nitrogen levels be low next spring? Probably, but, as GrowHows experience from its N-Min nitrogen testing service over the past 15 years clearly demonstrates, N levels can be surprisingly variable, not only between years, but also between soil types, regions and even individual fields within the same year. So soil sampling with GrowHow N-Min to measure the amount of nitrogen a crop will get from the soil over the entire growing season will be important in 2013.
One of the key conclusions from the independent HGCA Soil Nitrogen Supply research project which reported in 2012 was that Soil Mineral Nitrogen on its own is a relatively poor indicator of the amount of N that will come from the soil. However, using the GrowHow N-Min service which measures both SMN and AAN (Additional Available Nitrogen) shows a considerable improvement in accuracy.
Allison Grundy continues: However, this does not mean that every field needs to be sampled. Use N-Min on barometer fields to check base levels but target fields that are likely to have low or high SNS as these will benefit the most from using N-Min, she advises.
Based on current values of nitrogen and cereals, we have calculated that using N-Min can give a benefit of between 100/ha and 200/ha, so it definitely pays, she adds.