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 the environment. A 4-yr study (49 site-yr) was conducted to evaluate the performance of preplant (PPNT) and presidedress (PSNT) soil nitrate tests for predicting optimum N rates for corn at sites with inorganic and organic N inputs. Use of the PPNT on organic sites included the subtraction of standard book value N credits (BVNC). Soil samples were obtained 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 measured at all sites. The relationship between relative yield and PPNT or PSNT test values was usually improved by separating sites into medium (MYPS) and high (HYPS) soil yield potential categories. The PPNT and PSNT identified N sufficient sites more accurately on HYPS than on MYPS. Use of either test to predict N application rates for inorganic or organic MYPS resulted in more of the sites receiving correct N application rates and fewer sites receiving excessive rates of applied N; however, use of the tests increased the probability for underapplication 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 excessive N application by 67% (PPNT) and 56% (PSNT) on HYPS. For organic HYPS, 30% (PPNT-BVNC) and 54% (PSNT) more of the sites received correct N application rates, and excessive N applications were reduced by 39 and 54%, respectively. While both soil tests improved the prediction of optimum N rates 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. Separating nitrate test calibration data according to the yield potential of soils may improve the utility of the PPNT and PSNT for making N recommendations for corn when nitrate test values are in the N responsive range. Research QuestionEconomic and environmental concerns have led to increased research on several N availability indices to predict N requirements of corn on a site-specific basis. Of these indices, soil nitrate tests currently appear to have the greatest practical value for assessing N availability. This study evaluated the preplant (PPNT) and presidedress soil nitrate tests (PSNT) in 49 experiments at locations with a range of organic N inputs and soil yield potentials. The objectives of this research were to: (i) compare the PPNT and PSNT for predicting optimum N rates for corn following corn without recent organic N additions; and (ii) compare standard N credit recommendations, the PSNT, and the PPNT used in combination with standard N credits, for predicting optimum N rates for corn in organic systems such as second-year corn following alfalfa or where manure was recently applied. Literature SummaryPreplant soil profile nitrate tests are widely used in semi-arid regions of the USA. The PPNT has recently been incorporated into N recommendations in humid states such as Wisconsin, Michigan, and Minnesota. The lower annual precipitation of the Upper Midwest compared with other humid regions of the USA, and the fact that soils are usually frozen for 3 to 4 mo during the winter, promote accumulation and overwinter retention of residual nitrate. The PSNT is available to producers in several eastern states and Iowa. This test has the potential to reflect N availability from recent organic N additions, such as legumes or manure, since samples are taken 60 to 90 d after PPNT sampling. Study DescriptionPreplant and presidedress soil nitrate tests were evaluated for their ability to identify N sufficient sites and predict optimum N rates in several crop management systems for corn, including sites receiving N from inorganic and organic (manure and/or legume) sources. Forty-nine siteyear locations were selected during the 1989 through 1992 growing seasons to include soils with medium and high yield potentials. Corn yield response to applied N was measured at all sites. Nitrogen rates were 0, 50, 100, 150, and 200 lb N/acre in 1989 and 1990. In 1991 and 1992, rates were 0, 30, 60, and 90 lb N/acre where manure was applied and 0, 30, 60, 90, 120, 150, and 180 Ib N/acre elsewhere. Soil samples for nitrate analysis were obtained from the control (no N) plots in 1-ft increments from 0 to 3 ft before planting (PPNT) and when corn was 6 to 12 in. tall (PSNT). Applied QuestionsDoes the yield potential of soils affect the performance of soil nitrate tests? The relationship between PPNT values and relative yield was markedly improved when sites were separated by soil yield potential (Fig. 1a). Full scientific article from which this summary was written begins on page 561 of this issue. Critical PPNT (0–3 ft) values were 74 lb nitrate-N/acre greater for high yield potential soils than for medium yield potential soils. A good relationship between PSNT and relative yield was found for high yield potential soils but not for medium yield potential soils (Fig. 1b). Separating soils by yield potential also allowed improved identification of optimum N rates for corn based on soil nitrate tests. Both the PPNT and PSNT identified optimum N rates for corn more accurately on high yield potential soils than on medium yield potential soils. Fig. 1Relationship between preplant (a) or presidedress (b) soil nitrate tests and relative corn yield in Wisconsin, 1989 through 1992. MYPS, medium yield potential soil; HYPS, high yield potential soil. Can using a soil nitrate test, calibrated according to soil yield potential, improve N recommendations for corn? The use of the PPNT or PSNT improved N recommendations for corn on high yield potential soils and reduced excessive N applications without yield loss by up to 67% (PPNT) and 56% (PSNT) compared with standard N recommendation methods (Fig. 2). Soil nitrate tests increased the percentage of sites receiving correct recommendations for corn on medium yield potential soils compared with standard N recommendations, but the tests increased the probability of using below optimum N rates. Are there differences in effectiveness between the PPNT and PSNT? For sites with inorganic and organic N inputs on high yield potential soils, both tests reliably identified N sufficient sites and, predicted optimum N rates more accurately than standard N recommendations. The PPNT was most accurate in predicting optimum N rates for sites with inorganic N inputs (Fig. 2a), and the PSNT was most accurate in predicting optimum N rates for sites with organic N inputs OD high yield potential soils (Fig. 2b). Fig. 2Comparison of soil nitrate tests and standard Wisconsin N recommendations for predicting optimum N rates for corn at sites with inorganic (a) and organic (b) N inputs. STD, standard N recommendation; PPNT, preplant soil nitrate test; PSNT, presidedress soil nitrate test; MYPS, medium yield potential soil; HYPS, high yield potential soil.