Particle-size distribution of soil and the perception of texture

It has been suggested in this journal that, for uniformity, the texture diagram of the United States Department of Agriculture could be adopted in Australia. However, it is argued here that the Australian diagram should be retained because of the advantages given by its more systematic set of class limits with smooth transitions between classes, and its use of sloping, rather than horizontal, lines for clay content. This sloping trend accords with the behaviour of soil when tested for texture in the field. The principles underlying these features are discussed. Changes to the USDA diagram have brought it closer to the Australian diagram. The Australian diagram is unchanged but the fraction sizes chosen for it have been criticised. Raising the upper limit of the silt fraction from 20 mum to 50 mum would make its fractions uniform with those of the USDA. It would also double the average silt content and so give a better spread of texture classes. The diagram can be converted without losing its design advantages or changing its texture standards.