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 ( Zeu 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 R 2 , frequency of errors, and practical feasibility. A linear relationship R 2 = 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. Research Question Fertilizer N recommendations for most of the corn acres in Minnesota are based on expected yield, previous crop, and soil organic matter level. Environmental and economic concerns have prompted the possible need for refinement of these current fertilizer N rate recommendations. This study evaluated the effectiveness of including a soil N test component in the N recommendation procedure for corn. A 4‐yr, 54‐site correlation and calibration project was designed for this purpose. Literature Summary Soil nitrate‐N testing before planting is a long‐standing management strategy for the low‐rainfall areas in the Great Plains and western regions of the USA, including the western tier of Minnesota. In the humid climate regime of the Upper Midwest and Northeast, soil N tests have recently been incorporated into the procedure for making N recommendations in several states. One strategy recommends using a 1‐ft Sample taken when the corn is approximately 12 in. tall and analyzing for nitrate‐N. In contrast, another uses a spring preplant test to measure nitrate‐N in a 2‐ or 3‐ft depth. Study Description Nitrogen rate response experiments were conducted at 54 sites throughout the corn‐growing region of Minnesota from 1989 to 1992. At each site, N rates were applied either preplant or sidedress in 30 lb N/acre increments (including a no‐N control) with the maximum N rate based primarily on the previous crop. Soil samples were collected from the control plots at three sampling times and from four soil depths and analyzed for three N forms. The sampling times were just before planting, approximately 2 wk after planting, and when the corn was about 12 in. tall. The soil sampling depths were 12‐in. increments to a depth of 4 ft. Nitrate‐N, ammonium‐N, and hydrolyzable N were the N forms measured. Applied Questions Which soil N test parameters (i.e., time, depth, N form) were selected to use in Minnesota? A preplant, 0‐ to 2‐ft soil sample analyzed for nitrate‐N is the soil N test selected to be used. While the R 2 and error rate of this set of parameters is not superior to an early in‐season test taken to 2 ft (Table l), the 2‐ft preplant nitrate test has the advantages of (i) being able to estimate residual N separately from other N sources accounted for by current recommendations; (ii) not implying that sidedress N applications are necessary, thus having a greater potential for user acceptance. How can the soil N test information be integrated into fertilizer N recommendations? The preplant soil nitrate‐N test results were not well correlated to optimum N rates determined for each site in our study. Because a preplant test primarily accounts for residual N, one of several sources of available N for a corn crop, we determined that rather than using soil N test data to determine N rates, we would use the N test data to determine the fertilizer equivalent of residual N. This value was termed “soil N credit.” Soil N credits were calculated from our study's locations to be the difference between the standard N recommendation amount and the calculated optimum N rate. A useful linear model was developed by regressing the soil N credits against the soil N test values. This regression model was then separated into 3 ppm increments and shifted slightly so that soil N credit amounts used to adjust N recommendations were at least equal to the amounts predicted by the equation (Fig. 1). Recommendation The preplant, 2‐ft soil nitrate‐N test can be used to provide a soil N credit that can improve N recommendations for corn. Current University of Minnesota N recommendations will provide the initial N recommendation, and the soil nitrate test will be used to determine how much, if any, soil N credit should be subtracted from the initial recommendations. The effect of sampling time, sampling depth, and N form on linear‐plateau models with relative grain yields as the dependent variable, 1989–1992. Sampling Critical Error Time Depth N form R 2 value rate ft ppm N % PP 0–1 Nitrate‐N 0.36 19.5 22 0–2 Nitrate‐N 0.49 15.9 22 0–2 Ammonium‐N NF‐ ‐ ‐ 0–2 Hydrolyzable‐N 8 0.38 20.6 20 0–2 (Nitrate + ammonium)‐N 0.38 20.6 20 0–2 Nitrate‐N 0.49 14.5 20 VI 0–1 Nitrate‐N 0.50 20.8 26 0–2 Nitrate‐N 0.57 20.0 26 0–1 (Nitrate + ammonium)‐N 0.58 23.0 19 0–2 (Nitrate + ammonium)‐N 0.64 18.5 21 V4 0–1 Nitrate‐N 0.41 12.8 31 0–2 Nitrate‐N 0.44 20.0 20 0–1 (Nitrate + ammonium)‐N 0.53 19.5 35 0–2 (Nitrate + ammonium)‐N 0.58 17.0 29 PP = preplant; V1 = 1–2 leaf corn; V4 = 6 to 12‐in. corn. Phosphate‐borate extractable hydrolyzable N. The data did not statistically fit a model. Soil N credit amounts as a delineated, adjusted function of the preplant nitrate‐N concen