PRETREATMENT EFFECT ON DISPERSION OF CARBONATES IN CALCAREOUS SOILS

A pH-state procedure was previously developed to determine the effective particle-size distribution of soil carbonates. The results obtained by the pH-stat procedure are influenced by the accessibility of carbonate surface sites to the titrant and, therefore, provide a methodology for the comparison of carbonate dispersion treatments. In this study, the influence of dispersion treatment on the determination of effective particle-size distribution of soil carbonates was examined. The treatments consisted of agitation, sonication, or Na saturation of clay cation-exchange sites of calcereous soils representing three series (Austin: fine-silty, carbonatic, thermic Entic Haplustoll; Rio Grande: coarse-silty, mixed (calcareous), hyperthermic Typic Ustifluvent; Victoria: fine, montmorillonitic, hyperthermic Udic Pellustert). Shaking and sonication pretreatments resulted in increases in the experimentally determined proportion of effective fine-clay size soil carbonates and corresponding decreases in the effective silt- and sand-size carbonates, when compared with the untreated soils. As the intensity of either of these pretreatments was increased, the proportion of effective clay-size carbonates increased. Scanning electron microscopy indicated that the longer and more intense sonication treatments resulted in a higher degree of disruption or grinding of carbonate aggregates. Sodium saturation of the clay cation-exchange sites resulted in the most effective disaggregation of the phyllosilicate-carbonate complex, as indicated by the greatest proportion of effective clay-size and lowest proportions of effective silt- and sand-size carbonates. Sodium saturation may also result in a lower degree of disruption of carbonate particles or carbonate-cemented aggregates than may occur with intense sonication. An initial Na saturatio is, therefore, recommended as a pretreatment for the determination of effective particle-size distribution of soil carbonates.