This work includes the methodology for carrying out a proficiency testing program between Spanish laboratories that offer soil fertility analysis. Aliquots of each of the two large volume samples were prepared, taken from two surface horizons of soils with marked differences between them. Once the homogeneity of aliquots was tested, one sample of each soil was sent to each of the twenty-one participating laboratories, together with the instructions and a format for the submission of results. Throughout the process, special care was taken to ensure the confidentiality of the results of each individual participant. Once the responses were received, the results were analyzed, consensus values were assigned and each result was evaluated using the “z-score” indicator. The exercise was very well received by the participating laboratories, and has been very effective in characterizing the quality of the results issued. It has been shown that there are parameters with very homogeneous results (pH) compared to others with differences incompatible with the reliable agronomic use of analyses (granulometry, assimilable phosphorus, etc.). The urgent need to maintain this type of intercomparison exercise periodically, as an external quality control that increases the reliability of laboratories, is evident.
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