Layer charge characteristics of smectites from Vertosols (Vertisols) of New South Wales

Balwant Singh and Susan Heffernan

Abstract

Premature senescence in cotton has been attributed to K deficiency in the cotton soils of Australia. The availability, release, and fixation of K⁺ in soils are mainly dependent on the clay mineralogy and layer charge characteristics of 2 : 1 clay minerals. There is a little information on the mineralogy and charge characteristics of the cotton growing soils (Vertosols) of Australia. The aims of this study were to determine the clay mineralogy, the layer charge density, and layer charge distribution of some cotton growing soils by chemical and X-ray diffraction methods.

Most soil clays contain abundant smectite associated with small amounts of mica, kaolinite, and an interstratified mineral. The total layer charge as determined by the alkylammonium method ranged between 0.55 and 0.67 mol(–)/(O₁₀(OH)₂), indicating a high interlayer charge density. The layer charge of smectites from different valleys and for different size fractions was similar. The Greene-Kelly test showed that most of the charge originated in the tetrahedral sheet. The chemical analysis indicates that the smectite is an iron-rich beidellite, which has possibly formed from the weathering of mica.

Keywords: 2 : 1 layer silicates, Green-Kelly test, cotton soils, alkylammonium, K fixation, NH4 fixation

Australian Journal of Soil Research 40(7) 1159 - 1170 (2002) doi:10.1071/SR02017