In-field Dual-Frequency Characterization of Soil Magnetic Susceptibility Dynamics

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The ongoing adoption of precision management systems in agriculture requires detailed spatial information on the nature of soil moisture fluctuations, and their impact on the many biochemical reactions occurring in soils. This is especially important for the issue of carbon sequestration and greenhouse gas production. Various technologies, including high-resolution satellite imagery and surface elevation models, as well as specialized monitors on modern field implements, combined with tradition soil survey maps, are now regularly used to help establish the detailed field management zones. Non-contacting magnetic induction probes, such as models of the Geonics EM38, have been a popular instrument for characterizing spatial variations in soil electrical conductivity, which is interpreted in relation to soil moisture content and salinity levels. The EM38 is also capable of measuring the soil magnetic susceptibility, which is influenced by the amount and mineralogical composition of iron-containing minerals present. The close relationship between fluctuations in the moisture content and the nature of iron oxides (rust), throughout the soil, is widely-used to distinguish them according to their drainage class. This visual technique is limited in that field observations (of possible bluish gleying and reddish mottling) require excavating a small pit or augering to a depth of one metre - both of these procedures are laborious, and so not especially conducive to comprehensive mapping of areas. This research is focussed on evaluating the effectiveness of dual-frequency measurements of magnetic susceptibility, to improve our ability to delineate gleying and mottling at the field level. It is based on the well-established understanding that the crystalline nature of iron oxides influences their relative susceptibility, as the frequency of the magnetic field is changed. Geonics Limited, a Canadian company and international leader in electromagnetic field instrumentation, widely-applied in agriculture and the environment, brings more than 50 years of experience and expertise to this project.