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basic categories. Only the photographic and intercept techniques are appropriate for measuring crop residue cover
(Laflen et al. 1981).
Intercept techniques may be grouped into line-transect and point-intercept methods. Line-transect methods
measure the distance along a line covered by residue. In comparison, the point-intercept methods use a system of
cross-hairs, grid points, or dot matrices to define points where the presence or absence of residue is determined
Sometimes point-intercept and line-transect methods are combined. A line is placed and the intercept is read at
selected points along the line (Bonham, 1989). Accuracy of the line-point transect depends on the length of the line
and the number of points used per line. Commercially-available cables are typically 50 to 100 feet long and have
100 to 200 beads evenly spaced along the cable. The line-transect method is the current standard technique used
by the Soil Conservation Service (SCS) to quantify residue cover (Morrison et al., 1993).
A number of significant modifications of the line-transect method are being investigated. For example,
Morrison et al. (1993) described a "residue wheel" which has spikes that point close to spots on the surface for
observation and thus eliminates the need to stretch a line. While the wheel eliminates setting a line, it must be used
carefully to minimize possible bias incurred by the observer aiming the wheel.
Photographic methods consist of taking single vertical photographs or vertical stereographic pairs of
photographs and manually estimating the fraction of the soil covered by residue from the photographs. An important
advancement of the photographic technique is the computer-aided analysis of B&W, color, or multispectral video
images. Once a video image is captured, it can be quickly analyzed and classified in to soil and residue classes on
a computer using objective procedures. Classification errors occur when the spectral differences between classes (soil
and residue) are not sufficient for discrimination.
Field procedures for photography and video imaging require approximately equal time, however video
images may be immediately evaluated without the delay of film processing. Based on research experience, Morrison
et al. (1993) estimated that video image analysis procedures would require no more than 2 minutes per image as
compared to 20 minutes per slide for the minimum of three 200-dot screens. Problems arise primarily when the
contrast between the soil and the residue is small.
Current methods for quantifying the percent crop residue cover are tedious and somewhat subjective
(Morrison et al, 1993). Our overall objective is to develop new methods to quantify crop residue cover that are
rapid, accurate, and objective. In this paper, we discuss the potential for discriminating crop residues with soil using
fluorescence techniques and examine the changes in wheat residue composition and fluorescence during degradation.
1.3 Reflectance of Soils and Crop Residues.
Various remote sensing techniques have been used in attempts to replace the human visual judgment present in the
line-transect method with a sensor designed to identify residue based cm its reflectance characteristics. Unfortunately,
the reflectances of both soils and crop residues lack the unique spectral signature of green vegetation (Bauer, 1975)
and their reflectances typically increase monotonically with wavelength from the visible to the near infrared (Aase
and Tanaka, 1991; Baumgardner et al. 1985; Gausman et al., 1975). Crop residues and soils are often spectrally
similar and differ only in amplitude at a given wavelength
A variety of soil parameters and conditions including organic matter, moisture, texture, iron oxide content,
and surface roughness affect the spectral reflectance of soils (Condit, 1970; Stoner et al., 1980). Furthermore,
because many factors affect the reflectance of soil and residues, the reflectance of residue at a particular wavelength
may be higher or lower than the reflectance of the soil (Aase and Tanaka, 1991; Daughtry et al. 1993; McMurtrey
et al., 1993; Sterner et al., 1980). This makes discrimination of crop residues and soils difficult or nearly impossible
using conventional reflectance techniques alone. However, discrimination of residues and soils is possible using a
combination of visible and near infrared reflectance factors plus the wavelength of the maximum first derivative
(Dulaney et al., 1992).
1.4. Fluorescence of Soils and Crop Residues.
Most of the previous remote sensing techniques have relied cm measuring reflected radiation. McMurtrey et al.
(1993) demonstrated in the lab that crop residues fluoresce more than soils in a broad band centered at 440 nm when
illuminated with ultraviolet radiation at 337 nm. Chappelle et al. (1991) hypothesized that the fluorescence in the
blue region of the spectrum from plants was possibly due to fluorescence of li gnin , riboflavin, and NADPH. The
metabolic precursors of lignin, namely, ferulic acid, caffeic acid, and coumaric acid, may actually contribute mere
to the blue fluorescence than lignin. These compounds are abundant in plants but scarce in soils.
Daughtry et al. (1993) examined the fluorescence sp>ectra of recently-harvested and weathered com, soybean,
sorghum, and wheat residues plus 40 agricultural soils. The fluorescence of crop residues was a broad band
phenomenon with emission maxima at 420-495 nm for excitations of 350-420 nm. Soils had low intensity broad
