NIR spectroscopy, mineral nitrogen analysis and soil incubations for the prediction of crop uptake of nitrogen during the growing season

To predict the amount of N taken up in above-ground plant parts during the growing season, initial mineral soil N, a soil incubation method, soil organic matter and NIR data were compared as predictors. Soil samples were taken from 15 plots cropped with winter wheat on a farm in south-western Sweden. The plots were not fertilized with N during the 1997 growing season. N contents in above-ground plant parts were measured in mid-June and in mid-August. All methods were capable of predicting the crop uptake of N reasonably well. NIR data gave at least as good predictions as the best traditional method, initial soil NO3-N. The most important wavelengths, around 1400 and 1700 nm, and above 2000 nm, coincide with the wavelengths reported earlier to be important for the prediction of soil organic matter. However, the data suggest that other soil components influencing mineralization are also spectrally active. Since very few samples were taken, the studies need to be extended in order to be able to use the method in practice. It is recommended that further studies be instigated for the possibility of using the same NIR calibration over several years and to clarify the spatial regions that the calibrations can cover.