Proficiency Testing for Soil Fertility Analysis

In the context of the FAO Global Soil Partnership, the Spanish Society of Soil Science (Sociedad Española de la Ciencia del Suelo, SECS), the University of Zaragoza and the Agroenvironmental Laboratory of the Government of Aragon promoted initiatives for the harmonization of soil analysis methods in Spain. These included the development of an inventory of laboratories and the organization of proficiency tests of soil analytical results. The first test, carried out in 2019, showed significant discrepancies among laboratories, which led to methodological improvements for the second test in 2021. Twenty-six laboratories participated in the latter test, evaluating soil fertility parameters (organic matter, assimilable phosphorus, potassium and magnesium) and textural fractions (clay and sand). The analyses, carried out in triplicate, were evaluated using robust statistics and Zscore. The 2021 results showed that sample ‘HUERTO’ had a higher percentage of satisfactory results than “BIPEA,” mainly due to its greater standard deviation. The assessment identified problems arising from differences in analytical methods, especially for organic matter. A comparison of the two exercises revealed that, out of 17 laboratories analyzed, 10 improved their overall performance, 3 maintained it and 4 deteriorated. A positive trend was observed for assimilable phosphorus and magnesium, parameters that achieved satisfactory overall ratings, while sand and organic matter content showed more problematic results. Despite the improvements observed, only five out of the seventeen laboratories achieved an overall satisfactory rating, which underlines the need to regularly maintain and strengthen these proficiency tests. These results are particularly relevant within the context of the CAP Strategic Plan, which requires reliable data on the state of agricultural soils. This exercise has proven to be an effective tool for encouraging continuous improvement in the analytical quality of soil laboratories, contributing to the sustainable governance of soil resources.