Aggregate size distribution and stability of an oxisol under legume-based and pure grass pastures in the eastern Colombian savannas

Abstract

This study evaluated soil aggregate size distribution and stability of an Oxisol under improved grass-only or grass-legume pastures, established in previously native savanna. Three grass-legume combinations were included at various stocking rates. In all treatments and soil layers, soils were well aggregated, having more than 90% of their weight in macroaggregates (>250 µm). The addition of legumes to pastures did not affect the soil aggregate size distribution, although aggregates showed somewhat more stability against slaking. An increase in stocking rate negatively affected both average aggregate size and aggregate stability. Aggregates showed little or no dispersion of clay particles in any treatment. A positive correlation was found between wet aggregate stability and hot-water extractable carbohydrate concentration, supporting the hypothesis that these carbohydrates equate with plant-derived or microbial polysaccharides which glue soil aggregates together. It is suggested that determination of hot-water extractable carbohydrates may serve as a useful indicator of small differences in aggregate stability, even when these differences are not evident in the stability measurement itself.