This paper deals with the problem of determining the particle size distribution of selected organic soils from the vicinity of Rzeszów (Poland), using a laser diffractometer method, the knowledge of which will allow to determine the degree of differentiation or similarity of the tested organic soils in this aspect. The HELOS Laser Diffractometer manufactured by Sympatec GmbH was used for the tests. For proper analysis, the researches results in the form of graphs were grouped according to the content of organic substances in accordance with the standard classification. The conducted research was primarily aimed at presenting the grain differentiation and particle size distribution in terms of the applied method and comparing the test results of samples of selected, different organic soils, prepared using the same dispersion procedure and carried out in exactly the same test conditions, generated using capabilities of a diffractometer. Summing up, the laser diffractometer method presented in the article, although not fully verified in the case of organic soils, seems to be a the perspective method with capabilities allowing it to be nominated as an exceptionally useful method for the investigations of soft soils, including organic soils.

New ways of calculating narrow microparticle size distributions based on using the Tikhonov and the modified Twomey methods for the laser diffraction technique are presented. These allow to have reduced the broadening (over-smoothing) of the result occurring in these methods for narrow distributions both singular and their sum. The calculated singular distributions and their distribution sum were then approximated by a Gaussian function and a bimodal Gaussian function, respectively, using the Levenberg-Marquardt method. The angular distribution of scattering power was measured for polystyrene particles with radii of 0.676 µm and 1.573 µm, and for their sum. The tests were carried out for linearly polarized He-Ne laser light scattered by a dilute aqueous suspension of these particles. The results obtained were compared with those obtained with the nanoDS instrument (CILAS). It turned out that using the way based on the Twomey method, the parameters of the narrow distribution sought could be determined quite well.