Keywords: Chlorophyll tester, nitrogen mineralisation, 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 fertiliser 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 fertiliser N to a recommended level to ensure optimal N supply for the crop. To characterise N mineralisation 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 fertiliser 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 fertiliser 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 OsnabrÃƒÂ¼ck, Germany.
35 pages, 5 figures, 3 tables, 181 references