In situ characterization of soil clay content with visible near-infrared diffuse reflectance spectroscopy

Visible and near-infrared (VNIR, 400-2500 nm) diffuse reflectance spectroscopy (DRS) is a rapid, proximal-sensing method that has proven useful in quantifying constituents of dried and ground soil samples. Very little is known, however, about how DRS performs in a field setting on soils scanned in situ. The overall goal of this research was to evaluate the feasibility of VNIR-DRS for in situ quantification of clay content of soil from a variety of parent materials. Seventy-two soil cores were obtained from six fields in Erath and Comanche counties, Texas. Each soil core was scanned with a visible near-infrared spectrometer, with a spectral range of 350 to 2500 nm, at four different combinations of moisture content and pretreatment: field-moist in situ, air-dried in situ, field-moist smeared in situ, and air-dried ground. The VNIR spectra were used to predict total and fine clay content of the soil using partial least squares (PLS) regression. The PLS model was validated with 30% of the original soil cores that were randomly selected and not used in the calibration model. The validation clay predictions had a root mean squared deviation (RMSD) of 61 and 41 g kg(-1) dry soil for the field-moist and air-dried in situ cores, respectively. The RMSD of the air-dry ground samples was between the two in situ RMSDs and comparable to values in the literature. Smearing the samples increased the field-moist in situ RMSD to 74 g kg(-1). Whole-field holdout validation results showed that soils from all parent materials need to be represented in the calibration samples for maximum predictability. In summary, DRS is an acceptable technique for rapidly measuring soil clay content in situ for various water contents and parent materials.