AGGREGATE SIZE DISTRIBUTION AND STABILITY OF AN OXISOL UNDER LEGUME-BASED AND PURE GRASS PASTURES IN THE EASTERN COLOMBIAN SAVANNAS

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