Contents lists available at ScienceDirect Soil & Tillage Research journal homepage: www.elsevier.com/locate/still Shall we abandon sedimentation methods for particle size analysis in soils? M. Bittelli a, ⁎ , M.C. Andrenelli b , G. Simonetti c , S. Pellegrini b , G. Artioli d , I. Piccoli c , F. Morari c a Department of Agricultural and Food Sciences, University of Bologna, Bologna, Italy b CREA-AA, Council for Agricultural Research and Economics, Research Centre for Agriculture and Environment, Firenze, Italy c Department of Agronomy, Food, Natural Resources, Animals and Environment, University of Padova, Padova, Italy d Department of Geosciences, University of Padova, Padova, Italy ARTICLE INFO Keywords: Particle size analysis Comparative analysis Sedimentation methods Laser diffraction Digital image analysis ABSTRACT For many years papers have been published showing differences between sedimentation-based methods against laser diffraction. Differences were found especially in the fine texture ranges and regression equations were presented to convert data obtained between different methods. In this paper we aimed at understanding which method is closer to an independent measurement of particle size. We selected a new, automated image analysis technique as a reference method. Since with this new method each individual particle is photographed, its pixels counted and its shape analysed, we assumed this method as the reference method against which test the accuracy of sedimentation methods and laser diffraction. Comparison showed that laser diffraction was in better agree- ment with the independent optical methods, indicating that the sedimentation methods tend to largely over- estimate the finer fractions of the distribution. Considering the results presented in this research, and the methodological disadvantages of sedimentation methods, we propose to employ laser diffraction as a standard method for particle size analysis in soils. 1. Introduction Particle size distribution (PSD) of soils is a basic property that af- fects a large number of soil variables and processes, such as soil water retention, hydraulic conductivity, thermal properties and geo-me- chanical properties. A large variety of methods to measure the PSD have been developed (Allen, 1981; Gee and Or, 2002; Goossens, 2008). The pipette method (P) has been recommended as a standard for measurement in mineral soils (International Standards, ISO 11277, 2009). This method has been used for many decades and the obtained data populated soil maps and databases worldwide. Another method based on sedimentation theory is the Sedigraph (S) (Micromeritics In- strument Corp., Norcross, GA, USA), which measures the X-ray ab- sorption and results are equally expressed as percent by mass. Although sedimentation methods are still the standard, they have many dis- advantages: (a) small range and limited number of size classes when compared to other techniques such as laser diffraction, (b) lack of re- liable data at smaller sizes (< 2 μm) due to Brownian motion effects on sedimentation times (Loveland and Whalley, 2001), (c) long analysis time and, (d) assumptions about the homogeneous density and sphericity of all the particles (Clifton et al., 1999). Due to these lim- itations, many researchers investigated the potential of developing other techniques. Among the alternative available techniques, Laser Diffraction (LD) is becoming widely utilized, since it has several advantages (Allen, 1981). Sample analysis by LD is fast, covers a wide range of size classes, provides many data points allowing for obtaining a detailed PSD and the amount of soil needed for the measurement is small. Many re- searches have been performed in Laser Diffraction. Haynes (2008) studied the effects of the Refractive Index (RI) on PSD analysis. Eshel et al. (2004) performed several tests on the variation of PSD obtained from LD and reported that a value of RI = 1.53 was suitable for most soils. This is consistent with reporting of Jonasz (1987), who reported that the scattering cross-section for a particle increases with the real part of the RI. Ozer et al. (2010) presented similar values of RI and AC of 1.55 and 0.1 for laser diffraction in naturally soils. Moreover Eshel et al. (2004) showed “overestimation” of clay content with sedi- mentation methods when compared to laser diffraction. Studies have been performed to assess the most suitable medium of suspension and method of dispersion for a Malvern laser sizer on sandy soil (Chappell, 1998). Vdovic et al. (2010) investigated the effects of sample pre-treatment and performed a comparison among different methods. Storti and Balsamo (2010) investigated the effect of disper- sing methods and properties of the dispersion units for different vo- lumes and pump speed on the PSD results for sands. Újvári et al. (2016) discussed the importance of grain size analysis https://doi.org/10.1016/j.still.2018.08.018 Received 6 November 2017; Received in revised form 17 April 2018; Accepted 31 August 2018 ⁎ Corresponding author. E-mail address: marco.bittelli@unibo.it (M. Bittelli). Soil & Tillage Research 185 (2019) 36–46 Available online 07 September 2018 0167-1987/ © 2018 Elsevier B.V. All rights reserved. T