HOW MUCH NITROGEN IS AVAILABLE IN YOUR SOILS FOR CROPS THIS SPRING?

February 17, 2021



One of the most popular tests in the laboratory at this time of year is the analysis of soil mineral nitrogen levels. The agronomists and farmers who work with us are busy planning nitrogen applications for this year’s crops and to do this successfully, soil nitrogen availability must be assessed. To improve nitrogen use efficiency and therefore profitability, the assessment must be as accurate as possible.

As spring progresses, soil temperatures will rise above 5oc, allowing soil microbes to become active. They will start to mineralise the organic materials in the soil, releasing both nitrogen and sulphur for crop uptake. This may be from soil organic matter or from recent organic manure additions and crop residues from last autumn. Towards the end of 2020, soil temperatures were still high enough for mineralisation to have continued, but losses will have occurred over the winter with the heavy rains experienced across the country.

To help assess likely soil nitrogen supply (SNS), the AHDB Nutrient management guide provides typical figures to use. Tables are based on winter rainfall for low, moderate and high situations and on previous crop and soil type. They are reported as SNS indices, where index 0 equates to a supply of up to 60 kg N/ha, index 1 is 61-80 kg N/ha and index 2 is 81-100 kg N/ha. The figures are made up of likely soil mineral N, crop N to date and likely additional mineralisation through the season.

The AHDB tables can be a good guide where cropping history is known. Where additional organic manures have been applied, high nitrogen crop residues have been incorporated or there are high organic matter levels in the soil, they are less appropriate. In these cases, it is more effective way to start by measuring the soil mineral nitrogen levels and assessing the amount of nitrogen in crops in early spring. Here sampling leaf tissue along with assessing the green area index of the crop will indicate crop N content.

For a better understanding of the soil mineral nitrogen levels, core samples should be taken, ideally to 90 cm but to 60 cm will provide useful information on nitrate and ammonium quantities for planning nitrogen fertilizer applications. The ideal time for sampling is from now through to mid-March, preferably before the first nitrogen top dressings are applied. At this time soil nitrogen levels are generally at their lowest after winter losses.

Improving nitrogen use efficiency reduces nitrogen loss and increases profitability. Soil mineral nitrogen analysis from NRM is available from most agronomists or sampling can be subcontracted