After yet another wet winter, the results from this years' Deep Nitrogen testing is on one hand predictably simple. However, some key economic drivers have changed, and it is important to react appropriately.
After successive springs of ever lower soil nitrogen tests resulting from nitrate leaching, this year is no exception with average SMN levels across winter cereals and rape of just over 40 kg/ha (see graph below).
We took 57 individual samples across the South East at the end of February and early March and have analysed the results to understand optimum N dose rates in a range of cropping situations this spring.
This document should also form part of your Cross Compliance evidence to demonstrate that you are meeting the NVZ requirement to assess your Soil Nitrogen Status before applying fertiliser.
The past few months have been characterised by particularly mild weather and high rainfall. Crops established well in the autumn – albeit slightly later with most farms only commencing drilling at the very end of September.
Crops then produced high tiller numbers and dense ground cover and by Christmas disease levels were high, particularly mildew and brown rust in Crusoe and Cordiale.
Because of this mild autumn and winter, the amount of nitrogen held within both the soil and the crop itself – known as the Soil Nitrogen Supply (SNS) is again fairly high considering the low SMN figures being seen. There was a similar trend in 2015.
SMN is typically 20kg higher where manures were applied in the autumn, and the SNS showed that there was a similar amount extra held in the crop canopy too.
It is interesting that there is no evident difference between soil types this year. Last year showed that heavy soils typically held 15kg more N than average, and light soils a similar amount less. However , this spring the figures appear to be very similar.
How much fertiliser will I need to apply?
The amount of fertiliser to apply will depend, as always, on target yield and amount of N held within the canopy at GS 32, which is normally the end of April.
The optimum crop canopy size is a Leaf Area Index of 6 at ear emergence (6 units of leaf area for each unit, or square metre, of ground area).
Nitrogen applications should take into account the size of the canopy and the amount of N needed to produce it - and the likely yield of the crop.
Applications should also take account of Organic Manure that has been applied. The graph above shows all of our sample results this spring in order of the SMN level, and shows the effect of manure on soil nitrogen.
It is also important to consider variety and end market, particularly given the prevalence of Skyfall and Crusoe this season. Although the Breakeven Ratio of grain price to Nitrogen price has changed little (both have declined since last spring), the premiums on offer are so much lower that it is not economical to spend significantly on extra nitrogen.
Example: £7/t premium on 10t/ha crop = £70/ha
Additional N to achieve 13% protein = 40kg/ha, plus foliar urea
Cost of 40Kg/ha of 34% AN, plus Nufol = £55/ha
What impact does variety have?
In general protein levels were very good last year across the board. From our own on farm experience and from recent NIAB TAG data, we know that Skyfall requires higher levels of nitrogen to achieve similar grain proteins to Crusoe. For those with the new variety Trinity this is also likely to require an approach similar to Skyfall.
Having tried several alternative products to foliar urea (N20) over recent years, we remain unconvinced. Well timed soil/liquid applications through the season and N20 on varieties such as Skyfall and Trinity are likely to be most cost effective and successful.
"Assessing crop canopy size and potential yield is vital to optimising nitrogen rates"
Recent research indicates that Soil Mineral Nitrogen in rape is a poor determinant of how much nitrogen the crop needs to achieve optimum yield.
Crop Canopy size is a far better measure of crop N requirement and growers should be aiming for a target LAI of 3.5 at flowering.
As Agronomists, we base nitrogen applications on the amount of nitrogen needed by the crop to achieve this canopy ,and the yield expectation of the crop. Invariably this depends on pigeon damage and how early the crop begins to grow in the spring.
Key messages for 2016
Soil Mineral Nitrogen levels are slightly higher than last year, but still historically low
Where manures have been applied this should be taken account of in nitrogen recommendations
Consider carefully how likely you are to achieve milling specification on the farm and whether it is worth spending on additional Nitrogen
Tailor the recommendation to individual fields if necessary based on variety and yield expectation in wheat, and on crop canopy size and yield target in rape