A digital morphometric approach for quantifying ped shape

Ped shape is an important property with considerable influence over soil processes, such as root penetration, water infiltration, and solute transport. Despite the host of methods employed to quantify other soil morphological properties, ped shape quantification remains elusive. Existing methods attempting to quantify soil structure utilize laboratory techniques that have limitations on sample size and resolution. Our goal was to overcome these limitations by developing an approach to quantify ped shape using morphometrics created from published digital photographs of soil profiles and structure specimens. In addition, ped shapes from heuristic diagrams and three-dimensional (3-D) scans of peds were examined. The heuristic diagrams were quantified to assess ped shapes derived from common conceptualizations of soil structure, while the 3-D scans were quantified to assess the effect of ped orientation on shape measurements. Ped shape was quantified by manually outlining distinct examples of soil peds from high-resolution photographs and heuristic diagrams and calculating several morphometrics from the resulting silhouettes using image analysis software. A survey was designed to poll expert judgment to properly classify the shape of these peds. Using this method, we were able to transform typical categorical and subjective descriptions of peds into continuous quantitative shape data. The shape metrics, circularity and width to height ratio, exemplify the type of continuous variables that allow significant differences between ped shapes to be detected. This approach opens the door to analyzing soil structure at regional and continental scales through the analysis of existing photographs without the need to resample.