Algorithm to harmonize soil particle size data to the FAO/USDA system

Different countries often measure and express soil particle-size distribution using different delineations between the main textural components, clay, silt and sand content. In order to harmonize such diverse data so that a uniform textural classification system can be used, interpolation of the data is necessary. Here we provide an example algorithm written in MATLAB that helps harmonize such data country-by-country to the FAO-USDA particle-size classification system that defines clay content as the mass of solids (individual particles) that are <0.002 mm, silt as the mass of solids in the 0.002 – 0.05 mm size range, and sand content as the mass of solids in the 0.05 – 2 mm size range (USDA 1951; FAO 1990). This system considers particles sized above 2 mm as gravel or stones. The algorithm uses k-nearest neighbor type pattern recognition in a non-spatial context algorithm to achieve this goal (Nemes et al. 1999; Nemes et al. 2006). Note: The algorithm uses a pre-existing external reference data set to compare the current data with. That data set cannot be provided with the algorithm due to prior agreements about the use and availability of those data, but its description is provided on pages 125-127 in the report by Weynants et al. (2013), and the authors herein offer their collaboration with a future user in order to take advantage of this algorithm. References FAO, Food, and Agricultural Organization. 1990. Guidelines for Soil Profile Description. 3rd ed. Rome: FAO. Nemes, A., J. H. M. Wösten, A. Lilly, and JH Oude Voshaar. 1999. “Evaluation of different procedures to interpolate particle-size distributions to achieve compatibility within soil databases.” Geoderma 90: 187–202. http://www.sciencedirect.com/science/article/pii/S0016706199000142. Nemes, A., W. J. Rawls, and Y. A. Pachepsky. 2006. “Use of the Nonparametric Nearest Neighbor Approach to Estimate Soil Hydraulic Properties.” Soil Science Society of America Journal 70 (2): 327–36. https://doi.org/10.2136/SSSAJ2005.0128. USDA, United States Department of Agriculture. 1951. Soil survey manual, U.S. Dept. Agriculture Handbook No. 18. Washington, DC. Weynants, Mélanie, Luca Montanarella, Gergely Tóth, Arnold Arnoldussen, María Anaya Romero, George Bilas, Trond Borresen, et al. 2013. “European HYdropedological Data Inventory (EU-HYDI).” Luxembourg: European Commission EUR 26053 – Joint Research Centre – Institute for Environment; Sustainability; EUR – Scientific; Technical Research series – ISSN 1831-9424. https://doi.org/10.2788/5936.