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An analysis of the relationship of apparent electrical conductivity to soil moisture in alluvial recent soils, lower North Island, New Zealand : a thesis presented in partial fulfillment of the requirements for the degree of Masters of Philosophy (MPhil) in Soil Science at the Institute of Agriculture and Environment, Massey University, Palmerston North, New Zealand
Electromagnetic induction (EMI) sensors can be used in kinematic systems to provide rapid high-density measurement of apparent soil electrical conductivity (ECa) over large areas. In non-saline soils ECa has been used as a surrogate measurement for many soil properties including soil texture and moisture, critical properties in precision agriculture. However, complex interactions between soil properties and the irregular depth profiles of EMI measurements have prevented consistent interpretation of ECa in terms of soil properties. This study uses kinematic surveys and multi-height spot measurements of ECa with Geonics EM38 Mk2 and EM31 instruments together with field measurements of soil moisture and investigation of ECa theory to analyse the relationship of ECa to soil moisture in alluvial Recent Soils at two locations in the lower North Island, New Zealand. Soil samples from these locations were also analysed for bulk density, porosity, texture and the electrical conductivity (EC) of 1:1 soil pastes and extracts. Intact soil cores from one location were analysed for moisture retention properties. Results raise uncertainty about the function of EMI instruments, particularly the nature of temperature effects and the comparability of measurements by different instruments. Effects of soil solution conductivity on ECa were found to be significant though the soils studied were non-saline. Correlations of soil moisture with ECa in this study were varied and not in every case significant. The relationship of ECa to soil moisture in this study was too complex to allow simple use of ECa for measurement of soil moisture.