Inter-laboratory comparison of sulphur and nitrogen analysis in plants and soils

Ten United Kingdom laboratories participated in a study to evaluate variability of analysis for total concentrations of sulphur (S) and nitrogen (N) in plant materials and extractable S in soil samples. Four plant acid two soil samples were prepared and distributed to the participants. Results show that laboratories were in reasonable agreement for analysis of total N in plant materials, with coefficients of variation (CVs) in the range of 2.7-5.4%. In contrast, the inter-laboratory variability for total S was much higher, with CVs varying from 8.2 to 20.3%. The inter-laboratory variability for plant N:S ratio was similar to that for total S, indicating that the reliability of N:S was limited by the accuracy of the S analysis. Large differences (5-fold) between laboratories were reported for extractable S in soil, with CVs of 36-45%. The laboratories used different analytical methods for both soil and plant analyses, but method bias does not seem to explain the large inter-laboratory variability. Diagnosis of S deficiency based on S analysis may be of questionable validity if S analyses result:, for plant and sail samples are not accurate. To improve analytical reliability, more method development and standardization are needed.