| Summary: | Most cultivated upland areas of northeast Thailand are characterized by sandy and infertile soils, which are difficult to improve agriculturally. Information about the clay (%) and cation exchange capacity (CEC—cmol(+)/kg) are required. Because it is expensive to analyse these soil properties, electromagnetic (EM) induction instruments are increasingly being used. This is because the measured apparent soil electrical conductivity (EC<sub>a</sub>—mS/m), can often be correlated directly with measured topsoil (0−0.3 m), subsurface (0.3−0.6 m) and subsoil (0.6−0.9 m) clay and CEC. In this study, we explore the potential to use this approach and considering a linear regression (LR) between EM38 acquired EC<sub>a</sub> in horizontal (EC<sub>ah</sub>) and vertical (EC<sub>av</sub>) modes of operation and the soil properties at each of these depths. We compare this approach with a universal LR relationship developed between calculated true electrical conductivity (σ—mS/m) and laboratory measured clay and CEC at various depths. We estimate σ by inverting EC<sub>ah</sub> and EC<sub>av</sub> data, using a quasi-3D inversion algorithm (EM4Soil). The best LR between EC<sub>a</sub> and soil properties was between EC<sub>ah</sub> and subsoil clay (R<sup>2</sup> = 0.43) and subsoil CEC (R<sup>2</sup> = 0.56). We concluded these LR were unsatisfactory to predict clay or CEC at any of the three depths, however. In comparison, we found that a universal LR could be established between σ with clay (R<sup>2</sup> = 0.65) and CEC (R<sup>2</sup> = 0.68). The LR model validation was tested using a leave-one-out-cross-validation. The results indicated that the universal LR between σ and clay at any depth was precise (RMSE = 2.17), unbiased (ME = 0.27) with good concordance (Lin’s = 0.78). Similarly, satisfactory results were obtained by the LR between σ and CEC (Lin’s = 0.80). We conclude that in a field where a direct LR relationship between clay or CEC and EC<sub>a</sub> cannot be established, can still potentially be mapped by developing a LR between estimates of σ with clay or CEC if they all vary with depth.
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