LETTER TO THE EDITOR COMMENTS ON ‘‘A FAST METHOD FOR DETERMINING SOIL PARTICLE SIZE DISTRIBUTION USING A LASER INSTRUMENT’’ BY F. J. ARRIAGA, B. LOWERY, AND D. W. MAYS. SOIL SCI. 171:663–674 (2006) Particle size distribution (PSD) is one of the most fundamental physical properties of a soil, strongly affecting many physical and chemical soil properties. Commonly used methods for PSD determination, such as sieving and procedures based on sedimentation (e.g., the pipette and hydrometer methods), are both time and labor consuming. Therefore, the attempt to develop a fast and simple method for PSD determination, such as that proposed in the current article with the aid of a laser diffraction instrument (LD), is most welcome and should be encouraged. However, the authors have set themselves an additional goal, namely to relate and match PSD data obtained from the LD to those from the combined sieve-pipette method. We main- tain that such an approach, which attempts to convert LD-derived volume percent of a given size fraction to a pipette-derived mass percent (or vice versa), is inaccurate and should not be advocated, because it may possibly mislead readers, especially those unfamiliar with the finer details of the various methods used for PSD analyses. We base our contention on the following arguments. NONSPHERICITY OF SOIL PARTICLES A major problem associated with the accurate description of the PSD of soil material is the fact that soil particles are irregularly shaped particles. Hence, for simplicity and ease of analysis, particle size in soils is presented as an Bequivalent^ spherical particle, which allows the use of a single length (apparent diameter) as the descriptor. The impact of applying this simplification on the obtained PSD from different methods has been discussed in detail by Eshel et al. (2004) and is summarized as follows. In the case of sieving, the net outcome of the nonsphericity of soil particles results in the underestimation of the sand size fraction when wet sieving and sedimentation are combined for the determination of PSD. In sedimentation-based techniques (e.g., pipette), the nonsphericity of the particles affects the settling velocity, and the fine size fraction is, therefore, overestimated. In the LD analysis, the projected cross-sectional area of a nonspherical particle is larger than that of a sphere with an equal volume (Jonasz, 1991), which leads to a shift of the PSD toward its coarser fractions. It is evident from the above that the afore- mentioned simplification, arising from the non- sphericity of soil particles, leads to the dependence of the obtained PSD on the method used for its determination (Matthews, 1991). This dependence casts severe doubts as to the validity of the attempts to correlate PSD data obtained by one method to those obtained by a different method. Furthermore, it clearly indi- cates that there is no Bexact^ method (as referred to the pipette method by the authors in the Conclusion section of the article) but that the choice between methods depends simply on the balance between the pros and cons of each. REFRACTIVE INDEX Use of the LD method for determining soil PSD entails the employment of the Mie theory model. The Mie theory requires, as an input parameter, the refractive index (RI), which is a complex number comprised of (i) a real part (n r ), which represents the change in the velocity of light through the tested material compared with the velocity of light in vacuum, and (ii) an imaginary term (n i ), which represents the trans- parency and absorptivity of that material. The RI is, therefore, not just a Bfitting parameter^ that can be manipulated to bring closer different data sets, but a parameter of a physical meaning 413 0038-075X/07/17205-413–415 May 2007 Soil Science Vol. 172, No. 5 Copyright * 2007 by Lippincott Williams & Wilkins, Inc. Printed in U.S.A. Copyr ight © Lippincott Williams & Wilkins. Unauthor iz ed reproduction of this article is prohibited.