Potential for discriminating crop residues from soil by reflectance and fluorescence

Crop residues left in the field after harvest can be important in controlling soil erosion. Current methods for quantifying percent crop residue cover are tedious and somewhat subjective. There is a need for new methods to quantify residue cover that are rapid, accurate, and objective. We evaluated reflectance and fluorescence techniques for discriminating crop residues from a wide range of soils. Reflectance and fluorescence spectra of 37 agricultural soils (wet and dry) and of recently harvested and weathered corn (Zea mays L.), soybean [Glycine max (L.) Merr.] sorghum [Sorghum bicolor (L.) Moench], and wheat (Triticum aestivum L.) residues were measured in the lab. Reflectance factors in the visible or near-infrared wavelengths did not uniquely distinguish all soils from all crop residues. Crop residues may be brighter or darker than a given soil, depending on soil moisture and residue age. When illuminated with ultraviolet radiation, however, the crop residues fluoresced more than most of the soils. Fluorescence of crop residues was a broad-band phenomenon centered between 420 to 520 nm and induced by a relatively broad range of excitation wavelengths centered between 350 to 400 nm. More than 90 % of the crop residues < 2 yr old could be discriminated from 33 of 37 dry soils and 36 of 37 wet soils using fluorescence. The threshold for discrimination can be optimized for classification accuracy for each soil. Fluorescence techniques are less ambiguous than reflectance methods and are better suited for discriminating crop residues on soils. Futhermore, if properly implemented, fluorescence techniques can be used to quantify crop residue cover in the field