The effect of allophane on soil properties

Abstract

Allophane is a widely distributed clay mineral. Although characteristic of soils derived from volcanic ash, it has also been identified in Podzols and podzolised soils. Certain distinctive characteristics can be attributed to the spherical shape of allophane. The apparent specific surface is high and is decreased markedly on drying, thus enabling the external and internal surface areas and hence diameters of the spherules to be calculated. High phosphate retention is characteristics. Equations are presented showing that for allophanic soils there is a weaker correlation of phosphate retention with aluminum soluble in Tamm's solution than in non-allophanic soils. This is believed to be due to the dissolution of aluminum in hydrous oxides, organic complexes, as well as in allophane, all of which have differing capabilities to retain phosphate. Bulk density is low due to the high porosity resulting from the spherules remaining discrete. Very low bulk densities are found in perhumid climates. Shear strength of allophanic soils is generally less than in non-allophanic soils. This is due to lower cohesion since friction is not altered. Both field moisture and liquid limits tend to be high. Wilting point expressed on a unit-clay basis is also high and like liquid limit it decreases markedly on drying. In very wet climates the wilting point may exceed 100%.