Effects of different dispersion and fine fraction determination methods on the results of routine particle size analysis

Abstract

Particle-size analysis (PSA) methods to be used in routine situations need to be rapid, require no prior information about the sample and give precise results over a wide range of soil textures. Effects of three physical dispersion and two fine-fraction determination methods on PSA results were investigated over a wide range of soil textures to find the most appropriate technique for routine PSA. Interactions between physical dispersion and fine-fraction determination methods were also investigated. The reciprocating shaker produced significantly lower silt and fine sand and higher coarse sand (and clay, although not significant) values than either drink mixer or end-over-end shaker dispersions. This result was interpreted as the reciprocating shaker giving the most effective dispersion, with aggregated clay being dispersed to primary particles while coarse sand was not fragmented to fine sand or silt. The end-over-end shaker did not reliably disperse a heavy clay soil, and so cannot be recommended for routine use where similar soils may be encountered. When considered over all soils and dispersion methods, hydrometer clay and clay + silt values were higher than pipette values. These results were due to the effective depth of the hydrometer being greater, on average, than the depth of the pipette. However, there were interactions between dispersion and fine-fraction determination methods for the clay and clay +silt classes. Hydrometer values were greater than pipette values with drink mixer and end-over-end shaker dispersion, but were similar with reciprocating shaker dispersion. For the clay fraction, inferior dispersion given by the drink mixer and end-over-end shaker resulted in a significant mass of particles between the sampling depths of the pipette and hydrometer, causing the higher hydrometer values. For the clay +silt fraction, both drink mixer and end-over-end dispersion methods fragmented sands to a size which was recorded by the hydrometer but not the pipette. These interactions highlighted the requirement for effective dispersion where clay and silt are determined by the hydrometer, and may explain some of the conflicting observations of the precision of the hydrometer compared with the pipette. Reciprocating shaker physical dispersion combined with the hydrometer fine-fraction determination method was found the most appropriate PSA method combination for use in a routine situation.