Article reproduced from ADAS Crop Action:

With the high price of nitrogen fertilizer this year it is more important than ever to fully account for the nitrogen that will be available from the soil – the Soil N Supply (SNS). Assessing the SNS of a field is a crucial part of effective N planning and one of the required steps that must be completed as part of the Nitrogen Planning requirement (before any N is applied) for all fields that are inside an NVZ. It is normally estimated from empirical look up tables in the Nutrient Management Guide RB209.

 The amount of SNS in spring depends on the amount of N applied to the previous crop; offtake by the previous crop; losses of nitrate over winter from leaching; and mineralization of organic matter. This year there are good reasons to anticipate higher SNS levels than normal, hence welcome possible reductions to the fertilizer requirements of crops this spring.

N fertilizer use in 2021 on most arable crops is likely to have been around normal, with some people perhaps having applied more N late in the season to some crops in anticipation of higher yields and higher grain prices. Grain yields in 2021 were variable, and at a Defra reported national averages of 7.8 t/ha for wheat, 6.1 t/ha for barley and 3.2 t/ha for oilseed rape, were perhaps somewhat below expectations for many. This means that on many fields there will have been more nitrogen left in the field after harvest 2021 than expected.

Drilling conditions were generally good in autumn 2021, and crops have generally established well and grown over the mild autumn and winter. Many such crops now have captured substantial quantities of nitrogen, a large OSR crop with a green area index of 3 will contain around 150kg N/ha, worth £300/ha at current prices. Many well established early sown cereal crops now have GAI greater than 1 so will have >30kg N/ha in the crop already. Nitrogen already in the crop is not at risk of leaching, should be included in the estimation of SNS and doesn’t need to be provided by fertilizer. Crop N can be estimated for cereals and OSR from the tables below:

The relative dry autumn and winter so far (see Met Office Anomaly maps below) means that losses of nitrate to leaching are likely to have been lower than normal. Indeed the Crop Action weather data is still reporting a soil moisture deficit for medium soils in Cambridgeshire and Lincolnshire, and some more retentive soils in the East may not yet have fully recharged with water, with little substantial drainage to date. Check your over-winter rainfall and make sure you use correct SNS tables from RB209 tables, it is possible that some areas that are normally classed as moderate rainfall will be low rainfall this year.

Anomaly maps for monthly rainfall over autumn and winter, available from https://www.metoffice.gov.uk/research/climate/maps-and-data/uk-actual-and-anomaly-maps

The relatively mild winter also means that mineralization rates may have been higher, where there is substantial organic matter in the soils.

SNS can be measured by soil sampling to depth and analysing soil mineral nitrogen (ammonium-N plus nitrate-N). Whilst SMN sampling can be laborious and relatively costly, and results can be variable and must be treated with caution, it remains the best measure we have of available N in the soil. SMN sampling can prove most worthwhile in situations where N supply is uncertain, for example where there is a history of manure use. In years like this it can also be useful in confirming suspicions of N levels on representative fields across the farm. When N fertilizer prices are so high  the knowledge that N rates can be confidently reduced could be very valuable.  To get accurate results from SMN sampling it is important that sampling and sample handling is done with care, with samples being kept cool and analysed by the lab as soon as possible. Sampling must be done before any N fertilizer is applied. It is very important that the amount of N in the crop is estimated at the time of sampling and included in the estimation of SNS. The possible contribution from mineralization should also be considered.  AHDB Topic Sheet 115 gives useful guidance on taking and interpreting SMN samples.

Judging SNS is crucial to making the right decisions about how much N to apply.  ADAS has recently written advice for AHDB on how to adjust N recommendations for the current high N prices, available here and shortly to be updated.

Actions

• Estimate your SNS levels, accounting for previous crop (N applied & yield), soil type, over winter rainfall, crop uptake and mineralization
• Consider undertaking SMN sampling before applying N fertiliser on representative fields, or fields where expected SNS is uncertain