The “free iron oxide” fraction of soils is of interest for several reasons.  The oxide coatings of soils are indicative of soil function and an indicator of other soil parameters, such as weathering or drainage.  The oxide fraction also can be reactive and attenuate other soil constituents, such as phosphorus.  Iron oxide content is not available for every soil series in the United States.  It is believed that an estimate of the iron oxide content would be valuable.  In this study, the amount of iron in soil components is estimated using the relationships between the Munsell soil color parameters hue, value, and chroma along with the percentage of clay.  The soil survey database provides the data for making the estimates.  The database contains a wide range of aggregated attribute data, including map unit component horizon data.  This part of the database supplies the clay content data.  The site and pedon (non-aggregated) portion of the database also contains the observed horizon data for over 20,000 sites and pedons.  This part of the database supplies the color data.

A fuzzy system model using the National Soil Information System Interpretations Generator was developed using relationships between soil color and clay contents for select soils in the Northeastern United States.  The fuzzy sets were developed by plotting the relationships between one test dataset from the Soil Survey Laboratory database.  The general relationships were tested against a second test dataset.