Determining the clay/organic carbon ratio by visible near infrared spectroscopy

Abstract

The recently presented Dexter et al. (2008) threshold (ratio of clay to organic carbon (OC) of 10 kg/kg-1) is a good indicator for soil functional properties. However, the conventional analysis of OC and clay are costly and time consuming, thus an alternative method to quantify OC, clay or clay/OC ratio directly would be valuable. Visible near infrared spectroscopy (vis-NIRS) is a cost-effective method for soil analysis and was tested here for the prediction of clay/OC ratio. Soil samples from two agricultural fields in Denmark (N=115) were analyzed. Partial Least Squares regression (full cross-validation) was performed on 80% randomly selected samples to correlate soil spectra with OC, clay and clay/OC. The robustness of calibration models was tested on the remaining 20% of the samples. The soil from the two study sites vary greatly presenting clay/OC ratio between 1.20 and 10.43. Successful calibration results for OC, clay and clay/OC ratio were generated. The OC content was predicted with a RMSEP of 0.23%, R2 of 0.96, whereas the prediction of clay resulted in RMSEP=0.94%, and R2=0.94. The most successful predictive ability reported was of the clay/OC ratio (RMSEP=0.42, and R2=0.97) with the most important absorption bands related to both clay minerals (1421, 1910 and 2206 nm – OH bonds and 429, 720 nm – Fe oxides) and organic carbon (1730, 2160 nm and 2310 nm). The results of this study show that vis-NIRS can provide very successful and direct determination of Dexter index on a field scale through its correlation to both OC and clay