The SPAD 502 chlorophyll meter (Minolta Camera Co., Osaka, Japan) is a new instrument that has been introduced as a tool to improve N management. This study was conducted to evaluate the ability of the chlorophyll meter to detect plant N deficiencies in corn (Zeu mays L.) by identifying when it would be appropriate to supply N fertilizer in irrigation water. Nitrogen response studies were conducted on five irrigated sites in central Nebraska in 1991. Crop N status was monitored during the growing season and additional N was added to simulate fertigation when apparent N deficiencies were detected with the meter. Changes in N status over the season were determined relative to an adequately fertilized in‐field reference plot. Earlyseason (V6) N deficiencies were poorly correlated with yield because factors such as nitrate leaching, organic matter mineralization, and nitrate present in irrigation water modified the crop N supply during the growing season. Nitrogen deficiencies detected late (R4–R5) in the season were more highly correlated with yield than early season N stresses. Treatments that started with adequate fertilizer and then became deficient were corrected without yield loss. Young plants in a deficient state could not be corrected to full yield potential. Chlorophyll meters can be a valuable tool for N management of irrigated corn production when used to assess crop N status in the irrigated Great Plains. Research Question Corn production may lead to groundwater contamination by nitrate when N fertilizers are applied in excess of the crop's needs. Practices that will synchronize N fertilization with the crop's needs could help reduce the potential for groundwater contamination by nitrate without reducing yields. Literature Summary Monitoring leaf N concentration provides a means of identifying crop N status, but correlation of this measurement to grain yield is confounded by luxury consumption and requires time consuming lab analysis. Leaf chlorophyll measurements can also detect N deficiencies, but are not as sensitive to luxury consumption as other N measurements. The SPAD 502 chlorophyll meter provides an instantaneous means of evaluating chlorophyll content. Applied Questions Could the chlorophyll meter detect N deficiencies that resulted in reduced grain yields? The chlorophyll meter, when used with a reference strip (an area receiving adequate N), accurately detected N deficiencies that resulted in reduced grain yields. Using chlorophyll meter readings without an infield reference strip resulted in variability from such factors as different hybrids, soil types, and growth stage. Did all sites have the same pattern of N availability over the season? All sites responded differently. Some sites became more N deficient as the season progressed. In others, more N became available as the season progressed. Nitrogen availability at one site remained constant over the season. This variability may apply only to irrigated sites in the area studied. Could deficiencies detected by the chlorophyll meter be corr...

New tools that can rapidly quantify the N status of corn could be valuable in N fertilizer management practices. This study was conducted to compare light reflectance from corn leaves with other parameters used to detect N deficiencies. Light reflectance (400–700 nm) as measured from corn leaves in the laboratory with a Hunter tristimulus colorimeter was compared with Minolta SPAD 502 chlorophyll meter readings (light transmittance at 650 and 940 nm), leaf N concentrations, and specific leaf N (N content per unit area). Measurements were made on individual ear leaves collected from an irrigated corn N response trial with four hybrids and five N treatments. Light reflectance near 550 nm was the best wavelength to separate N treatment differences. Reflectance at 550 nm provided a stronger relationship with both leaf N concentration and chlorophyll meter readings than between chlorophyll meter readings and leaf N concentration. The measurement of light reflectance near 550 nm has promise as a technique to detect N deficiencies in corn leaves.

Techniques that measure the N status of corn (Zea mays L.) can aid in management decisions that have economic and environmental implications. This study was conducted to identify retiected electromagnetic wavelengths most sensitive to detecting N deficiencies in a corn canopy with the possibility for use as a management tool. Rdlected shortwave radiation was measured from an irrigated corn N response trial with four hybrids and five N rates at 0, 40, 80, 120, and 160 kg N ha-ĩ n 1992 and 0, 50, 100, 150, and 200 kg N ha-i in 1993. A portable spectroradiometer was used to measure reflected radiation (400-1100 nm in 1992, 350-1050 nm in 1993) from corn canopies at approximately the R5 growth stage. Regression analyses revealed that refiected radiation near 550 and 710 nm was superior to refiected radiation near 450 or 650 nm for detecting N deficiencies. The ratio of light refiectance between 550 and 600 nm to light reflectance between 800 and 900 nm also provided sensitive detection of N stress. In 1993, an inexpensive photometric cell, which has peak sensitivity to light centered at 550 nm, was also used to measure reflected radiation from a corn canopy. Photometric ceil readings correlated with relative grain yield (P < 0.001, 2 = 0.74), but more research will be required to develop procedures to account for varying daylight conditions. These results provide information needed for the development of variable-rate fertilizer N application technology.

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SummaryNitrogen is essential for crop production, but also cnnccibutes to eutrophication of surface water and degradation of drinking water quality. Modern corn production requires relatively large quantities of N, which are generally supplied by fertilizers. Over-application of N fertilizers and animal wastes frrquently results in nitrate leaching. Synchronizing N availability with crop N need offers the to protect the environment without sacrificing production. Tools are needed to rapidly and easily monitor crop N status to make timely decisions regarding fertilizer application. Analytical and optical techniques were . . evaluated with greenhouse grown corn at silking to evaluate several methods to monitor crop N status. A ourtable chloroohvll meter was used to measure chloroohvll content of leaves bv m e a n of transmittance Abbreviations: N = nitrogen; C = carbon; W = water stress; VI' = stage of corn growth after complete tassel emergence but beforr silk emergence.