Keywords: Chlorophyll tester, nitrogen mineralization, nitrogen recommendation, nitrogen splitting, on-farm tests, plant analysis, remote sensing, soil analysis.

Precision application of nitrogen (N) to arable crops is essential from an ecological perspective (e.g. groundwater contamination, N losses into the atmosphere), but even more important for the economic performance of a farm business. Today, farmers' decisions on N application rate are often based on yield expectation. However, yields are neither stable from year to year nor for a given field or subparts of a field. Therefore N application rates (total amount and timing) should be adjusted to the specific crop demand for a given field (and even within a field) under the specific growing conditions in that year.

Different methodological approaches have been suggested to improve N fertilizer recommendations for arable crops. For soil-based methods soil sampling is a prerequisite. Measurement of soil mineral N (SMN) in the soil profile either at the start of the growing period or in-season is a widespread approach in many countries in Europe as well as in North America. Based on data from field calibration trials, the SMN pool is supplemented with fertilizer N to a recommended level to ensure optimal N supply for the crop. To characterise N mineralization from different N pools in the soil (soil organic matter, plant residues, and organic manure) a large number of methods has been tested, but introduction into practical N fertilizer recommendation schemes has been rather limited. Plant analytical methods follow the concept that the plant itself should be used as indicator for the N supply from any source within the growth period. Up to now a number of methods have been proposed from quantitative laboratory analysis to semi-quantitative 'quick' tests carried out in the field. In-field methods like the nitrate plant sap/petiole test and chlorophyll testers have practical relevance at farm/field level for several crops, because with these methods an adequate adjustment of the N fertilizer application strategy within the season is feasible. During recent years several canopy sensors (mainly based on non-contact optical measuring techniques) have been introduced to practical farming. Using these monitoring systems farmers are enabled to move from a blanket N application rate per field to a real-time variable N application to cover in-field variability.

Hans-Werner Olfs, University of Applied Sciences, D-49090 Osnabrück, Germany.

35 pages, 5 figures, 3 tables, 181 references