USE OF A CHLOROPHYLL METER TO PREDICT SIDEDRESS NITROGEN REQUIREMENTS FOR MAIZE

Most N testing methods require collecting, processing and analyzing of either soil or plant tissue samples. In 1987 we began a study to determine if chlorophyll readings of maize (Zea mays L.) made in the field with a portable chlorophyll meter could be used to predict soil N availabilities and sidedress fertilizer N requirements. The meter was tested over a 3-yr period on 67 N response experiments located in central and southern Pennsylvania. At the six-leaf stage of maize the chlorophyll meter reading of the fifth leaf was an accurate predictor of whether maize would respond to sidedress N fertilizer. A chlorophyll meter reading of 43.4 meter units was found to be the best critical level to separate responsive and nonresponsive sites. The chlorophyll meter method was similar in its accuracy in separating N responsive sites from nonresponsive sites to several proposed soil N availability tests including the soil NO, concentration in the surface 20 cm of soil at planting, the UV absorbance at 200 nm of a 0.01 M NaHCO3 extract of the surface 20 cm of soil at planting, and the soil NO3 concentration in the surface 30 cm of soil 4 to 5 wk after plant emergence. The chlorophyll meter test was not well enough correlated with soil N supplying capability (r = 0.59) to determine sidedress N fertilizer rates for N responsive sites.