Significance of Organic Matter in Determining Soil Colors

Abstract

Quantitative relationships between soil color and organic matter content are only poorly understood, but they are of considerable practical importance in mapping and classifying soils, interpreting soil properties, and in designing sensors for agricultural equipment. We studied the color-organic matter relationships for Ap horizons from Indiana and Illinois soils to test the hypothesis that Munsell value and organic matter content are more closely related for soils occurring together in soil landscapes than for soils over a wide geographic region. Two sample sets were collected. Sample set 1 consisted of 105 Ap horizons from throughout Indiana, while set 2 consisted of 10 to 15 Ap horizons from each of 16 landscapes in Indiana and Illinois. Organic matter content was determined by dry combustion, and Munsell colors of both moist and dry samples were calculated from reflectance spectra. The relationship between Munsell value and organic matter content: (i) was poor for Indiana soils statewide (sample set 1), (ii) was predictable (r 2 > 0.9) within soil landscapes if soil textures did not vary widely, (iii) was linear within landscapes with silty and loamy textured soils but was curvilinear within landscapes with sandy-textured soils, (iv) was similar among landscapes having the same soil textures and parent materials, and (v) was not predictable if soil texture varied widely (sands vs. silts and loams) within the landscape. In a separate study, we measured the colors of various organic and inorganic fractions from the Ap horizons of four Indiana soils. The purified humic acid content was about 15 times the purified fulvic acid content for all four soils. The black humic acid, which masked the yellowish brown color of the fulvic acid, was responsible for the dark color of the soil organic matter.