Soil health indices in disagreement: A systematic review and comparison of soil health assessment methods

Soil health is commonly understood as the ability of soils to provide multiple ecosystem services such as primary production, climate mitigation, and water purification without meaningful trade-offs between them. Soil health is increasingly threatened by climate change and land management pressures, stressing the need to adequately assess and monitor it. However, soil health remains a conceptual term, with a wide array of quantification and interpretation approaches that create confusion, misapplication, and barriers to effective policy implementation. This paper identifies different methods of assessing soil health and makes a comparison between six representative assessment methods. To achieve this, we systematically reviewed 199 studies that cover a total of 212 soil health assessments. The methods of these assessments were categorized into a typology based on ten variables, which was subsequently analysed through clustering analysis. This clustering analysis identified four distinct groups of soil health assessments. For the comparison, Six representative assessment methods from three different clusters of soil health assessment were applied to a farm dataset of 87 sites. The analyses of the typology highlighted that soil health assessments conducted at smaller spatial scales typically express soil health as a single combined metric, whereas soil health assessments applied at larger spatial scales studies tend report separate scores for individual soil functions. The comparison of the six representative methods showed low agreement scores and poor correlations between selected soil health assessment methods. We conclude that soil health scores are not interchangeable across methodologies. In other words, different commonly used soil health assessment methods yield strongly contrasting results when applied to the same dataset. Further we noticed that the objective and approach for soil health assessments are often poorly documented. We caution against the use of aggregated soil health or multifunctionality metrics, as these methods obscure important trade-offs between different soil functions. We propose two distinct objectives for soil health assessment: 1. local soil function assessment and 2. broad-scale soil health evaluation and offer practical recommendations for selecting suitable methods aligned with these goals.