Impact of surfactants on aggregate and colloidal stability of two tropical soils

Abstract

This study was undertaken to evaluate the modifications in water-stability of soil aggregates treated with anionic and nonionic surfactants. Two texturally contrasting soils from southern Nigeria (an Entisol and an Ultisol), and five surfactants (three anionics and two nonionics) were evaluated using four application rates (0, 0.1, 1.0 and 10.0 g/kg). Each treatment was replicated three times. The three anionic surfactants were sodium dodecylsulphate (AS 1), sodium dodecylbenzenesulphonate (AS 2) and sodium 1-hexadecanesulphonate (AS 3) whereas the two nonionic surfactants were polyoxyethylene = oleylether (NS 1) and polyoxyethylene sorbitomonoleate (NS 2). On both soils increasing rates of anionic surfactants progressively decreased aggregate stability in water at the macro and colloidal aggregation levels. Application rates >1.0 g/kg were significantly different from the control and the 0.1 g/kg treatments. At the highest application rate AS decreased percent water stable aggregates >0.5 mm by 13.8% and 10.2%; increased dispersible clay by 11.3% and 23.0% and decreased aggregated clay 24.5% and 23.7% on the Entisol and Ultisol, respectively. The nonionic surfactants had opposite effects with aggregate stability significantly enhanced as application rates increased. On the average NS increased percent water-stable aggregates by 66.0% and 13.0%; decreased dispersible clay by 18.4% and 24.0% and increased aggregated clay by 30.6% and 26.7% respectively, on the Entisol and Ultisol. Differences in aggregate stability within each surfactant group were not significant at P=0.05 . The possible acting mechanisms of these surfactants and soil management implications of these results are discussed.