Dispersed clay and organic matter in soil: their nature and associations

Clay dispersion in soil results in structural instability and management pr oblems. The aim of this study was to determine whether or not the easily di spersed colloidal materials direr in their properties from colloidal materi als that do not disperse easily. Soil samples from the topsoil of sodic and non-sodic variants of an Alfisol under irrigated pasture (Kyabram, Victori a, Australia), and from the topsoil and subsoil of a sodic Alfisol under cu ltivation (Two Wells, South Australia) were fractionated into easily disper sed, moderately dispersed, and difficult to disperse clay, and silt, sand, and light fractions. As a proportion of total clay, easily dispersed clay c ontent was greatest in the subsoil, and least in the Kyabram topsoils. In t he topsoils, easily dispersed clay had larger particle size and lower catio n exchange capacity than difficult to disperse clay, suggesting that high s urface area and charge lead to increased inter-particle interactions and lo wer dispersibility. Easily dispersed clay had lower organic C contents than difficult to disperse clay. Organic matter was examined by C-13 nuclear ma gnetic resonance, and the spectra were interpreted using major groups of bi omolecules as model components. In all soils, organic matter in the easily dispersed clay fraction contained a high proportion of amino acids, suggest ing that amino acids or proteins acted as dispersants. Difficult to dispers e clay contained a high proportion of aliphatic materials in the topsoils, and carbohydrate in the subsoil, suggesting that these materials acted as w ater-stable glues. Selectivity for Na (K-G) was negatively correlated with organic C content in the clay fractions. In the Kyabram soils, K-G was grea ter in easily dispersed clay than in difficult to disperse clay. In Two Wel ls soil, clay with high K-G appeared to have already moved out of the topso il, into the subsoil. This work showed that variability in the nature of or ganic matter and clay particles has an important influence on clay dispersi on in sodic and non-sodic soils.