Spectrophotometric color measurement in situ in well drained sandy soils

Abstract

A method for measuring spectral reflectance of samples in place on the face of a soil pit was developed for well drained, sandy soils. Spectrophotometer readings were taken directly on the pit face immediately after scraping a fresh, flat surface to ensure uniform soil moisture content. Measurements taken in this manner were compared to field-based visual estimates and to laboratory-based spectrophotometric measurements of sieved, air-dried samples. While overall reflectance was shown to decrease with increasing moisture content in the laboratory, little change was noted over the 1–7% moisture content range encountered for most samples in the field. The interaction between the “granular materials cover” (a glass window used to protect the integrating sphere) and the “specular reflectance included” viewing geometry resulted in hue shifts for some samples at higher moisture contents, suggesting that the “specular reflectance excluded” mode is more appropriate for in situ measurements of soil samples. A moderately strong correlation exists between spectrophotometer measurements and visual estimates of Munsell parameters in the field, but the spectrophotometer provides a higher degree of precision. Soil features smaller than the 8-mm diameter instrumental measurement area are averaged with the surrounding matrix, however. The research suggests that for well drained, sandy soils, a spectrophotometer can be used in situ to provide reliable, repeatable measurements of spectral reflectance.