The Taxadjunct Problem

Quantitative definitions of taxa and inclusion of the soil series as a category in soil taxonomy cause about three‐fourths of the series‐named pedons sampled, analyzed, and correlated in the USA to be taxadjuncts to the named series. Imposing the limits of soil taxonomy that circumscribe soils from without frequently divides natural soil bodies, i.e., collections of recurring contiguous pedons that are more similar to each other than they are to adjacent soils. We used the pedon data for six soil series to explore causes and alternatives to placement of pedons as taxadjuncts. Eight to 11 pedons per series were identified in the field, and sampled. Of the total of 56 pedons, 59% were taxadjuncts. We tried the two common approaches to handling these: (i) ignore misses that are just outside the limits of taxonomic criteria, or (ii) attribute near misses to errors in precision, which assumes that the error is not normally distributed. Neither approach is valid nor eliminates all of the taxadjuncts. We considered (i) dropping the seires as a taxonomic category, (ii) adopting the cartographic series proposed by Knox, or (iii) classifying the central concept of the series but allowing characteristics to range across the limits between two families, or between two classes of any higher category. We propose option iii because, if accepted, (i) natural soil bodies would not be subdivided by artificial boundaries, (ii) soil taxonomy would be retained to facilitate technology transfer, (iii) the USDA‐SCS prohibition against publishing data for taxadjuncts in soil survey reports would be nullified, and (iv) the exchange of information about the use and management of series would be facilitated.