USE OF A CHLOROPHYLL METER AT THE EARLY DENT STAGE OF CORN TO EVALUATE NITROGEN SUFFICIENCY

A late-season chlorophyll meter test for corn (Zea mays L.) could provide growers with a convenient evaluation of that year's N management program, along with information useful for N fertilizer rate decisions in subsequent growing seasons. Chlorophyll meter (SPAD 502) readings were taken late in the growing season on corn ear leaves in replicated plots of 357 N treatments from 93 N-response experiments conducted in Pennsylvania. At sites not affected by severe late-season drought or disease, chlorophyll meter readings taken at the 1/4 milk line stage separated N-deficient from N-sufficient treatments with 93% accuracy using a critical level of 52.0 SPAD units. If meter readings were normalized at each site by comparison with a site-specific high-N reference reading, chlorophyll meter readings of ear leaves made from the late milk to the middent stage of corn could separate N-deficient from N-sufficient treatments with 92% accuracy using a relative SPAD reading critical level of 0.93. For treatments with meter readings below the critical level, relative grain yields and the additional N that would have been needed for economic optimum grain yield were correlated with chlorophyll meter test values. For treatments with a meter reading above 52.0 SPAD units, the amount of excess N that may have been applied could not be accurately predicted. For corn tested at the 1/4 milk line stage, we suggest an N-sufficiency range of 52.0 to 56.0 SPAD units, above which there is a 70% probability of an excess of >50 kg N ha(-1) having been applied.