Illinois Fertilizer
Conference Proceedings
January 28-30, 1991
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H.M. Brown, R.G. Hoeft, and E.D. Nafziger 1
Nitrogen recommendations in much of the CornBelt are based on expected crop yield with adjustments for previous crops and manure applications. However, documented reports of near optimal corn yields, with little or no supplemental N, demonstrate the need for improved methods of determining N rates. Such improvements would contribute to the economic well-being of the farming community and minimize the potential for a negative impact of NO3-N on surface and groundwater.
Soil profile NO3-N levels have been used for several years as a guide for fertilizer recommendations in the more arid environments of the western CornBelt. Adaptation of such predictive procedures for more humid environments has received attention in recent years. Bundy and Malone (1988) have demonstrated the value of utilizing NO3-N concentration in the top 2 or 3 feet of soil in early spring to adjust N rates in Wisconsin. In Michigan, Vitosh et al. (1989) recommends that farmers reduce N rate by one pound for each pound of NO3-N found in the top 2 feet of soil. Blackmer et al. (1989) reported that the procedure of Magdoff et al. (1984) shows promise for Iowa soils. According to that procedure, no N need be applied when NO3-N in the surface 12 inches of soil collected when corn plants are 6 to 12 inches tall exceeds 21 ppm. A full rate of N (based on expected yield) is suggested at levels below 10 ppm, and an adjusted rate is suggested at levels between 11 and 20 ppm. The objectives of the project reported in this paper were to evaluate the accuracy of four nitrogen recommendation systems for predicting nitrogen rates necessary for optimum corn production.
Experiments were established at 20 locations throughout Illinois. Useable data were collected at 18 of the sites as adverse weather created crop growth problems at two of the locations. Sites were selected to provide a range in soil texture (sandy loam to clay loam), yield potential (98 to 167 bushels per acre), previous crop (continuous corn and corn following soybeans), and manure application (0 to 200 lbs available N as manure). Two of the sites had a history of manure applications in prior years, but not for the 1990 crop year.
Soil samples were collected for NO3-N analysis from each of the sites in April at depths of 0-6, 6-12, and 12-24 inches. Nitrate values (ppm) for the upper 24 inches were multiplied by a factor of eight to estimate total pounds NO3-N per acre.Additional samples were collected when corn was 6-12 inches tall from the top foot of soil at each location. Individual plots 10 ft. (4 rows) x 50 ft. in size were' arranged in a randomized complete block design using 4 replications. When corn was 12,18 inches tall, N was applied at 0, 20, 40, 60, 80, and 10O% of recommended rates. The recommended rate for each soil type was determined by taking expected yield x 1.2 lb N/bushel, minus adjustments for previous crop and manure application. The nitrogen was injected as urea-ammonium nitrate solution (28% N) using a 60-inch knife spacing.
Soon after N application, the center two rows of each plot were thinned to a uniform plant population. At silking, ear leaf samples were collected for N analysis. Grain yield was determined at physiological maturity by harvesting 30 feet of each of the center two rows. Grain samples were collected for N analysis. Economically optimum yield was calculated for each location as that point on the quadratic response curve where the slope was equal to a cost:price ratio of 0.053 ($0.12/lb of N; $2.25/bu of corn). Where there was no response to N, the optimum N rate was considered to be zero.
The four nitrogen recommendation systems evaluated were:
| NO3-N ppm | N Recommendation lb N/Acre |
| <10 | Yield goal x 1.2 lb |
| 11 to 12 | Yield goal x 1.2 x 0.8 |
| 13 to 15 | Yield goal x 1.2 x 0.6 |
| 16 to 18 | Yield goal x 1.2 x 0.4 |
| 19 to 20 | Yield goal x 1.2 x 0.2 |
| > 21 | No nitrogen |
Soil NO3-N content in the upper 24 inches of soil in April ranged from a low of 9 to over 300 lbs/acre (Table 1). The lowest value was observed on a sandy soil in east central Illinois, and the highest on a silty clay loam that had received over 10 tons of manure per acre the previous fall. The average preplant NO3-N values were as follows: central Illinois fine textured soils - 33 lbs/acre; northern Illinois fine textured soils - 56 lbs/acre; southern Illinois fine textured soils - 39 lbs/acre; sandy soils 24 - lbs/acre; and manured soils - 195 lbs/acre.
Soil NO3-N content in the top 12 inches of soil in early June ranged from 0.3 to 55.4 ppm, with the highest value coming from the field that contained 300 lbs N/acre in the top 2 feet at the earlier sampling. All of the fine textured, central Illinois locations contained less than 10 ppm NO3-N , and all had a significant response to applied N. Two of the five Northern Illinois locations contained less than 10 ppm NO3-N , but only one responded to applied N. One indicator of N responsiveness - the yield without fertilizer N expressed as a percent of the maximum yield in the trial - was positively correlated with soil NO3-N level at the early summer sampling, but most of the soil NO3-N levels were low, and so had little effect on recommended N rates.
Nitrogen recommendations derived from expected yield x 1.2 lbs N/bushel minus adjustments for previous crop and or manure (Illinois) would have resulted in an average recommendation 55 lbs N/acre in excess of the economically optimum rate for each location (Table 1). The extremes in over-recommendation occurred at sites 2, 4, 5, 13, 14, 17, and 19, where the recommended rate averaged 142 lbs N/acre higher than the amount needed for optimum yield. Sites 4 and 5 were located in an area of the state that received very low rainfall and consequently had low yields in the prior three years. As a result, there may have been an accumulation of more nitrates at a deeper depth than sampled, but still within the rooting zone. There was little or no response to applied N at five locations (2, 13, 14, 17 and 19). At these sites, all located in northern Illinois, the corn was planted early and encountered severe stress from excess moisture for several days in mid to late May prior to the time of N application. When one looks only at responsive sites, the average N recommendation exceeds the optimum by 18 lbs/acre.
Inclusion of the adjustment based on the 12-inch N03-N concentration (calibration proposed by Blackmer et al. 1989) resulted in an average recommendation 61 lbs N/acre in excess of the optimum. Removal of the five non-responding sites from the average reduced the excess to 30 lbs/a. Adjustment in N rate based on the 12-inch N03 concentration had little influence at those sites where excessive rates would have been applied using expected yield x 1.2 lbs N/bushel.
Subtraction of the amount of N contained in the top 2 feet of soil in early spring from the normal N recommendation (calibration proposed by Vitosh et al. 1988) resulted in a recommended rate 36 lbs N/acre higher than the optimum when averaged over the 18 sites. This system would have resulted in a significant over-application at only four sites (2, 13, 14, and 17). Even though those sites had low profile N03 levels, there was no response to applied N. Elimination of the five non-responding sites from the average reduced the excess to 6 lbs/acre.
Eleven of the 18 sites had yields less than expected for the soil type. These lower yields were due in part to the excess in soil moisture in the early part of the growing season, cooler than normal temperatures and severe infestation of corn borer. When averaged over all locations, 0.58 lb of fertilizer N produced one bushel of corn at the optimum N rate. Elimination of the five non-responding sites would increase that to .76 lbs N/acre. This would imply that soils mineralized more N or that N was more efficiently utilized during 1990 than was the case during the time period when the factor of 1.2 lbs N/bushel was derived.
Incorrect N recommendations may result in lost income because of the purchase of excess N without a yield response or from a missed opportunity resulting from the failure to apply enough N to attain optimum yield (Table 2). The data in Table 2 were determined by subtracting net income to fertilizer at the recommended rate from net income to fertilizer at the experimentally determined optimum N rate.
Setting $5.00 per acre as an acceptable error, the Illinois recommendation system would have resulted in over-application at 9 locations and under-application at 1. Using that same criterion, the Michigan and Iowa recommendations resulted in over application at ,6 and 13 locations and under application at 1 and none respectively. The modified Michigan recommendation would have over-applied at 5 locations and underapplied at 4 locations. All of the 5 non-responding sites (2, 13, 14, 17, and 19) were counted in the over-application for each of the 4 recommendation systems. Considering all 4 recommendation systems, under-application was of less economic consequence when averaged over all locations than was over-application. When the 5 non-responding locations are eliminated, over-application is still of more economic consequence than under-application except for the revised Michigan system in which there were no overapplications.
Results from this first year have shown that use of N recommendation based on yield goal times 1.2 lbs N/bushel minus an adjustment for previous crop and manure application was reasonably accurate in predicting correct N rate for 8 out of 13 responding sites. Use of NO3-N in the surface 12 inches of soil when corn was 6-12 inches tall, was reasonably accurate for 5 of the 13 responding sites. When the NO3-N level in the top 2 feet of soil prior to planting was used to adjust the recommendation, only 3 of the 13 responding sites were incorrectly predicted.
Blackmer, A.M., D. Pottker, M.E. Cerrato, and J. Webb. 1989. Correlations between soil nitrate concentrations in late spring and corn yields in Iowa. Jk Prod. Agric. 2:103-109.
Bundy, L.G. and E.S. Malone. 1988. Effect of residual profile nitrate on corn response to applied nitrogen. Soil Sci. Soc. Am. J. 52:1377-1383.
Magdoff, F.R., D. Ross, and J. Amadon. 1984. A soil test for nitrogen availability to corn. Soil Sci. Soc. Am. J. 48:1301-1304.
Vitosh, M.L., D.D. Warncke, D.R. Christenson, and J.G. Hahl. 1988. Soil nitrate testing: a guide for adjusting Michigan nitrogen recommendations for corn. Proceedings of the 18th N. Central Extension-Industry Soil Fertility Workshop, Nov. 9-10, 1988. Potash & Phosphate Institute, Manhattan, KS.
