Analysis of Aerial Photography for Nitrogen Stress within Corn Fields

Spatial variability of soil parameters within fields complicates N fertilizer recommendations for corn (Zea mays L.) production. Thus, the ability to identify differences in crop N status within corn fields could lead to elticiencies in N fertilizer application and decreased ground water pollution. In this study, we digitized aerial color photographic transparencies using an eight-bit scheme to generate digital counts for the red, green, and blue primary colors in the photographs at the R5 growth stage and related them to grain yield. Digital count responses were relative to the N treatment in which grain yield plateaued. Experiments were conducted in 1992 and 1993 for four irrigated corn hybrids with five N rates on a 6-ha field near Shelton, NE. Red and green digital counts relative to those for the high N treatment provided better prediction of yield response than relative blue counts in both years. In 1993, black-and-white photographs taken with a filter centered around 536 nm also predicted yield response to N well (r2 = 0.93). These findings permit the use of low cost aerial photographs to characterize variability in crop N status throughout entire fields. Vr ARIOUS TYPES OF PLANT STRESS have been identified using remote sensing techniques (Jackson, 1986). One of the first studies was conducted by Colwell (1956) using aerial photography (one form of remote sensing) to look at the prevalence of disease in wheat. Aerial photography has also been used to detect insect damage and nutrient and water deficiencies (Wildman, 1982). Because of the number of different stresses detectable photographically, specific stresses need to be verified by ground observations (truthing) or by inclusion of known reference conditions within the photograph. Quantitative interpretation of aerial photographs is aided by digitizing the image and computerized processing of the resulting digital counts. Digitized photography has been used, for example, to evaluate the ground cover of plant canopies (Gerbermann et al., 1976; Thomas et al., 1988) and crop residues on soil (Meyer et al., 1988). Aerial photography is a way to cover large areas quickly and produce images that are spatially uniform in lighting and viewing angles. Recently, low cost digital color imaging methods have become readily available. The green color of plant canopies is most frequently associated with leaf chlorophyll content, which is positively correlated with N concentration for many agricultural plants (Wolfe et al., 1988). The chlorophyll content, in turn, affects the amount of light absorbed or reflected (AI-Abbas et al., 1974; Thomas and Gausman, 1977; Maas and Dunlap, 1989). Light reflectance in the visible wavelengths (400-700 nm) increases with N deficiency (Walburg et al., 1982; Hinzman et al., 1986; Takebe et T.M. Blackmer, J.S. Schepers, and G.E. Varvel, USDA-ARS and Dep. of Agronomy, and G.E. Meyer, Dep. of Biological Systems Engineering, Univ. of Nebraska, Lincoln, NE 68583. Joint contribution of the USDA-ARS and the Neb. Agric. Res. Div., Journal Series no. 11038. Received 7 June 1994. *Corresponding author (Email: [email protected] ). Published in Agron. J. 88:729-733 (1996). al., 1990; Blackmer et al., 1994), because chlorophyll is an efficient absorber of visible light. Although reflectance from a corn canopy depends on many factors, including incident lighting, background, and physiological condition of the crop (Colwell, 1974), N deficiencies can detected by calibrating photographic responses within fields against responses from areas measured as nonlimiting in N. It is not practical for producers to establish a series of N rate areas in a field for each corn hybrid for calibration purposes. Schepers et al. (1992) proposed the use of areas within fields that receive ample or excess fertilizer N as reference targets when using chlorophyll meters to monitor crop N status. They found that chlorophyll meter readings (a ratio of light transmittance at 650 and 940 nm) first increased with fertilizer N application rates and then plateaued as N became ample. Apparently, as N rates increased, other nutrients and/or metabolic processes became limiting. The chlorophyll content corresponding to luxury consumption of N depends on development stage (Blackmer and Schepers, 1995). Even though there can be luxury consumption of N, leaf chlorophyll content reaches an upper limit. This upper limit changes during the growing season with the age of the leaf tissue. Once N supply to corn is adequate, the reflectance of corn canopies changes little as the N supply increases further (Blackmer et al., 1996). The use of a within-field reference also permits the separation of N stress from other stresses that may also be detected photographically. Variable rate fertilizer applicators, now available commercially, have created the need for better methods of characterizing spatial patterns of N availability within fields. Our objectives were to test color aerial photography for measuring the variability of crop N deficiency and to relate color responses as detected in transparencies to differences in N status within fields. MATERIALS AND METHODS Studies were conducted in 1992 and 1993 on continuous corn N response trials with four hybrids on irrigated plots at the Management Systems Evaluation Area (MSEA) project near Shelton, NE. Corn was planted in late April and early May in plots of eight rows (91-cm row spacing) by 15.2 The corn was fertilized at planting with NH4NO3 broadcast at rates of 0, 40, 80, 120 and with 160 kg N ha-I in 1992 and 0, 50, 100, 150, and 200 kg N ha-1 in 1993. Pioneer brand 3162, 3379, 3394, and 3417 hybrids were planted at approximately 65 000 seeds haI in four replications. The experimental design was a split-plot randomized complete block with hybrid as the whole plot and N treatment as the subplot. Aerial photographs (using 35-ram film) were taken in late August in both 1992 and 1993 from a height of approximately 1000 m under similar conditions and camera settings. In 1992, Kodak Gold 400 color film was used to collect images; in