Distinguishing Soil Horizons of Alfisols Using MIR, Vis–NIR, and XRF Spectra

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ABSTRACT We studied horizonation of 150 Alfisols on a research farm in Wisconsin, USA. Most of these Alfisols had developed in loess covering dense glacial till; common horizons were an Ap over a glossic horizon (E/Bt or Bt/E) over a Bt covering the glacial till (2Bt). In addition to field identification, we distinguished the horizons by combining soil properties and spectra (Vis–NIR, MIR, and XRF) using a random forest (RF) model. A total of 503 soil samples from 150 pedons were analyzed. Soil samples were scanned using visible–near infrared (Vis–NIR) (350–2500 nm), mid‐infrared (MIR) (4000–600 cm −1 ), and X‐ray fluorescence (XRF) (0–50 keV) spectrometers. Higher silt, pH, soil organic carbon (SOC), Ca, K, P, Si, Ti, Zn, and Zr contents were measured in the Ap horizon, whereas the glossic horizons were characterized by higher CIE L * (lightness) and b * (yellow‐blue) values. The underlying Bt horizon had higher clay, Al, and Fe contents, while the 2B(t) horizon was distinguished by higher sand, coarse fragments, and CIE a * values. The combination of soil properties and spectra (Vis–NIR, MIR, XRF) achieved a higher overall accuracy for predicting soil horizons compared to their individual use. The Ap horizons were best predicted using MIR or XRF (R 2 = 0.98), the glossic horizons using combined Vis–NIR and XRF spectra (R 2 = 0.98), the Bt horizons using MIR (R 2 = 0.90), whereas the 2B(t) horizons were best predicted using combined spectra (Vis–NIR + MIR + XRF) (R 2 = 0.50). In the absence of soil data, these spectra can be used with high accuracy to distinguish A and various B horizons of Alfisols.