Sensitivity of chlorophyll meters for diagnosing nitrogen deficiencies of corn in production agriculture
Chlorophyll meters have been widely used to diagnose N deficiencies in research trials and to help refine N fertilizer recommendations in production agriculture. The diagnoses are based on the assumption that chlorophyll meters can detect a wide range of N deficiencies under field conditions according to relationships between chlorophyll meter readings (CMRs) and yield responses to N fertilization that are established from small-plot trials. We conducted a field-scale study to evaluate the sensitivity of chlorophyll meters when they are used to detect N deficiencies in corn (Zea mays L.) production. The multiple year–location data were gathered in three cornfields where N fertilizer was applied at different rates in strips that crossed several soil map units. Grain yields were related to CMRs taken on corn leaves in June, July, and August of 1998–1999. Results showed that the chlorophyll meters could detect severe N deficiencies early in the season, but small and moderate deficiencies of N could not be diagnosed with reasonable certainty until it was too late to make in-season fertilization. Two types of discontinuity are discussed in this paper as likely causes for the limited sensitivity of the chlorophyll meters. Our study suggests that problems associated with the use of chlorophyll meters to diagnose deficiencies of N in production agriculture are much greater than their use to help interpret results of well-controlled research trials where yields are also measured.