Using soil organic matter fractions as indicators of soil physical quality

The objective of this study was to evaluate the use of chemical and physical fractions of soil organic matter (SOM), rather than SOM per se, as indicators of soil physical quality (SPQ) based on their effect on aggregate stability (AS). Chemically extracted humic and fulvic acids (HA and FA) were used as chemical fractions, and heavy and light fractions (HF and LF) obtained by density separation as physical fractions. The analyses were conducted on medium-textured soils from tropical and temperate regions under cropland and pasture. Results show that soil organic carbon (SOC), SOM fractions and AS appear to be affected by land use regardless of the origin of the soils. A general separation of structurally stable and unstable soils between samples of large and small SOC content, respectively, was observed. SOM fractions did not show a better relationship with AS than SOC per se. In both geographical regions, soils under cropland showed the smallest content of SOC, HA and carbon concentration in LF and HF, and the largest HF/LF ratio (proportion of the HF and LF in percent by mass of bulk soil). With significant associations between AS and SOC content (0.79**), FA/SOC (r = -0.83**), HA/FA (r = 0.58**), carbon concentration of LF (r = 0.69**) and HF (r = 0.70**) and HF/LF ratio (r = 0.80**), cropland showed lowest AS. These associations indicate that SOM fractions provide information about differences in SOM quality in relation to AS and SPQ of soils from tropical and temperate regions under cropland and pasture.