In Michigan the presidedress nitrate test (PSNT) (Magdoff et al., 1984) is used to adjust nitrogen (N) recommendations for corn and sometimes sugar beet. Many growers do not use the PSNT because the presidedress soil sampling time does not conveniently fit into their operation. Preplant nitrate tests do not provide a good estimate of plant available N because of the relatively wet weather conditions during Michigan springs. Another drawback to the PSNT is that it tends to recommend N on soils which have manure histories and are non-responsive to N fertilization.

The Illinois nitrogen soil test (INST) measures a portion of amino sugar-N and provides an estimate of N that may become available during the growing season (Mulvaney et al., 2001). The University of Illinois is suggesting that when the INST is greater than 250 ppm N for a 1- foot soil sample, corn will not respond to fertilizer N in central or northern Illinois (University of Illinois Technical Note 02-01 (rev. d)). Soil samples for the INST may be taken prior to planting. A preplant soil test that may be used to determine non-responsive fields would likely see wider adoption in Michigan than the PSNT.

Two studies have been started in Michigan to calibrate the INST with the optimum N fertilizer rate for corn and sugar beet. The objectives of this paper are: 1) To provide initial results of the field calibration studies; 2) To discuss differences in results between laboratories.