Magnetic susceptibility for use in delineating hydric soils

Field indicators are used to identify hydric soil boundaries and to delineate wetlands. The most common field indicators may not be seen in some soils with thick, dark, mollic epipedons, and do not form in Fe-poor soils. This study evaluated magnetic susceptibility (MS) meter as a field tool to determine hydric soil boundaries. Five Mollisol-dominated sites formed in glacial deposits in Illinois were evaluated along with one Ultisol-dominated site formed in Coastal Plain sediments of North Carolina. Measurements of volumetric MS were made along transects at each site that extended from wetland into upland areas. One created wetland was evaluated. Field indicators were used to identify the hydric soils. Results showed that volumetric MS values were significantly (P < 0.01) lower in hydric soils than in nonhydric soils formed in glacial deposits. Volumetric MS values also decreased slightly with depth. In a created marsh, significant differences in MS were found between hydric and nonhydric soils at 15 to 30 cm but not at shallower depths where topsoil from a wetland had been artificially applied. No significant (P > 0.15) differences in MS were found for Coastal Plain hydric and nonhydric soils where MS values were low (<10 × 10⁻⁵ SI). Critical MS values that separated hydric and nonhydric soils varied between 20 × 10⁻⁵ and 30 × 10⁻⁵ SI for the loessal soils evaluated in Illinois. Such critical values will have to be determined on site using field indicators until specific values can be defined for hydric soils within a given parent material. With a critical MS value in hand, a wetland delineator can make MS measurements along transects perpendicular to the envisioned hydric soil boundary to quickly and quantitatively identify it.