EVALUATION OF CALIBRATION METHODS FOR INTERPRETING SOIL-SALINITY FROMELECTROMAGNETIC INDUCTION MEASUREMENTS

The EM-38 electromagnetic induction sensor of Geonics Ltd. (Canada) is a most useful instrument for rapid field identification and mapping o f soil salinity. Interpretation of instrument measurements in terms of meaningful parameters of soil salinity is difficult, however, due lar gely to the non-uniform response distribution with depth. Various mode ls have been proposed that allow the conversion of measurements made o n this instrument to the electrical conductivity of the bulk soil (ECa , as measured with the four-electrode probe) or EC of the saturation e xtract (ECe). Seven of these models were evaluated in this study by co mparing predicted with measured values. Four allow the estimation of E Ca at 0.30-m depth intervals down to 0.90 m, two allow the estimation of a single valued ECe, weighted for instrument response with depth, a nd one estimates ECe for 0.30-m intervals down to 0.90 m, The performa nce of the models varied greatly, and likely reasons for poor predicti on are discussed. For the model that produced the most accurate estima te of ECa, the estimated values were, based on 95% confidence limits, within 0.8 dS m(-1) of the values predicted using the regression equat ions. For the models that predict weighted ECe, the corresponding valu e was typically about 2.2 dS m(-1). While salinity measurements made w ith the EM-38 are not highly accurate, the strength of the technique i s that measurements of reasonable accuracy can be made very rapidly. C ategories of soil salinity for large areas can be readily established, which represents useful information for salinity management and certa in research applications.