Precision Technologies to Determine Optimum Nitrogen Management in Europe.

Nitrogen (N) requirements of small grain cereals in North West Europe commonly range between 100 and 300 kg ha^‑1 because grain yields are large and growing conditions are variable.  Despite much research over 170 years, N management in Europe is still subject to great imprecision.  Multi-site experiments in the UK show that official advice only predicts within 50 kg ha^‑1 of the optimum N amount in 50% of cases.  What is more, as shown by six recent chessboard experiments, variation is commonly as great within fields as between fields; optimum N amounts within areas of 4-5 ha varied by 100 kg ha^‑1 or more.  Prediction of optimum N amounts is problematic because, whether between or within fields, variation may principally arise through any of (i) crop N demand, (ii) soil N supply, or (iii) fertiliser N recovery.  These components are also commonly correlated; in particular, crop N demand often relates positively to soil N supply.  Precision technologies thus have the daunting challenge of not only predicting all three components of fertiliser N requirement, but also of predicting the associations between them.  Where precision technologies are employed in NW Europe it is common practice for inter-field variation to be predicted ‘manually’ and for N applications to be adjusted from this pre-set level according to intra-field variation in canopy reflectance indices such as NDVI or GDVI.  Our current research is directed at improving interpretation of canopy reflectance data and augmenting these with predictions of crop N demand based on past grain yields.  However, there are currently no leads to guide prediction of the extensive variation in fertiliser N recovery, so this variation commonly remains ignored.  We contend that improved predictions of fertiliser N requirements depend on the networking and coordination of field-scale experimentation, so that soil-by-management interactions become better understood.