Establishing a standard protocol for soil texture analysis using the laser diffraction technique

Authors: Stacanelli Pires, Isis; Nouwakpo, Sayjro; Bjorneberg, David - Dave; Rogers, Christopher; Vitko, Lauren

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/9/2024
Publication Date: 8/3/2024
Citation: Stacanelli Pires, I., Nouwakpo, S.K., Bjorneberg, D.L., Rogers, C.W., Vitko, L.F. 2024. Establishing a standard protocol for soil texture analysis using the laser diffraction technique. Soil Science Society of America Journal. https://doi.org/10.1002/saj2.20738.
DOI: https://doi.org/10.1002/saj2.20738

Interpretive Summary: This study presents results of experiments conducted to develop a standard protocol for soil texture analysis using the laser diffraction method. Optical methods including laser diffraction have been increasingly used to measure soil texture and particle size distribution. However, they have not been adopted yet as a routine methodology mainly due to the difficulties in comparing their results to more commonly-used techniques (i.e., sedimentation methods). The experiments compared fractions of sand, silt, and clay measured using the laser diffraction method to those obtained using the traditional sieve-hydrometer method. Soil texture class determined by laser diffraction agreed with the sieve-hydrometer method in 98% of the runs when the following parameters were used: (1) Refractive index of 1.44 - 0.100i, (2) 180 seconds of sonication, (3) sand sieving prior to analysis, and (4) sample dispersion by shaking the sample for 1 hour with 5% of sodium hexametaphosphate. We observed that adding the entire sample to the analyzer (1 g of soil in 100 mL of dispersant) while keeping the appropriate levels of transmittance through dilution (transmittance matching) is a better way of sample addition in comparison to sub-sampling with a pipette, especially for coarser soil samples. This work proposes a standard operation procedure that may broaden the adoption of laser diffraction analysis as a routine soil texture methodology.

Technical Abstract: Optical methods including laser diffraction have been increasingly used to measure soil texture and particle size distribution. However, they have not been adopted yet as a routine methodology mainly due to the difficulties in comparing their results to more commonly-used techniques (i.e., sedimentation methods). Many attempts exist in the literature to find an agreement between methodologies with relative success. In this work, we aim to improve the agreement between methodologies by adjusting parameters of the laser diffraction analysis, including sample treatment (chemical dispersion, carbonate removal, sand separation), mode of sample addition (sub-sampling versus transmittance matching), and analysis parameters (time of sonication, refractive index). Soil texture class determined by laser diffraction agreed with the sieve-hydrometer method in 98% of the runs when the following parameters were used: (1) Refractive index of 1.44 - 0.100i, (2) 180 seconds of sonication, (3) sand sieving prior to analysis, and (4) sample dispersion by shaking the sample for 1 hour with 5% of sodium hexametaphosphate. We observed that adding the entire sample to the analyzer (1 g of soil in 100 mL of dispersant) while keeping the appropriate levels of transmittance through dilution (transmittance matching) is a better way of sample addition in comparison to sub-sampling, especially for coarser soil samples. This work proposes a standard operation procedure that may broaden the adoption of laser diffraction analysis as a routine soil texture methodology.