Crop Nitrogen Use – Testing or Guessing? – William Waterfield (Andover)

The price of nitrogen has hit the national headlines and has more than doubled in the last 18 months.  So, what factors should be considered in preparing the nutrient plans for the next year or what strategy should be adopted to mitigate the impact of this cost increase in the longer term.

For the growing season ahead many of the factors are already in place.  The crop is mainly in the ground; the previous crop is obviously known; you may have bought the fertiliser.  If you have forward sold the crop all the better and some of the risk has been eliminated.  In the longer term, for example 3 years and beyond, there are more options to affect the crops nitrogen needs but for the year ahead some careful consideration needs to be taken.

Being realistic about the potential of the crop is key.  Feeding the crop for the historic yield is probably a good place to start rather than basing on hope and expectation.  Add to this an understanding that the economic optimum level of artificial nitrogen will depend on the price of the nitrogen and the price of the crop being grown.  Optimum rates for some farms may have changed by more than 40 kg/ha.  In determining the Crop Nitrogen Requirement and calculating next years’ fertiliser requirements it is important to remember what that may have changed from last year.  Include the rotation or applications of organic manures in the autumn or the presence of a cover or catch crop.  Winter rainfall and soil type have a considerable impact on the crop available Nitrogen in the spring, so making informed adjustments is critical.

Nitrogen should not be looked at in isolation.  Soil pH probably has the biggest effect on the efficiency of Nitrogen use, along with sulphur.  Whilst many arable farmers routinely apply sulphur to arable crops, it is often missing from grass and forage crops although for many forage crops sufficient sulphur is applied when slurry and FYM are used.  Where slurry is being applied it essential to get it tested rather than relying on book values.

On sites where there has been a change in cropping or large amounts of organic manures have been applied or after grass, it is worth calculating the Soil Nitrogen Supply.  The Soil Nitrogen Supply is a combination of an estimate of what is already in the crop plus the Soil Mineral Nitrogen combined with an estimate of the mineralisable soil nitrogen.  These calculations lead to an estimate of the SNS which can then be used to determine the level of Nitrogen required.  Soil Organic matter is an important source of nitrogen and as a rule on soils with more than 4% Organic matter an extra 10kg of N is released per each 1% increase in SOM.

Tests for soil mineral nitrogen needs to be carefully planned with samples normally taken in February or early March and across the rooting profile (normally at three depths).  Samples need to be chilled and sent by courier to the labs.  The results from this test will then inform the decision around how much Nitrogen will be released by mineralisation and hence the level of Soil Nitrogen Supply and by reference to the tables in RB209 an appropriate rate of N can be decided upon.

However, with the price of fertiliser now being in excess of £1.40/kg of Nitrogen the RB209 recommendation should be adjusted depending on the value of the crop.  At current fertiliser prices and with wheat at £200/tonne, the economic maximum would be to reduce by 20 Kg N/ha.  If the crop was worth £150/tonne then the reduction would be 30 kg N/ha from the levels recommended by RB209 which for an 8 tonne crop on a medium soil with an SNS index of 1 would be 220 kg N/ ha. 

Time to stop the guessing and cash in on the overuse of expensive nutrients in the past.  A Nutrient Plan may cost £5-10/ha depending on farm size, but N fertiliser costs alone will increase by £180/ha for a winter wheat crop!  Contact your local FACTS qualified FCG consultant or William Waterfield at w.waterfield@fcgagric.com to develop a nutrient plan and start saving on purchased fertiliser, even if forward bought cheaply.