Expanded measurement uncertainty of soil parameters derived from proficiency-testing data

Basic parameters for the calculation of the expanded measurement uncertainty (U) of 33 soil parameters were derived from the data base of proficiency tests of the Association of German Agricultural Analytical and Research Institutes (VDLUFA) over the years 1993-2005. The underlying statistical model was the regression of the standard deviations of the means for all participating laboratories on the mean of all laboratories. In general, the linear model fits U very well. Only in three cases, a curvilinear equation showed a better significance. The calculated regressions were nonsignificant only in the cases of copper (using the Westerhoff method), pH, and the percentage of sand in the soil samples. All regressions were tested for plausibility using the Horwitz equation (HORRAT). Given a medium range of concentrations, practically all analytes fell well within a ratio of 2, only the results for manganese (Schachtschabel method) did not comply. Plausibility could be proven also for higher-concentration ranges with the single exception of boron (hot-water extraction). At very low-concentration ranges, for some of the analytes a HORRAT value of 2 was exceeded, presumably due to the proximity of the limit of detection in conjunction with some methodological problems. For all of these cases, a concentration limit for the reliability range of the regression for the analytes was calculated, at which the HORRAT values just met the critical ratio of 2. Laboratories are recommended to integrate the regression functions for U into their specific laboratory information and management system (LIMS) or other documentation systems in order to specify U for the measured values. Results are also considered as a solid basis for the evaluation of legally defined limits in agricultural fertilization of soils.