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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 and 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 results for plant and soil samples are not accurate. To improve analytical reliability, more method development and standardization are needed.