Determination of clay contents in soils containing allophane

Abstract

The clay contents of soils containing allophane were derived by separating clay fractions at the appropriately value and correcting the amount of clay so indicated by means of specific surface measurements ts on the clay and residual silt plus sand fractions, as: described in a previous indication. Where the soil appeared to be derived from fairly homogeneous parent material, clay contents of topsoils and subsoils as given by this procedure were very similar, but coarse-iextured soils low in clay and high in organic matter showed somewhat more clay in the topsoils. Measurement of specific surface by absorption of acetic acid from toluene solution led to values of clay content that were similar to those obtained when specific surfaces were measured by adsorption of nitrogen if the topsoils were d medium to high clay content. For these topsoils the relationship of clay fraction constituents to pH for optimum dispersion was the same as for subsoils, but topsoils low in clay and high in organic matter would disperse only in alkaline medium. The resulting day fractions from these latter topsoils showed high contents of amorphous silica and of residual organic matter. Some correbtion between clay percentage and the amount of phosphorus absorbed from a 2.5 percent solution of ammonium phosphate at pH 5 was obtained when the clay fractions contained only allophane or allophane plus minor amounts of gibbsite.