Critical Evaluation of the Use of Laser Diffraction for Particle-Size Distribution Analysis

m) by wet sieving from the clay and silt (particles 50 m). The sand fraction is determined by drying and Determination of soil particle-size distribution (PSD) by sieving, weighing the material remaining on the sieve. The silt hydrometer, and pipette methods as well as by laser diffraction (LD) suffers from inherent flaws, mainly due to the difficulty in defining and clay fractions are then determined by the pipette the size of irregularly shaped particles. Therefore these methods yield method (Gee and Bauder, 1986; Janitzky, 1986). These only estimates of PSD. The objective of this study was to determine classical methods suffer from numerous disadvantages. whether a functional relationship exists between the PSDs obtained For example, the sieving method is useful only in the by the combined sieve-pipette method and those obtained by LD. size range of 2000 to 50 m (Gee and Bauder, 1986). Samples from 42 California soils were analyzed. For the LD measureA major source of error in the hydrometer method is ments a Beckman–Coulter LS-230 apparatus with a 750-nm laser beam simply the inaccuracy in the hydrometer reading (Gee that measures particles in the range of 0.04 to 2000 m was used, and Bauder, 1979). The sedimentation-based methods employing the Mie theory for the PSD calculations. Values of 1.5 require relatively large samples (10–20 g for the pipette and 0.2 for the real part and the imaginary term of the refractive and 40 g for the hydrometer) and their capacity for index (RI), respectively, gave satisfactory results for the optical model calculations. Volume percentage of the clay-size fraction obtained by resolution to subgroups of sizes is limited. In addition, LD was generally lower than mass percentage of the clay fraction the sedimentation-based procedures are time-consumderived by the pipette method. The opposite trend was noted for ing, especially for the determination of 2m size fracthe silt-size fraction. Coefficients of determination for the regression tion, while for particles 1 m the results are increasequations for the clay, silt, and sand fractions determined by the two ingly unreliable because of the effect of Brownian methods were 0.702, 0.689, and 0.821, respectively. Good agreement motion on the rate of sedimentation (Allen, 1981; Lovebetween measured and calculated LD values for one size class was land and Whalley, 2001). accompanied by poor agreement between measured and calculated Numerous techniques for the determination of PSD, values for the other. The LD method provides a continuous PSD other than these classical ones, were developed, mainly curve, which enables a detailed data analysis and a flexible application for industrial applications such as the determination of of different particle-size dependent classification systems. the homogeneity of powders and gels (Buurman et al., 1997). Among those techniques, LD (or laser light scattering) was occasionally applied to soil material (e.g., P distribution is one of the most fundaCooper et al., 1984; Levy et al., 1993; Buurman et al., mental physical properties of a soil, defining, for 1997; Muggler et al., 1997; Konert and Vandenberghe, example, the soil’s texture, and strongly affecting many 1997; Chappell, 1998; Beuselinck et al., 1998). In this physical and chemical soil properties. It is typically prelatter method the forward diffraction of a laser beam sented as percentage of the total mass of soil occupied by the particles is used to determine their size distribuby a given size fraction. Determination of the soil PSD tion. The angle of diffraction is inversely proportional is not a trivial task because of the heterogeneity of the to particle size, and the intensity of the diffracted beam shape and density of particles. at any angle is a measure of the number of particles with a specific cross-sectional area in the beam’s path. Particle-Size Distribution Determination Two optical models are commonly used to calculate Methods Employed in Soil Science PSD, the Fraunhofer diffraction model and the Mie The classical techniques for determining PSD in soils theory. The former is based on the approximation that include sieving and procedures based on sedimentation, the laser beam is parallel and the detector is at a distance such as the pipette and hydrometer methods (Gee and that is very large compared with the size of the diffractBauder, 1986). The pipette and the hydrometer methods ing particle. The Mie theory is a solution of the Maxwell give comparable results (Liu et al., 1966; Walter et al., equations (i.e., a set of four fundamental equations gov1978) provided similar pretreatment techniques are erning the behavior of electric and magnetic fields) deused. However, the pipette method, requires the addiscribing propagation of the electromagnetic wave of tional step of first separating the sand fraction (50–2000 light in space. The theory provides a solution for the case of a plane wave (i.e., the wavefronts of which are G. Eshel and M.J. Singer, Dep. of Land, Air, and Water Resources, planes) on a homogeneous sphere of any size (Jonasz, University of California, Davis, CA 95616; G.J. Levy and U. Mingel1991). In addition, it takes into account phenomena grin, Institute of Soil, Water, and Environmental Sciences, Agriculother than diffraction (e.g., transmission through the tural Research Organization (ARO), The Volcani Center, P.O. Box 6, particle) and therefore requires knowledge of the RI of Bet Dagan 50250, Israel. Received 9 Apr. 2003. *Corresponding author ([email protected]). Abbreviations: LD, laser diffraction; PIDS, polarization intensity difPublished in Soil Sci. Soc. Am. J. 68:736–743 (2004).  Soil Science Society of America ferential of scattered light; PSD, particle-size distribution; RI, refractive index. 677 S. Segoe Rd., Madison, WI 53711 USA