Laser Grain-Size Determination in Soil Genetic Studies 1. Practical Problems

Grain-sizing by laser diffraction cannot replace the classical combination of sieving and sedimentation as long as correlations between the methods have not been established for many populations of samples. Nevertheless, grain-size determination by laser diffraction has a great potential for use in soil science, e.g., for detailed comparison of samples from the same origin to establish homogeneity of parent materials, for the study of texture changes caused by weathering, and for changes in aggregation. In particular, the amount of information relating to fine fractions and the continuous distribution curves makes the method very valuable. Correct use for clay-size fractions depends on the choice of optical properties for the measured material; these have to be obtained for each kind of material and change with pretreatment of the samples. Laser diffraction will always provide more detailed information than can be obtained by sieving and sedimentation. Practical problems relate primarily to homogeneity/sample size and flocculation. Ways to overcome such problems are discussed.