Scoping for scale-dependent relationships between proximal gamma radiometrics and soil properties

- Spatial variability of proximal gamma radiometrics and soil properties are studied.
- Wavelet analysis identified scale and location specific variability.
- Wavelet coherency quantified scale and location specific relationships.
- + ve correlations were observed for 40K:CEC > 400 m and for Total Count:SOC at > 800 m.
- Scale-location specific variability information can be used in digital soil mapping.

Soil varies from location to location as a result of complex combinations of soil forming factors and processes operating at different scales and intensities. The scale-dependent variability in soil properties causes challenges for the characterization and mapping of soils. With the advent of technologies, proximal sensors such as gamma radiometrics can characterize soil spatial variability and the digital mapping of soil properties. In this study we aim to characterize scale-dependent variability of proximal gamma radiometrics and its complex relationship with soil properties at different scales using wavelet transform and wavelet coherency. We measured gamma radiometrics (γ-ray potassium (40K), uranium (238U), thorium (232Th) and Total Count (γ-TC)) using a hand-held device in the field every 25 m along a 3.0 km transect. At each point we measured elevation (m) and land use and collected soil samples to determine key soil properties (electrical conductivity EC1:5, pH (CaCl2), soil organic carbon (SOC), cation exchange capacity (CEC), exchangeable potassium (exch. K), and Colwell P). Wavelet spectra identified significant (P < 0.05) variations at different scales and locations for both γ-ray and soil properties. There were significant localized variations with changes in elevation and land use also important. In most cases a 'global' Pearson correlation between the γ-ray and soil properties was not significant. In contrast, wavelet coherency provided a more powerful scale- and location-specific relationship between γ-ray, elevation and land use with selected soil properties. For example, a strong correlation (18% of the area was significant) was detected between land use and soil EC1:5 at all scales (< 100-800 m) in a low drainage depression. This correlation was due to the localized mobilization of salts and has implications for the identification of saline land. At large spatial scales (> 800 m) there was significant positive correlations between γ-TC and SOC (36% by area). At medium scales (~ 400 m) there was significant and positive correlations between 40K and CEC (23% by area), but only along the first half of the transect which was underlain with Bushy Creek Granodiorite. These findings provide an improved understanding of the spatial variability of γ-ray-soil relationships. The approach used shows the scope and significant potential to use wavelet analysis of gamma-soil property relationships that are scale-specific and would be useful for digital soil mapping. Moreover, scale-specific spatial information of soil properties aids understanding of soil formation processes and it illustrates an approach to improve future soil sampling strategies.