Evaluating methods for measuring residue cover

ABSTRACT: The percentage of soil surface covered with crop residue can be estimated from slide photographs using point sampling methods such as dot screens, but the accuracy of these methods is unknown. Our objective was to develop a benchmark method for accurately measuring residue cover. Slide photographs of residues were projected on the screen of a slide viewer, and tracings of residues were made on acetate film taped to the screen. We measured the area of tracings with computerized image analysis. Results were compared with those obtained by projection of slides onto dot screens, and by image analysis of videotaped pictures of real residues. Measurements made with dot screens and with image analysis of tracings were not significantly different. Image analysis of videotaped pictures underestimated residue cover because we were unable to discriminate completely between soil surface and crop residue based solely on gray-level. The tracing method, although time consuming, is useful for testing the accuracies of other residue-cover measurement methods.