Neutralization Curves of the Colloids of Soils Representative of the Great Soil Groups

Further studies concerning the nature of colloidal soil acids have been made by means of titration curves. The colloids used were from representative profiles of various great soil groups. The colloids were electrodialyzed, and various increments of sodium hydroxide added. When approximate equilibrium was reached, pH determinations were made. Colloids from the different soil groups differ widely in their fundamental acid character. The colloids of the Hastings profile, a dry land soil, have a relatively strong acid character. This is shown by the initial pH of the electrodialyzed colloid, by the base required to produce pH 7* and by buffer action in the region of high pH. There is some tendency toward a break In the curve in the region around pH 7> but such a break is not very impressive. The Carrington colloids of the prairie group and the Miami, representative of mature profiles of the Gray-Brown Podzolic group, differ from the dry land colloids more in degree than in fundamental difference of character. The laterltic Ruston colloid differs in all fundamental respects from the dry land colloids in that it is a much weaker acid, as shown by the various criteria. The Trenary colloids from a typical Podzol profile vary widely In character throughout the different horizons. Their organic matter content is high and appears to have much stronger acid properties than any of the inorganic soil colloids. The Inorganic colloids are, in some cases, nearly freed from the organic fraction by treatment with hydrogen peroxide. The inorganic colloids are weaker than the acid of bentonite, and are stronger than the acid of a halloysitic colloid whose silica-alumina ratio is very nearly 2. The stronger soil colloid acids are no more acid than silicic acid as regards pH, but have much greater base exchange capacity than silicic acid or any possible simple mixture of silicic acid and aluminum hydroxide.