Black walnut suitability index: a Natural Resources Conservation Service national soil information system based interpretive model

—Suitable site conditions are essential for productive growth of black walnut (Juglans nigra L.). Field officers at the Natural Resources Conservation Service (NRCS) in the Midwest are often asked, “What is a good walnut soil?” Current NRCS information available to most field offices rates soils only as “suitable” or “unsuitable” for black walnut. To refine the precision of this categorization and more effectively answer this question, we developed a quantified suitability model that uses 10 soil and site factors from the National Soil Information System database (NASIS). Our interpretive model generates a black walnut suitability index rating for each soil component within each soil survey map unit. Soil properties in the model are effective soil depth, available water capacity, water table depth, percent clay, percent sand, pH, and surface rock fragments. Site properties are flood frequency and duration, landform, and historic native vegetation. Linear and nonlinear functions are used to convert NASIS property values to numeric scores, which are then weighted according to relative importance in the model. Output ratings are on a continuous scale from 0.0 to 1.0, with higher values indicating better suitability. On the basis of these numeric values, the soils are grouped into the following six suitability classes: unsuited, poorly suited, somewhat suited, moderately suited, well suited, and very well suited. The final Black Walnut Suitability Index provides a rational, objective method of rating soils based on their inherent potential for black walnut growth.