REFLECTION OF RADIANT ENERGY FROM SOILS

Data secured with spectrophotometers show that surface moisture content, organic matter, and particle size strongly influence the reflectance and absorptance of solar radiant energy by soils. At all wavelengths measured, on all samples, reflectance decreased and absorptance increased as moisture content increased. By using Gates' direct solar energy distribution curve, one would expect that increasing the moisture content on a Newtonia silt loam from 0.8 to 20.2 percent would increase absorption of radiant energies by at least 14.2 percent of that in the equivalent direct solar beam. The plot of moisture content against reflectance indicates the possibility of using reflectance methods for surface moisture determinations. Results were especially good at 1900 mu, a moisture-sensitive wavelength. The oxidation of soil organic matter increased the reflectance from all samples measured. Again using Gates' distribution curve, 8.2 percent more of the energy in the equivalent direct solar beam might have been reflected by the oxidized Newtonia silt loam sample. With both kaolinite and bentonite clays, reflectance increased exponentially as particle size decreased. The magnitude of reflectance change was very similar with both clays. By applying the kaolinite reflectance data to Gates' direct solar energy distribution curve, one would expect energy absorption to increase by at least an additional 14.6 percent of that in the incident beam when the particle size is increased from 22 mu to 2650 mu. /Author/