Open access publications in the SLU publication database

Abstract

The advantage of using near infrared spectroscopy to increase sample point density in farm soil mapping relies on the number of conventional laboratory analyses for the calibrations being kept to a minimum. The objectives with the present study was to compare the performance of small farm scale calibrations (25 samples) to a larger national soil library (396 samples) and to test if a site specific sample set selected from the national library, consisting of the 50 samples spectrally most similar to those of the local sites could increase performance. In addition the national library and the selected subsets were augmented with up to the 25 local calibration samples to test whether that would have any additional effect on prediction errors and bias. Calibrations were made for predicting within-field variation in clay, silt, sand, soil organic carbon, pH and phosphorus. Selecting a subset of samples from the national library did not improve the results compared to using the entire national library. However, spiking both libraries with local samples reduced the RMSEP considerably, mainly due to a decrease in bias and often resulted in comparable results to the local calibrations. There was a tendency for better clay and SOC predictions when spiking a reduced national library compared to spiking the entire national library, sometimes even resulting in better predictions than using the local calibrations. However, using local calibrations seems to be the best alternative for predictions of soil parameters at the farm or filed scale, even with as few samples as 25.