Surface structure in gray clays of northwestern New South Wales in relation to micromorphology, cation suite and particle size attributes

Abstract

Considerable differences in surface structure (0-100 mm) were observed in the field in two adjacent areas of grey clays near Narrabri, N.S.W. The absence of any differences in clay mineralogy and granulometry of the sand fraction supported the field assessment that both types of soil were similar in provenance. A transect of soil profiles including seven with poor structure and five with well-structured surface horizons was examined. The field observations of structure were supported by photographs of the surface, and water entry after rain. Micromorphological examination showed that closely spaced porphyric to adporphyric fabric in the poorly structured soils contributed to poor structure, highlighting the importance of textural attributes. The well-structured soils had a more widely spaced porphyric fabric. A measure of dispersibility depending on clay content and exchangeable plus soluble Na, Ca and Mg tallied very well with the field assessment of soil structure. Five groups were obtained from a euclidean distance/flexible sort strategy on the basis of cation suite, carbon content and particle size attributes. The groups identified areas of poor structure very well and the groups appear to be discriminated mainly on the basis of differences in Na, Ca and clay content. Treating the transect as a continuum of soils of very poor structure at site 1 grading to very good at site 12 showed that greater values for Ca, K, and clay were associated with good structure and greater values for Mg, C and silt were associated with poor structure. The sodium adsorption ratio and ionic strength of the soil solution were not on their own good predictors of structural behaviour possibly due to the independent contribution of Ca and Mg in this respect.