LASER GRAIN-SIZE DETERMINATION IN SOIL GENETIC-STUDIES .1. PRACTICAL PROBLEMS

Grain-sizing by laser diffraction cannot replace the classical combina tion of sieving and sedimentation as long as correlations between the methods have not been established for many populations of samples. Nev ertheless, grain-size determination by laser diffraction has a great p otential for use in soil science, e.g., for detailed comparison of sam ples from the same origin to establish homogeneity of parent materials , for the study of texture changes caused by weathering, and for chang es in aggregation. In particular, the amount of information relating t o fine fractions and the continuous distribution curves makes the meth od very valuable. Correct use for clay-size fractions depends on the c hoice of optical properties for the measured material; these have to b e obtained for each kind of material and change with pretreatment of t he samples. Laser diffraction will always provide more detailed inform ation than can be obtained by sieving and sedimentation. Practical pro blems relate primarily to homogeneity/sample size and flocculation. Wa ys to overcome such problems are discussed.