A gamma‐ray spectrometry approach to field separation of illuviation‐type WRB reference soil groups in northern Thailand

Abstract

According to the World Reference Base for Soil Resources (IUSS Working Group WRB, 2006), the differentiation of Acrisols and Alisols is based on the cation-exchange capacity of clay, which cannot be directly determined in the field, but needs expensive and time-consuming soil-chemical analyses. This is an unsatisfactory situation for pedologists, who urgently require a rapid field method to distinguish illuviation-type reference soil groups (Alisols, Acrisols, Luvisols, Lixisols).In this study, we tested the ability of gamma-ray spectrometry to separate major WRB reference soil groups in the field. The underlying hypothesis is that Alisols and Acrisols are distinguished by their clay mineral composition, which should be reflected by geochemistry and consequently gamma-ray radiation (i.e., K-containing illite vs. K-free kaolinite). Highly significant differences in their gamma-ray spectrum for K, Th, and U were found for limestone and its soils. Especially the K and Th signatures allowed a clear separation of Acrisols and Alisols. In general, the surface radiation was sufficient to separate these soils. Best results were revealed considering parent rock and the whole soil profile. This means by using a portable radiometer and a pH meter, all illuviation-type reference soil groups could be distinguished in this case. If applicable at other sites, this approach could enormously reduce expenditures for soil-chemical analysis needed to assist soil classification.

Gamma‐ray spectrometry as versatile tool in soil science: A critical review

“…In conclusion, especially eU measurements are intrinsically defective and should be cautiously interpreted. Several authors ( De Meijer and Donoghue , ; Rawlins et al, ; Schuler et al, ) judge 40 K and eTh as reliably detectable. TC measurements give less information on element content, but can be used as indicator where the signal intensity, e.g ., of 238 U and 232 Th is low ( Hyvönen et al, ).…”

“…Inter alia, Beckett (), Schuler et al. () or Priori et al. () used tree analysis to group soil‐derived gamma spectra.…”

“…Schuler et al. () introduced ground‐based gamma‐ray spectrometry in combination with conservative field pH measurements for an in situ distinction in Northern Thailand. While the Th/K ratio was used as surrogate for CEC clay and, thus, clay mineral composition, the pH indicated the base saturation.…”

“…Gamma‐ray measurements, thus, can help to differentiate between bedrock and soil, and detect weathering intensity, textural properties, and nutrient status of the latter ( Cook et al, ). Gamma‐ray spectrometry has emerged as a helpful tool for rapid soil mapping ( Schuler et al, ), soil map refinement ( Reinhardt and Herrmann , ), soil characterization ( Beamish , ), and precision farming ( Van Egmond et al, ).…”

Magnetic susceptibility and natural gamma radioactivity as indirect proxies for characterization of sandstones and limestones of the Sabinas basin

“…Results of research in other regions indicate that this process can generate secondary minerals rich in Th and K (e.g. clays), due to the degradation of feldspars and micas (Gbadebo, 2011;Schuler et al, 2011).…”