Impact of sewage sludge addition on Dexter soil quality index

Assessing soil quality is essential for sustainable land use and agricultural productivity, particularly in the case of degraded, sandy, or low-organic-matter soils that suffer from poor water retention and reduced biological activity. One promising tool in this context is the Dexter soil quality index (S-index), which focuses on the relationship between volumetric water content and soil structure stability. Unlike general soil quality indices (SQI) that often integrate chemical or biological indicators, the S index offers a more physically grounded and quantifiable parameter for evaluating soil physical quality and water availability. It is particularly valuable for monitoring changes following organic amendments such as sewage sludge. The aim of this study was to evaluate the influence of sewage sludge application on the Dexter S-index, as an indicator of improved soil usability in terms of volumetric water content and retention capacity. The research was based on controlled pot experiments involving three mineral soil types: sandy- loam clay, loose sand, and clayey silt. Each soil type was amended with sewage sludge at rates equivalent to 50, 100, and 200 Mg∙ha−1. The results showed a clear increase in the S-index with increasing sludge doses across all soil types. The greatest relative improvements were observed in light-textured soils (0.042), where the original S values were lowest (0.018). This confirms the positive effect of sewage sludge on enhancing soil structure and water availability, particularly in sandy soils with inherently low retention capacity.