“Why do soils change at county lines?”

Fred Young,Ph.D.
USDA-NRCS
Columbia, MO
fred.young@mo.usda.gov
573-876-9427

     If you clicked on this link, you’ve encountered the phenomenon, on soil series distribution maps, of abrupt soil series changes along county lines. Of course, soils in the field do not change at county lines (unless the county line is a river or other natural feature).

     The county line discontinuities are the result of the past administrative structuring of the soil survey program in Missouri. Soil surveys have been conducted on a county-by-county basis. This has caused county line discrepancies for the following reasons:
      Often, adjacent counties were mapped years or even decades apart. For example, mapping in Scotland county was completed in 1967, whereas Clark county to the west was completed in 1991, by a different generation of soil scientists. Although there were probably no significant changes in the soils over that time, there were significant changes in both land management technology and in the science of soil survey.
      Many soils change gradually with distance, and placing a line between them is challenging. For example, in northwest Missouri, the Marshall series changes gradually to the Sharpsburg series along a gradient from west to east. County-based soil mappers were unable to examine soils regionally, and mapped the dominant soil in the county, even as they recognized that the soil was grading into something different. This is why both Sharpsburg and Marshall series show discontinuities along some county lines.

For soil survey users with multi-county projects, these county line changes can be confusing and frustrating. It may be cold comfort, but please bear the following in mind:
     Each county soil survey represents the very best information obtainable according to the technology and resources available at the time of the survey.
     Soils that join at county lines are nearly always “similar” soils. For example, Marshall soils grade into Shapsburg soils, so that there are only minor differences between a Marshall soil in eastern Holt county and a Sharpsburg soil in western Nodaway county.
     The National Cooperative Soil Survey in Missouri has now initiated the Modernization Phase, which will be conducted on a regional basis. County boundary discrepancies will, some day, be a thing of the past.