Estimation of Clay and Soil Organic Carbon Using Visible and Near‐Infrared Spectroscopy and Unground Samples

Core Ideas The spectra of unground samples can be used to predict soil organic C (SOC) and clay. Vis‐NIR spectroscopy models for ground samples can predict SOC and clay of unground samples. Using unground samples will save time and labor when estimating SOC and clay. Visible and near‐infrared spectroscopy (Vis‐NIR) can accurately predict soil organic C (SOC) and clay from the spectra of air‐dried and ground (DG) samples. However, grinding generally requires a lot of time and labor. In this study, air‐dried and unground (DU) samples were used to exploit the time and accuracy advantages of Vis‐NIR. The reflectance of 117 samples using DG and DU pretreatments was measured in the laboratory. Five spectral pretreatments were used: no‐pretreatment (NP), Savitzky–Golay (SG) smoothing, first derivative (FD), standard normal variate transformation, and multiplicative scatter correction (MSC). When calibrations and validations used the same sample pretreatment (DG or DU), good predictions ( R 2 > 0.75; RPD > 2.0) could be obtained for both SOC and clay. There were no sample × spectral pretreatments interaction effects on the prediction accuracy, whereas the main effects of the pretreatments were significant for clay but not for SOC. However, when calibrations derived from the DG samples were applied to the DU samples, the prediction accuracy decreased compared with that of the DG samples regardless of the SOC and clay. In this case, good predictions could be acquired for clay using the FD spectra and satisfactory predictions for SOC (0.60 < R 2 < 0.75; 1.4 < RPD < 2.0) were obtained from the NP, SG, and MSC spectra. These results indicate that it is viable to predict SOC and clay content using DU samples with acceptable accuracy while saving a lot of time and labor compared with that needed for DG samples.