PRE-SIDEDRESS SOIL NITRATE TEST FOR SWEET CORN

Research was conducted with sweet corn (Zea mays L.) to evaluate the pre-sidedress soil NO3 test (PSNT) originally developed for use on field corn on a wide range of New Jersey soils. Soil NO3-N concentrations reflected differences in N availability due to manure or preplant N application. The relationship between soil NO3-N concentration and relative yield of marketable ears was examined using Cate-Nelson analysis to define the PSNT critical level. Soil NO3-N concentrations > 25 mg . kg(-1) were associated with relative yields at greater than or equal to 92%. The success rate for the PSNT critical level was 85% for predicting whether sidedress N was needed. Including NH4-N in the soil analysis did not improve the accuracy of the soil test for predicting whether sidedress N was needed. Although the PSNT is quite accurate in identifying N-sufficient sites, it appears to offer only limited guidance in making N-fertilizer rate predictions. The PSNT is most useful on manured soils, which frequently have sufficient N. The test likely will help decrease the practice of applying ''insurance'' fertilizer N and the ensuing potential for NO3 pollution of the environment.