Soil nitrogen testing is set to be critical this spring for arable growers as heavy autumn rainfall may have led to reduced fertility, especially in light soils and waterlogged fields.

Autumn rainfall in England and Wales was nearly twice normal levels in some areas, and even the drier eastern arable areas saw an extra 25% of rain, leading to nitrogen being lost over a range of soil types, agronomists say.

Soil nitrogen levels are notoriously variable between years, soil types and regions, but after such heavy rainfall most agronomists suggest more soil testing could be worthwhile to tailor fertiliser applications to crop needs.

Daniel Kindred, crop scientist at consultancy group ADAS, says soil nitrogen levels are likely to be low due to the heavy rains, so testing could be useful although it has limited value on shallow and lighter soils where nitrogen levels are usually low.

“This year soil testing would be a useful check as nitrogen levels could be inadequate as it appears to have been raining since fertiliser applications were made last spring,” Dr Kindred told Farmers Weekly.

Allison Grundy, agronomist at fertiliser group GrowHow UK, warns that cold and wet weather in the last four months of 2012 will mean nitrogen has been lost from soils, so testing is even more important this year.

“This year we have seen so much rain that the goal posts have changed, and we recommend that growers would be better off testing,” she says.

Dr Grundy suggests not every field needs to be sampled, but what is important is to test fields that are likely to have either low or high levels of nitrogen in the soil system. She claims an N-Min test can give a benefit of £100-200 per hectare based on the prices of nitrogen fertiliser and grain.

This test, available through fertiliser distributors, can costs around £95-£100 a sample, with on average about four samples being taken per farm, and can be undertaken up to the first nitrogen dressing in early spring.

ADAS says sampling across a few fields can give a useful check of nitrogen levels in the soil, or Soil Mineral Nitrogen (SMN), compared with field assessments based on soil type and previous cropping.

In addition, sampling can be used to measure the amount of nitrogen that is likely to be released following mineralisation or the breakdown of organic matter in the soil during crop growing, known as Additional Available Nitrogen (AAN).

Dr Grundy says independent research has shown a measure of SMN on its own is a relatively poor indicator of the amount of nitrogen that will come from the soil, but measuring SMN and AAN together, as in her group’s N-Min test, has shown more accurate results.

For further information see the HGCA study Estimating Soil Nitrogen Supply, Topic Sheet 115/Summer 2012, www.hgca.com/

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