Aggregation and Aggregate Stability in Forest and Range Soils

Abstract

AbstractA simple method for measuring aggregation and aggregate stability across soils of widely varying structure and texture is presented. The method uses standard techniques of particle‐size analysis to measure yield of fine particles (<26‐µm estimated spherical diam.) after three types of dispersion treatments: shaking in water or in sodium oxalate (Ox), and sonication in sodium pyrophosphate (PPi). PPi sonication, which provided maximum dispersion, served as our benchmark; it was substantially more effective than the commonly used stirring in hexametaphosphate. Aggregation was then calculated as yield of fines after PPi sonication minus yield of fines after the water shake, the minimum dispersion treatment. Aggregate stability was measured by comparing yield of fines after the Ox shake, the mild dispersion treatment, with that after the water shake; aggregate instability was then calculated. The five soils studied varied markedly in both aggregation and aggregate stability, suggesting that mechanisms of aggregate stabilization differ markedly across soil types. Despite these differences, our method of quantifying mild dispersion by shaking with Ox, which should have negligible effect on subsequent microbiological studies, relative to complete dispersion by PPi sonication should be consistent and applicable over a reasonably wide range of soil types.