Prediction of Mineral Fines to Soil Organic Carbon Ratio with Visible Near-Infrared Spectroscopy (vis-NIRS).

The ratio of mineral fines (clay<2 µm and (clay+fine silt)<20 µm) to organic carbon (OC) is important for evaluating soil physical and chemical properties with respect to e.g. tilth conditions, clay dispersibility and PAH sorption affinities. Studies have suggested that a clay/OC ratio around 10 gives the saturation level of clay and OC complexation, with non-complexed clay being present at a ratio above 10, and non-complexed OC being present below 10. Likewise, a (clay+fine silt)/OC ratio around 20 has been suggested to indicate a shift in soil physical properties.

Visible near-infrared spectroscopy (vis-NIRS) is a fast method, with little or no -sample pre-treatment, which has shown capabilities of replacing time-consuming conventional soil analysis methods. The light absorption is measured in the visible near-infrared range from 350 to 2500 nm, where specific wavelengths are assigned to species of organic and inorganic soil components.

The ability of vis-NIRS to determine the ratio of clay/OC and (clay+fine silt)/OC was evaluated in this study. A total of 417 soil samples from seven Danish fields, with a large textural gradient (clay: 0.03-0.45 kg kg^-1, organic carbon: 0.011-0.084 kg kg^-1), was analysed for particle size distribution and measured with a vis-NIRS spectrophotometer in the spectral range from 400 to 2500 nm (DS2500, Foss, Hillerød, Denmark). Partial least squares (PLS) regression was used to correlate spectral data to the clay/OC ratio and (clay+fine silt)/OC ratio on a representative calibration data subset. PLS models were then tested using a validation set. Results of both the calibration and validation yielded very successful predictions of the clay/OC ratio and (clay+fine silt)/OC ratio, and thus illustrates the potential applicability of vis-NIRS as a replacement/alternative for conventional soil analysis methods.