Modified centroid for estimating sand, silt, and clay from soil texture class

Authors: Levi, Matthew

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/16/2017
Publication Date: 6/30/2017
Publication URL: https://handle.nal.usda.gov/10113/5863767
Citation: Levi, M.R. 2017. Modified centroid for estimating sand, silt, and clay from soil texture class. Soil Science Society of America Journal. 81:578-588. https://doi.org/10.2136/sssaj2016.09.0301.
DOI: https://doi.org/10.2136/sssaj2016.09.0301

Interpretive Summary: Soil texture class and clay percentage are collected as a standard practice for many land management agencies (e.g., NRCS, BLM, USFS, FAO) and clay content is frequently estimated with acceptable accuracy (± 3-5%). I tested the concept of a modified centroid approach (mcent) for soils that have clay estimates for 75,736 soil samples from the National Cooperative Soil Survey pedon database that also had measured hydraulic soil properties. Comparisons were made using the Rosetta pedotransfer function to test modeled values using gcent, mcent, and measured particle size in comparison to measured values of water content at field capacity ('330) wilting point ('15000,) and available water holding capacity. The mcent approach produced a continuous distribution of values for sand and silt whereas the gcent model resulted in a stair-step pattern for each of the 12 texture classes. Rosetta generally underestimated water content compared to measured values. Comparison of all samples showed larger R2 and smaller error for '15000 compared to '330. Results support the practice of collecting field estimates of clay percentage along with texture class to expand the use of soil profile data for modeling soil processes that will advance our understanding of soil contributions to ecosystem function.

Technical Abstract: Models that require inputs of soil particle size commonly use soil texture class for input; however, texture classes do not represent the continuum of soil size fractions. Soil texture class and clay percentage are collected as a standard practice for many land management agencies (e.g., NRCS, BLM, FAO) and clay content is frequently estimated with acceptable accuracy (±5%). When clay and texture class is known, sand and silt can be constrained to a narrow range that may differ from the geometric centroid of a given texture class (gcent). I tested the concept of a modified centroid approach (mcent) using 75,736 soil samples from the National Cooperative Soil Survey pedon database that also had measured hydraulic soil properties. Comparisons were made using the Rosetta pedotransfer function (PTF) to test modeled values from gcent, mcent, and measured data in comparison to measured values of water content at field capacity ('330) wilting point ('15000,). The mcent approach produced a continuous distribution of values for sand and silt whereas gcent produced a stair-step pattern for each of the 12 texture classes. Rosetta underestimated water content. Unbiased root mean square error was smaller for mcent than gcent for seven and ten of the 12 soil texture classes for '330 and '15000, respectively. Results support the practice of collecting field estimates of clay percentage along with texture class to expand the use of soil profile data for both spatial and non-spatial modeling of soil processes that will advance our understanding of soil contributions to ecosystem function.