Using Magnetic Susceptibility to Delineate Hydric Soils in Southeastern Coastal Plain Soils

Abstract

Identifying hydric soil boundaries is one component in delineating jurisdictional wetlands. Evaluating hydric soil field indicators using borings along transects is an invasive and arduous task. Magnetic susceptibility (MS) has been used successfully as a geophysical method for determining hydric soil boundaries by detecting a drop in soil magnetism when iron minerals mobilize or are recrystallized in alternating reducing and nonreducing conditions. Magnetic suseptibility has not, however, been fully evaluated in the soils of the southeastern coastal plain, where iron minerals are uncommon and a substantial amount of wetland delineation work is performed. Five relatively pristine and two disturbed wetland sites in northern Florida were rapidly evaluated by recording uncorrected volumetric MS measurements (n = 12 hydric; n = 12 nonhydric) on 1 by 7 yd transect grids centered on the hydric soil boundary as determined by field indicators. At the more sensitive “×0.1” instrument setting, instrument drift obscured any true differences in the populations of hydric and nonhydric MS values. Two additional sites in southeastern Georgia and two of the original sites were selected for a more intensive survey (n = 30 random hydric; n = 30 random nonhydric) volumetric MS measurements recorded inside a 3 by 20 yd transect block centered on the hydric soil boundary. Values were air‐corrected for temperature drift, and the hydric and nonhydric populations were compared. All four sites showed significant (P < 0.004) differences in both population means and medians. However, the necessity for air correction, statistical analysis, and the lack of a unique MS value separating hydric and nonhydric populations make the use of this technique for delineating the precise hydric soil boundary impractical in this region. Mass specific MS measurements of the surface soils at two sites had a marginal correlation (R2 = 0.78) with total and ammonium oxalate extractable iron, and trace amounts of iron minerals—most likely maghemite and goethite—were magnetically separated from filter residue and unaccounted for in the total iron analysis. This sporadic occurrence of ferrimagnetics has an overriding but difficult‐to‐quantify influence on the effectiveness of this method in the southeastern coastal plain.