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 (Zea 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. Early-season (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.