Quality Control in Soil Analysis Through Interlaboratory Proficiency Tests

Soil analysis is an important source of information in agriculture and environmental monitoring. However, analytical laboratories are subject to an increasing number of regulatory and accreditation requirements regarding the reliability of their test results. Participation in interlaboratory proficiency tests provides an essential tool to demonstrate the analytical performance of laboratories. This paper describes the results obtained from four proficiency tests evaluating 27 laboratories applying 12 standardized methods to measure key soil properties (moisture, pH, P2O5, exchangeable cations, organic carbon, and nitrogen) in two different batches of agricultural soil tested in blind replicates under time-different intermediate-precision conditions. This involved laboratories blindly analyzing two soil samples over a period of 17 and 28 months, respectively, with the aim of assessing the long-term consistency of laboratory performance. Various statistical and graphical methods, including Algorithm-A-based robust estimation of the assigned value, z-scores, z*-scores, and Youden plots, were used to assess individual laboratory performance. More than 80% of the participating laboratories showed satisfactory performance in all these tests. The results of this study highlight the robustness of the time-different blind testing in controlling the analytical performance of laboratories in soil analysis. Beyond conventional proficiency testing schemes, the present study provided laboratories with a unique framework to demonstrate their ability to sustain a high level of analytical performance, under intermediate-precision conditions, over an extended period.