DEVELOPING A SOIL-NITROGEN TEST FOR IMPROVED RECOMMENDATIONS FOR CORN

Accurate, reliable N recommendations are important if farmers are to attain optimum economic benefits while minimizing the potential for negative impacts of N on the environment. This study was conducted to determine the feasibility and reliability of including a soil N test to adjust N recommendations for corn (Zea mays L.) and to evaluate which soil test parameters should comprise the test. Nitrogen rate response experiments were conducted at 54 sites throughout the corn-growing region of Minnesota from 1989 through 1992. Analysis of the data indicated that nitrate-N concentration in the surface 2 ft taken in the spring before planting provided a sound, useable relationship for a soil N test that was evaluated in terms of R2, frequency of errors, and practical feasibility. A linear relationship R2 = 0.51) was measured between soil nitrate-N in the proposed N test and soil N credit. Soil N credit was calculated for each site-year by subtracting the optimum N rate, which was measured experimentally, from the tabular N recommendation presently used in Minnesota. Based on the slope of this regression, the interpretation is that for each unit of nitrate-N measured in the soil test, a slightly lower amount would be quantified as a soil N credit. Minnesota corn N recommendations can be refined using a soil N test by subtracting the soil N credit values, which are a function of the proposed soil test, from the standard tabular N recommendations. No adjustment in N recommendations is made when soil nitrate-N is < 6 ppm while N is not recommended at concentrations > 19 ppm. The test appears most helpful when corn is the previous crop, when the soil texture is either medium or fine, and when some carryover of residual nitrate is expected. This approach, using the current N rate recommendations adjusted by the soil nitrate test, should facilitate easier and quicker adoption of a soil N test by Minnesota crop producers.