Soil quality and soil health indices: Conceptual boundaries, purpose-oriented construction, and standardized interpretation

Soil quality and soil health are central concepts for sustainable agriculture, ecosystem service provision, and environmental governance. Despite decades of development, persistent methodological bottlenecks include blurred conceptual boundaries, weak indicator-function linkages, insufficiently transparent scoring and weighting choices, and over-reliance on single composite scores. This critical review clarifies the distinctions between soil quality and soil health, separates these conceptual constructs from their operational index expressions, and synthesizes the methodological logic of index construction. Bibliometric mapping is combined with a structured narrative synthesis of conceptual, methodological, and operational literature. On this basis, we develop a purpose-oriented operational framework that links assessment goals and target soil functions to indicator screening, minimum data set selection, scoring and threshold calibration, weighting and integration, and structured multi-layer interpretation. For indicator selection, we extend the conventional physical-chemical-biological classification with four functional response classes: inherent background, state, rapid response, and direct process indicators. This dual classification captures both disciplinary domain and diagnostic role. We further establish a 10-item minimum reporting checklist and demonstrate its applicability using two contrasting published assessment architectures. Together, the framework and checklist provide a coherent architecture for moving soil quality and health indices from data aggregation toward functionally grounded, interpretable, and decision-relevant assessment. Future progress depends less on additional index formulas than on explicit purpose, functional alignment, transparent methodological assumptions, and well-defined applicability boundaries.