Biological and Photometric Measurement of Light Transmission Through Soils of Various Colors

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Abstract
We measured light penetration photometrically and biologically through black, brown, brick-red, tan, and gray-white soils. Less than 0.005% of the incidence light penetrated 4 mm of the dark-colored or 10 mm of the gray-white soils. Light-requiring (LR) and light-indifferent (LI) tobacco [Nicotiana tabacum] seeds were used for biological measurement of light through 2, 4, 6, 8, and 10 mm of each of the five soils. The LR seeds were used to measure light-penetration effects on germination, and LI seeds were used as controls to determine whether the seed had sufficient reserves for seedlings to emerge from the various depths. The germination studies were conducted in a controlled environment with daily 12-h light periods from cool-white fluorescent lamps at 20 C. About 99% of the LR seeds germinated in light and 0% when covered with 4 mm or more of the dark-colored soils. Germination for the LI seeds was ca. 98% in light or when covered with 4 mm of any color of soil.