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

Abstract

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 (SQI_w) to evaluate SQ across a land-use – soil type – climate gradient. Soil profile data (n = 47) covering semi-arid, sub-humid, and humid climates, three soil types (Inceptisols, Vertisols, and Alfisols) and three major land-use 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 (SQI_m), physical quality index (SQI_p), chemical quality index (SQI_c), and biological quality index (SQI_b) from the respective SQ indicators using the weighted additive index method. The thematic SQ indices were integrated to create SQI_w for surface and subsurface soils. The SQI_m showed a strong relationship with SQI_p and SQI_c and a moderate relationship with SQI_b, indicating that the thematic SQ indices can be employed to evaluate soil quality in resource-limited regions or countries. The SQI_w 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 SQI_w is a crucial advancement in SQ assessment, and the SQI_m adds a new dimension. The proposed SQI_w 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.