Calibration methods for measuring the color of moist soils with digital cameras

The study of soil color is important for soil classification and as a diagnostic criterion reflecting a range of soil processes and soil properties. Field measurements of soil colors are affected by moisture content and other environmental and lighting condition. Here we evaluated calibration procedures to obtain colorimetrically corrected soil profile images on field moist soils using a gray card or histogram of reference soil images. In the laboratory, we further evaluated the colorimetric calibration methods on moist soil samples with varying moisture content and texture using an external scale (color scale with air-dry soil standards with known colors) or an internal scale (moist reference soil samples). The colors measured by a spectrometer were used as true colors of moist soil samples. The CIE L*a*b* color difference (Delta Eab*) was used to evaluate the colorimetric accuracy between colors obtained from different devices. The results showed that the soil colors were greatly affected by soil moisture content. With the decrease in soil moisture, a significant increase in lightness (L*) and yellowness (b*) was observed. The field experiment showed that the colorimetrically accurate soil profile images at natural moist condition cannot be obtained by calibrating the digital image using a gray card or using the image of another camera. The laboratory experiments showed that colorimetrically accurate images can be obtained for sandy loam soil at a moisture content of less than 12% using the external standard. For soils with different texture, color characteristics, and moisture content (5-40%), colorimetrically accurate images of the samples can be obtained by using the internal scale. We conclude that the proposed calibration methods can be used to obtain colors of moist soils.