Mobile Gamma-ray spectrometry for soil texture mapping

Particle size distribution in soil, or texture, is a property essential for understanding processes driving soil water dynamics, fertility and conservation. However, soil texture mapping, using traditional soil sampling and analytical techniques, is labor intensive, time consuming and hence expensive. Having a more rapid and low-cost proximal sensing technique to accurately map soil texture would be a big step forward, in particular at a sufficiently detailed spatial scale for providing advice on soil management at field scale.For the development of such proximal sensing techniques assisting in soil texture monitoring, a study was carried out by the Joint FAO/IAEA Centre at the Petzenkirchen catchment of the Hydrological Open-Air Laboratory (HOAL), located 100 km west from Vienna in Lower Austria. As sensing technique for determining texture, with emphasis on the topsoil (0-30 cm), Gamma-Ray Sensor (GRS) technology was selected. A Medusa MS-350 portable GRS was used to measure the spatial activity concentrations (Bq.kg-1) of 40K (potassium), 238U (uranium), and 232Th (thorium) over 20 points across the studied catchment. These activity concentrations were then linked with soil texture parameters of interest, such as silt, clay, and sand, at the same positions. In total 200 soil samples (10 soil samples for each of the 20 points) were taken for soil texture determination.Preliminary results showed the best correlation between 40K radionuclide concentrations and clay (R2 = 0.51), and silt (R2 = 0.46). Thus, spatial monitoring of 40K with mobile GRS shows potential for the monitoring of clay and silt. However, correlations with other radionuclides concentrations such as 238U and 232Th were weak with R2 coefficients less than 0.16.Further studies are now required to assess ways to improve 40K based predictability of soil texture and validate the applicability of this approach in a more generic way, i.e. a wide range of soil textures. This validation will then enable the further development of this nuclear technology for effective and efficient ground based and air-borne soil texture determination.