Use of EM38 and Gamma Ray Spectrometry as Complementary Sensors for High-Resolution Soil Property Mapping

Apparent soil electrical conductivity (ECa) is related to soil properties such as clay and water content, clay mineralogy, and depth to textural contrast – and hence to plant-available soil water storage capacity (PAWC). High spatial resolution sensing of ECa, coupled with local field calibration, has been used to map expensive-to-measure soil properties, interpret yield maps, locate leaky areas for water and nitrate, and manage the land. Multiple factors affecting ECa is a weakness of the method. Salinity interferes with data interpretation, and the method cannot distinguish between sandy soils and gravels which have similar and low ECa. Therefore soil depth and PAWC cannot be estimated in shallow soils over gravels. Gamma ray spectrometry is relatively new to soil sensing and has shown promise to estimate clay content, PAWC, soil depth, and other soil properties. It is insensitive to common salt, but again it is difficult to interpret gamma ray emission data alone, as clays and gravels result in similarly strong signals. This work provides an approach to overcoming the weaknesses of the single-sensor data by developing a rule-based method for dual EM38–gamma radiometric sensor interpretation to infer soil properties. Simple rules are developed and used to identify soil types (ranging from coarse-textured sands to clay and areas of shallow soils <40 cm deep) and soil acidification risks. The rules are guided by the grower’s soil map and validated with published maps of soil pH and depth. The dual-sensor method overcomes the weakness of the single-sensor data and has the potential to compensate sparsely sampled measurements and estimate their spatial distribution at high resolution in complex field situations without the need for expensive and extensive direct sampling and measurements.