A novel and comprehensive soil quality index integrating soil morphological, physical, chemical, and biological properties

The increased focus on soil quality (SQ) aims to conserve land resources and arrest land degradation. However, there are several unknowns regarding which indicators can most effectively indicate specific SQ outcomes and ecosystem functioning. For the first time, this study aims to integrate the soil morphological properties and earthworm population with physical and chemical properties and propose a comprehensive soil quality index (SQIw) to evaluate SQ across a land-use - soil type - climate gradient. Soil profile data (n = 47) covering semiarid, sub-humid, and humid climates, three soil types (Inceptisols, Vertisols, and Alfisols) and three major landuse systems (grassland, plantation, and annual field crops) were used in this study. As a novel approach, we used a combination of expert opinion and principal component analysis to select 12 soil quality indicators (five morphological, two physical, three chemical, and two biological properties) and developed four thematic SQ indices, viz., morphological quality index (SQIm), physical quality index (SQIp), chemical quality index (SQIc), and biological quality index (SQIb) from the respective SQ indicators using the weighted additive index method. The thematic SQ indices were integrated to create SQIw for surface and subsurface soils. The SQIm showed a strong relationship with SQIp and SQIcand a moderate relationship with SQIb, indicating that the thematic SQ indices can be employed to evaluate soil quality in resource-limited regions or countries. The SQIw differentiated the effects of climate, soil type, and land use management on soil quality and showed a strong correlation with crop yield, enabling the comparison of production systems. The integration of the earthworm population to SQIw is a crucial advancement in SQ assessment, and the SQIm adds a new dimension. The proposed SQIw could be a potential precursor for emerging consensus towards a generalised and comprehensive SQI, which can be effectively used for SQ monitoring across varied land use, soil types, and climate regions.