Abstract

ABSTRACT Particle size distribution (PSD) is a soil physical property influenced by organo-mineral complexes (or-mic) in silt and clay fractions. In Andosols, these fractions are underestimated due to the presence of or-mic in soil micro-aggregates. This study estimated quantitatively the organo- mineral fraction by comparing two methodologies of granulometric analysis (sequential dispersion and sieve-sedimentation) in Andosols and Fluvisols from southern Chile. Or-mic were calculated through specific dissolution extractions (Al, Si, Fe). In addition, pHNaF, and soil organic carbon (SOC) were determined. Correlations between Alo (0.74), Alp (0.71), pH in NaF (0.41), and the or-mic were found. Or-mic were higher in Andosols, varying between 1.5 and 5.2%, which led to an underestimation of silt and clay fractions of the soil in PSD analysis and, thus, an overestimation of soil texture due to the continuous formation of micro-aggregates. Therefore, to properly determine soil PSD, it is essential to estimate or-mic, particularly in highly reactive soils, which are readily capable of binding organic compounds in their mineral fraction.