Use of a Chlorophyll Meter to Monitor Nitrogen Status and Schedule Fertigation for Corn

The SPAD 502 chlorophyll meter¹ (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 QuestionCorn 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 SummaryMonitoring 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 QuestionsCould 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 corrected soon enough to prevent yield loss? 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.