Discriminating crop residues from soil by shortwave infrared reflectance

Quantifying crop residue cover on the soil surface is important for evaluat ing the effectiveness of conservation tillage practices. Current methods of measuring residue cover are inadequate in characterizing the spatial varia bility of residue cover over large fields. The objectives of this research were to determine the spectral reflectance of crop residues and soils as a function of water content and to evaluate the limits of discrimination that can be expected. Spectral reflectances of corn (Zea mays L.), soybean [Gly cine mar (L.) Merr.], and wheat (Triticum aestivum L,) residues plus five d iverse soils were measured over the 400- to 2400-nm wavelength region at a wide range of moisture conditions in the laboratory. Reflectance factors fo r scenes with varying proportions of crop residues and soils were simulated . The spectra of dry crop residues displayed a broad absorption feature nea r 2100 nm, associated with lignin and cellulase, that was absent in spectra of soils. The relative depth of the cellulose-lignin absorption feature, d efined as the cellulose absorption index (CAI), was positive for all crop r esidues, except those saturated with water. In contrast, all soils had nega tive CAI values. Water significantly altered reflectance spectra of wet cro p residues, but it did not prevent the discrimination of crop residues from the soils using the CAI. The wide range of CAI values expected for dry and moist conditions makes quantification of crop residue cover feasible. This reflectance technique appears promising for field- and regional-scale surv eys of crop residue cover and conservation tillage practices.