Soil color measurements from reflectance spectra: Applications to the study of iron-oxide-soil color relationships

The color of a soil provides information on the soil\u27s past history and clues to its physicochemical properties. Procedures were developed to measure the visible reflectance spectra of dry and wet soil powders on a Cary 17D spectrometer, and to calculate colors from these spectra. Accuracy and precision were checked by measuring the colors of chips of the ISCC-NBS Centroid Color Chart and chips of the Munsell Soil Color Chart. Measured Munsell values were not significantly different from nominal values, while measured hue and chroma differed from nominal hue and chroma, by an average of 0.25 units, a small but significant amount. Average deviations for duplicate measurements of Munsell hue, value, and chroma of 12 silt loam and silty clay loam samples were $\u3c$0.1 unit of hue, value, or chroma. Precision was similar for both dry and moist samples. The influence that hematite and goethite pigments have on soil color was studied by plotting the color of simulated soil material on the Munsell hue-chroma plane. Saturation of hue and chroma occurred when the pigment concentration was $\geq$16%, and the absolute hematite content was $\geq$8%. Munsell value decreased with increasing pigment concentration. Prediction of the color of these mixtures was attempted using Kubelka-Munk analysis (K-M). Predictions from K-M analysis were reliable for pigment concentrations between 0 and 16%. K-M analysis can make it easier to study color pigment relationships by reducing the number of spectroscopic measurements which need to be made. It is not known why soils become redder as one moves from cooler climates of the upper latitudes to warmer climates of lower latitudes. Spectroscopic color measurement techniques were used to study a transect of well-drained, loess-derived, Wisconsinan-aged soils collected along the Mississippi River, from Minnesota in the north to Mississippi in the south. There was a small but consistent trend to redder hues in the Bt horizons as latitude decreased. The redder hues correlated with an increase in the oxalate extractable-Fe (Fe$\sb{\rm o}$). Fe$\sb{\rm o}$ is an indicator of the presence of ferrihydrite in soils. The data support the hypothesis that ferrihydrite in soils is a necessary precursor of hematite formation