Estimating crop residue cover by blue fluorescence imaging

Crop residues, the portion of the crop left in the field after harvest, can be an important management factor in controlling soil erosion. Current methods for quantifying crop residue cover use tedious manual sampling methods or visual comparisons with photographs. There is a need for new methods to quantify residue cover that are rapid, accurate, and objective. Scenes with known amounts of crop residue were illuminated in the lab with long-wave ultraviolet (UV) radiation and fluorescence images were measured and recorded with a video camera equipped with a mico-channel-plate image intensifier and fitted with a 453-488 nm bandpass filter. Six agricultural soils were used as backgrounds for the weathered soybean residue. Residue cover was determined from the proportion of the pixels in the image with fluorescence values greater than a threshold. Soil pixels gave the lowest fluorescence or brightness responses in the images and the residues the highest, so that brightness values of the scene spanned nearly the full range of the 8-bit video data. The images were classified in brightness categories that related to within 2% (absolute units) of measured residue cover regardless of the soil type or moisture condition (dry vs. wet). Therefore, fluorescence images can be used to provide percent residue cover in the lab, but portable equipment and procedures for use in the field still need to be developed. (C) Elsevier Science Inc., 1997.