Soil Taxonomy and Geomorphology: Better Correspondence Means Better Soil Maps.

Like most soil classification systems, Soil Taxonomy attempts to categorize the landscape by placing properties related to bioclimatic factors at the top of the system, with properties related to geology, geomorphology and topography nearer the bottom. At the lowest and most detailed levels, soil series and phases relate most closely to topography and parent material – both of which are normally fairly predictable using local geomorphic knowledge. We argue that changes made to Soil Taxonomy should facilitate soil mapping efforts, first and foremost. Taxonomic units are most easily mapped, and mapping them is done most accurately, when they match local topography. Genesis considerations must be included in the discussion of how to modify Soil Taxonomy, but should be secondary to the goal of creating better maps with more taxonomically pure soil map units. The disconnect between high resolution soil maps and Soil Taxonomy – which inevitably leads to map error - occurs because the taxonomic categories do not readily match the topography. To this end, we propose that categories of the system be reexamined and redefined, so that series concepts can better fit the five, traditional slope position categories – summit, shoulder, backslope, footslope and toeslope. We present a quantified model that accurately, objectively, and efficiently segments landscapes into these five categories. Comparisons between the hillslope categories, as developed by our model, and NCSS maps confirm that most map unit boundaries do not correspond well to these natural topographic breaks. Small tweaks to Soil Taxonomy, coupled with incorporation of base maps created by our model into routine mapping protocols, would represent a painless and rapid means of making better soil maps, and improving existing ones. The historical progression of soil mapping, coupled with the development of LiDAR data, illustrates that the next frontier in mapping involves topography. Soil Taxonomy needs to be ready for these coming advancements, and one of the easiest ways to do so is to be able to objectively match map boundaries to topographic breaks.