Humic substances and clay minerals in organically-amended semiarid soils

Unsuitable agricultural practices have resulted in a permanent soil degradation in some semiarid areas of southeastern Spain. Because addition of exogenous organic matter has been shown to generally cause an improvement in quality of these degraded soils, the objective of this work was to evaluate the efficacy of amendment with fresh sewage sludge, composted sewage sludge, and a mixture of fresh sludge and a commercial humic substance on restoring the quality of a degraded soil 7 years after a one-time amendment with 25 kg/m organic matter. The soil used in the study was a sandy Xeric Torriorthent. Because the experimental soil plots had a slope of 15%, the effect of soil slope on these properties was also determined. Soil samples were collected from the surface layer (0-20 cm) of the bill top (shoulder) and bottom of the hill (foot slope) portions of each plot. Chemical properties, mineralogy of the clay fraction, and humic and fulvic acid contents and composition were analyzed. The exogenous organic matter was apparently incorporated into the native soil humic and fulvic acids, resulting in a significant improvement of the overall soil chemical quality. Mineral composition and amounts and structure of humic acids varied between the shoulder and foot slope portions of soil plots, suggesting an effect of soil erosion on the distribution of these substances. Organic matter amendment seemed to limit soil runoff, but more research is needed to evaluate humic substance-clay mineral interaction effects on the formation of more stable soil aggregates and on the resulting impact on soil erosion at these sites.