Comparison of the performance of nine commercial laboratories for testing nitrogen and sulphur in plants and soil: a second study

Abstract

Nine UK laboratories participated in a second inter-laboratory study to evaluate the variability of analysis for concentrations of total Sulphur (S) and Nitrogen (N) in four plant materials, and extractable S in three soil samples. Results for total S varied considerably, with inter-laboratory coefficients of variation (CV's) that ranged from 13.4 to 19.8%.

For total N, the results were less variable and laboratories were in reasonable agreement with CV's which ranged from 4.3 to 6.6%. With the soil samples the inter-laboratory variability was even greater, with CV's ranging from 16.7 to 46.2%. The levels of inter-laboratory variability in this second study are similar to that found in the first study and show no improvement.

To obtain greater consistency between laboratories, analytical methods need to be more closely controlled. This could be achieved by improved standardisation of methods, wider use of reference materials and regular participation in quality assurance schemes.