Assessing Bt Horizon Character in Sandy Soils

Using Ground-Penetrating Radar:

Implications for Soil Survey


D.      L. Mokma, R. J. Schaetzl, E. P. Johnson and J. A. Doolittle


            In sandy soils, the cumulative thickness of lamellae (textural bands) is the primary criterion used for the determination of an argillic horizon (Soil Survey Staff, 1975; Miles and Franzmeier, 1981).  Such soils present challenges in mapping because (i) the presence of absence of this diagnostic horizon is not reflected at the soil surface and (ii) Psammentic Hapludalfs and Alfic Udipsamments, sandy soils whose classifications depend on thickness of lamellae, occur in both consociations and complexes with each other.  Productivity is affected by the cumulative thickness of and depth to the lamellae, in part because these bands markedly increase water-holding capacities of sandy soils.  To optimize production and increase land-use efficiency, land owners often need to know which soils is dominant in a field. 

            In other sandy soils, finer materials, rather than lamellae, may occur in the subsoil.  Although some measure of the mean depth to these fine materials can be accomplished by “point” data, variability in depth is difficult to determine from the limited number of auger observations usually made during soil mapping.  As in soils with lamellae, thickness of the sand over finer material greatly influences productivity.  (Warncke et al., 1985). 

            Ground-penetrating radar (GPR) is a field technique that has been used successfully to study organic soil thickness (Shih and Doolittle, 1984; Collins et al., 1986), depth to bedrock in mineral soils (Olson and Doolittle, 1985; Doolittle et al., 1988), depth to and lateral extent of argillic horizons (Collins and Doolittle, 1987; Truman et al., 1988) and to update soil surveys (Schellentrager et al., 1988).  The GPR may have potential in gathering data to assist soil scientists in mapping the sandy soils described above.  The objectives of this study were to determine the usefulness of GPR for assessing the character and depth of Bt horizons in sandy soils and to estimate map unit composition.