Particle-size and analytical considerations in the mineral-magnetic interpretation of soil loss from cultivated landscapes

Prior research has highlighted the potential for using soil magnetism as a rapid estimator of total cumulative soil loss on cultivated land. In this study, the influences of particle size, choice of magnetic measurement and initial conditions on a proposed model linking surface soil magnetism and soil loss are described and evaluated. Output model curves are generated for several parameter sets corresponding to different model assumptions and physical scenarios. Output is also compared to a new high-resolution surfacial magnetism data set from a cultivated site, Gilbert Farm in northwestern Alabama (USA), to further evaluate the model. Magnetic susceptibility values at Gilbert Farm range from 150 to 220 (×10−8) m3 kg−1 in a control soil profile. The original model output ranges from 180 to 192 (×10−8) m3 kg−1 and many magnetism values in the cultivated area fall outside this range. Introduction of a particle-size sorting correction factor decreases the range of output values and results in even greater numbers of out-of-range observations. However, there is evidence suggesting that particle-size distributions in the upper soil at this site are not strongly affected by differences in water erosion mechanism. Lower tillage depths cause magnetic susceptibility values to come in range, but may be unrealistic and create a twin-value problem for complex magnetic profiles. Use of the remanence (SIRM)/volume susceptibility (κlf) ratio profile, with values ranging from 35 to 120 mA m−1/10−5 SI, instead of magnetic susceptibility as used in the original model, was found to eliminate out-of-range and twin value. Anomalous magnetism values were identifiable and treatable on the high resolution magnetism maps, but other types of natural spatial variation remain at issue.