Beyond heavy metals: Integrating safety and agronomic performance in the classification of organic amendments

Nutrient-containing organic amendments, such as compost, have gained attention as sustainable alternatives to conventional fertilizers, aligning with global efforts to promote a circular economy and sustainability principles. However, classification of organic amendments in the European Union generally focuses on heavy metal concentrations. Thus, the nutritional quality of the products is generally not considered, and information regarding the fertilizing capacity is therefore incomplete, despite being important. This paper reports a study of the heavy metal concentrations of organic amendments registered in Spain between 2007 and 2024. Multivariate chemometric analysis, including principal component analysis and hierarchical cluster analysis, revealed significant variability in the metal concentrations and fertilizing potential among the 338 amendments considered. Most of the products had intermediate concentrations of heavy metals; however, the overlap in concentrations in many of the categories indicates the need for criteria that also include information on the fertilizing capacity. Therefore, a new three-component product identification code (3c-ID) is proposed to classify organic amendments by integrating fertilizing potential (C, N, P, K), C:N ratio and heavy metal concentrations. Based on the results obtained, the application of this code reveals that only 18.05% of amendments exhibit high fertilizing capacity (Fertility score = 3), while 40.24% and 41.72% show medium (Fertility score = 2) and low capacity (Fertility score = 1), respectively. In addition, 77.81% of amendments present balanced C:N ratios (10–20). Moreover, a clear overlap in heavy metal concentrations among regulatory classes was observed, indicating limited discriminatory power of the existing classification. These results demonstrate that the proposed system enables a more differentiated and function-oriented classification of organic amendments, improving the identification of products with similar safety profiles but contrasting agronomic potential. Moreover, the use of the new, updated classification system for amendments will contribute to improving soil health and reducing dependence on synthetic fertilizers.