SOIL-YIELD POTENTIAL EFFECTS ON PERFORMANCE OF SOIL NITRATE TESTS

Improved N recommendation methods for corn (Zea mays L.) are essential for increased profitability and reduction of potential N losses to th e environment. A 4-yr study (49 site-yr) was conducted to evaluate the performance of preplant (PPNT) and presidedress (PSNT) soil nitrate t ests for predicting optimum N rates for corn at sites with inorganic a nd organic N inputs, Use of the PPNT on organic sites included the sub traction of standard book value N credits (BVNC). Soil samples were ob tained before planting (PPNT) and when corn was 6 to 12 in. tall (PSNT ), and analyzed for nitrate-N, Corn yield response to applied N was me asured at all sites, The relationship between relative yield and PPNT or PSNT test values was usually improved by separating sites into medi um (MYPS) and high (HYPS) soil yield potential categories, The PPNT an d PSNT identified N sufficient sites more accurately on HYPS than on M YPS, Use of either test to predict N application rates for inorganic o r organic MYPS resulted in more of the sites receiving correct N appli cation rates and fewer sites receiving excessive rates of applied N; h owever, use of the tests increased the probability for underapplicatio n of N, especially on inorganic sites, For inorganic HYPS, 89% (PPNT) and 67% (PSNT) received correct N application rates compared with only 22% with standard N recommendations. These tests also reduced excessi ve N application by 67% (PPNT) and 56% (PSNT) on HYPS. For organic HYP S, 30% (PPNT-BVNC) and 54% (PSNT) more of the sites received correct N application rates, and excessive N applications were reduced by 39 an d 54%, respectively, While both soil tests improved the prediction of optimum N Fates for inorganic and organic HYPS compared with standard methods, the most reliable prediction of optimum N rates were with the PPNT for inorganic HYPS and with the PSNT for organic HYPS. Separatin g nitrate test calibration data according to the yield potential of so ils may improve the utility of the PPNT and PSNT for making N recommen dations for corn when nitrate test values are in the N responsive rang e.