A geomorphological investigation of soils forming on the Humboldt terrace in western Tennessee

Soils forming on the Humboldt terrace in western Tennessee were investigated for their genesis and classification. Diatoms within the profiles were used to determine whether the Humboldt terrace originated in a fluvial environment, which is the accepted theory, or a tidal environment.

A total of seven sites were chosen on the Milan Agricultural Experiment Station. Five of the sites were located in an upland position on the Humboldt terrace, while two other sites were located on a lower lying flood plain. The five terrace sites were sampled to a depth ranging from 3.5 to 5 meters. The flood plain sites were sampled to a depth of no more than 2 meters. Standard soil survey field and laboratory techniques were performed on the sites for particle size, cation exchange capacity, base saturation, pH, total elemental concentrations, free iron oxides, dithionite extractable iron, hydroxylamine manganese, and KCl exchangeable aluminum. Diatom identification was completed using standard separation techniques, a 950X microscope, and scanning electron microscopy. The soils were subsequently classified according to Soil Taxonomy.

A sequence of loess over alluvium was found on the five terrace sites. Separate depositions of Peoria, Roxana, and Loveland loess were found on all of the five Humboldt terrace sites. A paleosol of the Loveland loess deposition was present on all five sites located on the Humboldt terrace. Total loess thickness ranged from 312 to 370 centimeters. Diatoms were not found at the loess/alluvium interface, thus the fluvial depositional environment theory remains the standard. The soils classified at the Great Group level as Udifluvents, Eutrochrepts, Hapludalfs, and Fragiudalfs.