Particle-size classes are intended to form the bridge between pedologic and engineering soil classifications. If texture class and rock fragment content alone could do this, then there would be no need for the particle-size classes. Realize that some soil texture classes (such as loam) could be in the coarse-loamy, fine-loamy, coarse-silty, or fine-silty particle-size classes depending on total clay content, fine sand or coarser content, and rock fragment content. Even the percent carbonate clay (CO3-clay) could affect the class placement. There is also the mineralogy of the sand and silt fractions (i.e. substitute classes such as ashy or medial) that must be taken into account.
Those variables aside, there are several other properties and factors that need to be considered. The first factors considered are those needed to determine the particle-size control section (see Key to the Control Section for Particle-Size Classes on page 297 of the 10th edition of the Keys to Soil Taxonomy). These are the soil depth; the assumed taxonomic placement in Order, Great group, and Subgroup; the diagnostic horizons and characteristics present or absent; the pre-assumption of whether a strongly contrasting particle-size class occurs in the soil; and the critical depths and horizon boundaries. All of these factors must be considered first before the Key to Particle-Size and Substitute Classes is even used to determine a particle-size or substitute class.
So using just texture classes and rock fragment content is much too simplistic an approach. There is no computer software that can properly assign particle-size classes to soils using just these two properties.
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Particle-Size Classes
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