Fig. 2. Predicted vs. measured clay content of the validation data set for (a) air-dried ground, (b) air-dried in situ, (c) field-moist in situ, and (d) field-moist in situ smeared samples. Clay content predictions were made from models built with partial least square regression using the first derivative of visible near-infrared reflectance (VNIR) spectra (350–2500 nm). RMSD is root mean squared deviation.