Ultrasonic Dispersion to Investigate the Role of Mineral-Organic Interactions in Aggregation Processes.

Application of ultrasonic energy is a common laboratory tool in the dispersion of soil aggregates. In this study, we used ultra sonic energy (100 to 2000 J cm^-3) to explore the strength of mineral-organic associations in six soils that varied in a range of soil mineral properties. At first we removed the coarse fraction (>250 µm), the remaining residue (<250 µm) was subsequently mixed with water and ultrasonically dispersed by application of 100, 200, 400, 500, 1000, 1500 and 2000 J cm^-3 energy. After dispersion the suspension was centrifuged, the supernatant was decanted and filtrated and the solid residue was freeze dried. We determined the amounts of water extracted organic carbon (OC), Fe, Al, Ca, Mn, and Mg in the filtrates. The extracted OM and solid residues were further characterized by Fourier Transformed Infra Red spectroscopy and Scanning Electron Microscopy. First results show a linear increase in amount of dissolved organic carbon with increasing amounts of ultra sonic energy up to 1500 J cm^-3 indicating maximum dispersion of soil aggregates at this energy level. In contrast to Mn, and Mg, the amounts of dissolved Ca, Fe and Al generally increase with increasing ultra sonic energy amounts up to 1500 J cm^-3. If the increases in the dissolved Fe and Al amounts result from the disintegration of poorly crystalline pedogenic oxides, a positive relationship to oxalate soluble soil Fe and Al contents can be expected. Since the studied soils are free of carbonates, we hypothesize that the dissolved Ca may originate from Ca ions involved in aggregation processes.