Regional Differences in the Validity of the Concept of Innate Soil Productivity

A reliable index of “innate soil productivity,'’perhaps stored in a computerized geographic information system, would be of great value in choosing crops or tillage techniques, designating prime farmland, assessing land for tax purposes, aligning utility corridors, targeting price-support and soil conservation programs, and making other land use decisions. One problem with the creation of a productivity index is the fact that a given tract of land can often be used for different crops with yields that may not be well correlated with each other. In that case, the relative value of two tracts of land will depend on what is grown on them, a situation that allows a change in world market conditions to alter the fairness of a zoning map or tax structure. To help guide people trying to incorporate soil information in their regional data bases, this paper presents a map of the probable validity of a single-number index of soil productivity in a sample of 200 counties across the United States. This map shows some striking regional disparities in the validity of that kind of productivity index; these disparities seem to be related to a complex set of factors, including local crop mix, soil traits, climatic calendar, and management techniques, as well as some error inherent in the data source and the analytical methods used to make the map. Unraveling the variables that affect soil productivity in a given county is a difficult task, but it is necessary if we are to avoid some of the adverse economic and political consequences of an invalid measure of soil productivity.