How much residual nitrogen is available in the ground?
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?
We have had an awful lot of rain since then. 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, East Anglia, has had 125% of its usual rainfall, and in the run-up to Christmas we saw even more rain.
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Leaching is bound to have been a problem on lighter soils, but where soils have been waterlogged de-nitrification is likely to be the main problem. On all soil types the cold and wet will have meant very little mineralisation. Every way you look, nitrogen is being lost.
So will nitrogen levels be low next spring? Probably but, as experience from our 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 soil mineral nitrogen and additional available nitrogen, there is a considerable improvement in accuracy.
But this does not mean that every field needs to be sampled. Use N-Min on barometer fields to check base levels, as these will benefit the most from using N-Min.
Based on current values of nitrogen and cereals, we have calculated that using N-Min can give a benefit of between £100 per hectare and £200 per hectare – so it definitely pays.
Across the South West region rainfall last year averaged 250.8mm, which was 142mm more than the norm.