Gamma-ray spectrometry is a fast and cost-efficient tool for carrying out spatial mapping of physical parameters related to soil properties. Therefore there is a rising interest in the method for application in digital soil mapping (DSM). However, the concentration of detected radioactive nuclides, gamma K (gK), U and Th, can be influenced by mineralogy and texture as well as further chemical properties, such as pH or organic carbon content (C_org). In many existing studies, the main influencing property of measured gamma-ray concentration in soils is the texture.

Our investigation focuses on a field site with homogeneous texture at the Static Fertilizer Experiment in Bad Lauchstädt. However, the plot is characterized by high range in C_org and plant-available potassium (paK) due to specific long-time fertilization. In our study we tested gamma-ray spectrometry as a potential method for mapping C_org and paK at a test site with very low variation in texture in dependency of pH value. The survey was realised using stop-and-go measurements with a mobile gamma-ray spectrometer.

We found a negative correlation between pH value and Th for pH < 7. Above this value no correlation can be observed. We have seen no relationship between gK and U/K with C_org, as well as between gK and paK, as found in previous studies. A negative correlation for C_org and paK content with Th at pH < 7 was observed. A pH of 7 was found to be a threshold. Above this pH level we found no correlations. We show that gamma-ray data can be used as indicator for mapping of C_org in areas with low variations in texture below a pH of 7.

In our study gK and U were influenced by soil water content, which is important to be considered for interpretation of data measured at different time steps in humid climates.

Our study shows the complexity of relationships between gamma-ray nuclides and soil properties, even for a field site with homogenous texture. In general, we recommend for application of gamma-ray spectrometry in DSM, e.g. at catchment scale, it is necessary to record the pH and the soil water content at the time of measurement.