Evaluating methodological parameters to quantify particle size of organic soil material with laser diffraction

The recognition that texture is a “master soil property” points toward the need for actual quantification of particle size in organic soil material. Using a multi‐wave particle size analyzer, fibric and sapric soil samples were circulated in deionized water through a closed aqueous loop at 9.6 L min−1, and the following methodological parameters were investigated: pre‐treatment, circulation time, and refractive index. Our results show that pre‐treatment for organic soil samples is dependent upon the degree of decomposition; the intact and dispersed PSDs for fibrous samples were similiar, whereas the PSDs for sapric samples showed a shift from 500–2000 (intact) μm to 5–100 (dispersed) μm. Circulation time was investigated using mean particle diameter and specific surface area. We demonstrated that as circulation time increased, the mean particle diameter decreased and the specific surface area increased out to 30 min as mechanical dispersion and/or fragmentation of organic particles occurred. However, circulation time after 5 min is not significantly different in terms of mean particle diameter. To investigate refractive index, 12 optical models were created. When determined across all intact samples, uncertainty was low within individual bins, with a maximum value of 0.07 ± 0.04% v/v. For dispersed samples, uncertainty increased within the silt sized region and had a maximum value of 0.17 ± 0.07% v/v. This study demonstrates that the particle diameter of organic soil material can be measured by LD with comparable certainty as that of mineral soil material using the methodological approach used in this study. Core Ideas Established baseline laser diffraction parameters to investigate the diameter of organic soil material. Uncertainty induced by particle refractive index (RI) assumptions is evaluated across the entire fine earth fraction for organic soil material. Linear relationships between particle RI assumptions and size class volumetric concentrations are established in organic soil material. Mean particle size diameter and specific surface area did not significantly differ after 5 min of circulation. Results indicate that the number of intact fibers and organic matter content influence the requirement for chemical dispersion.