Illinois
Fertilizer Conference Proceedings
January 26-28, 1998
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R.G. Hoeft, E.D. Nafziger, R. L. Mulvaney, L.C. Gonzini, and
J. J. Warren1
Hypoxia and nitrate levels in excess of the public health standard of 10 mg/l in public water supplies have drawn renewed interest to improving fertilizer N use efficiency. This interest comes both from farmers who are concerned about the environment and economics and from the general public who are concerned about the environment and maintaining an abundant but reasonably priced food supply. Nitrates in high concentrations in water supplies may create health problems for infants and according to oceanographers, may lead to undesirable low levels of oxygen, thus creating problems for shrimp and other marine life in the Gulf of Mexico. As agriculturalists, we want to do everything possible to maximize the efficiency of N fertilizers, while growing as much corn as possible. Doing this will improve the economics of corn production as well as minimize environmental problems.
A study by Keeney and DeLuca (1993) demonstrated that nitrate concentrations in the Des Moines River between 1980 and 1990 were similar to those observed during 1945, even though fertilizer consumption in the watershed had increased from 440 to 240,000 tons of N per year. They concluded that intensive agricultural practices in both 1945 and the late 1980's were the major source of nitrates to the river, rather than just the increase in N fertilizer consumption. However, the strong correlation between an increase in N concentration in the Mississippi River and an increase in fertilizer consumption suggests that fertilizers contribute significantly to the problem.
Minnesota research has demonstrated that use of N rates above recommended levels will contribute significantly to an increase in nitrates in tile line waters. Similarly, Illinois data have shown that long-term use of greater than recommended N rates will contribute more nitrates to tile line waters than when recommended N rates are used.
Some Illinois producers may be unknowingly contributing nitrates to water supplies (Brown et al., 1993). On-farm research identified 13 of 77 fields in which corn was non-responsive to fertilizer N. There was evidence to indicate that these fields had a previous history of high levels of fertilization and/or manure application. Based on these results, along with work by Torbert et al. (1992) showing that excess fertilizer N is assimilated into organic N compounds, and work by Stevens et al. (1997) that shows that these compounds mineralize more easily than native organic matter, we have theorized that these non-responding sites likely had adequate N release from the soil to meet the needs of the crop. Continued application of optimum or above optimum N rates on these fields will enhance the potential for nitrate movement through tile line drainage.
A survey of Champaign County producers indicated that nearly 70% were applying 40 lb N/acre or more above the recommended level. The reasons for such overapplication are numerous, but one frequently mentioned reason is lack of confidence in taking credits for legume and/or incidental N contributions.
The objectives of this project are to:
Ten experimental sites having clearly defined tile systems that drain only that field or a known portion thereof were identified in early 1997. Work is underway to identify the two remaining sites and to locate a new site needed to replace one of the initial ten.. Unfortunately, even though we had been told that one of the tile systems was easily identified, close inspection revealed that neither the extent nor the outlet of the tile line system could be found. Tile line monitoring systems were installed in the spring of the year at 8 locations. Each of these systems records water flow rates and collects water samples on a predetermined schedule based on flow rate. At 4 of the locations, air temperature, soil temperature, and soil moisture are collected every 5 minutes. Past cropping records including yield, time and rate of N application, and crop rotation were recorded for each site (Table 1). Other than in the small plot area of the field, the farmers have been encouraged to continue to apply the same rate of N and to continue to manage the field in the same manner as in the past.
Small plot nitrogen rate studies were established at 7 of the sites using ammonium sulfate in 40 lb N/acre increments. At four of the locations, no additional fertilizer N was applied by the farmer; at two of the locations, DAP had been applied at the rate of 200 lb/acre (36 lb N/acre); and at one location DAP was applied at the rate of 250 lb/ acre (45 lb N/acre). Thus the total N fertilizer application ranged from 0 to 240 lb /acre at four locations, from 36 to 276 lb /acre at two locations, and from 45 to 285 lb /acre at one location. The ammonium sulfate was applied at each location near the time that the farmer made his application. At three locations, 15N-labeled ammonium sulfate was applied to microplots within each N rate plot at the same rate of N.
Corn was planted in mid-to late April and thinned to 29,000 plants per acre at the V-4 growth stage. SPAD readings were collected from all plots once in June, twice in July, and once in August. At maturity, grain was hand-harvested for yield determination. At physiological maturity, whole plant samples were collected from the microplot areas that had received the 15N labeled fertilizer and from the 0, 80, 160, and 240 lb N/acre rate plots that had received unlabeled ammonium sulfate. Following harvest, soil samples were collected to a four-foot depth from all plots and analyzed for inorganic nitrogen. The soil samples collected from the microplots that had received N15 were analyzed for both inorganic and organic N and for 15N.
Application of N increased yield from 50 to 130 percent in the 7 experiments in 1997 (Figure 1). The larger responses that occurred in 1997 compared to 1996 were likely the result of markedly different weather conditions in the 2 years. In 1997, cooler and drier conditions prevailed throughout the early part of the growing season (Table 2). While the April temperatures were below normal in both years, the May and June 1996 temperatures came back up to near normal, but the 1997 temperatures remained significantly cooler than normal during those months. These lower temperatures combined with the low rainfall in April and June 1997 likely reduced mineralization rates and increased the response to fertilizer N.
Prior history of application did not appear to be related to either the unfertilized yield or the optimum N needed in 1997 (Table 3). Thus, data from 1997 do not support our hypothesis that N application in previous years affect response to N.
Tile line samplers were installed at four locations where N was spring applied, one location where N was sidedressed, and three locations where soybeans were grown. Peaks in nitrate concentrations were observed in early and late May at one location (Figure 2C), late May at one location (Figure 2A), and in early June at the other two locations that had spring applied N (Figure 2 and 2 D). When the N was sidedressed, there were peaks in nitrate concentration in late May to mid-June (Figure 2E). When soybeans were the current crop, one location had consistently high nitrate concentrations throughout the growing season (Figure 2G), one had consistently low concentrations (Figure 2H), and one had peaks similar to those observed when corn was grown with spring applied fertilizer (Figure 2F). Flow weighted nitrate concentrations were unrelated to either previous N use or current cropping, i.e. soybean or corn. The highest concentrations tended to be associated with the sites where the field N application rate exceeded the optimum N rate by 0.5 lb N/bushel (sites 2725 and 3717). In 1997, time of N application appeared to have no influence on nitrate concentration in tile line waters.
Table 1. Characteristics of the experimental sites.
Table 2. Early season temperature and moisture levels.
Table 4. Effect of time and rate of N application on
weighted N concentration in tile line effluents.
Figure 1. Effect of nitrogen rate and application timing
on corn grain yield.
Figure 2. Nitrate concentrations in tile drainage water.
1R.G. Hoeft and E.D. Nafziger are Professors, Dept. of Crop Sciences; R.L. Mulvaney is a Professor, Dept. of Natural Resources and Environmental Sciences; L.C. Gonzini and J.J. Warren are Senior Research Specialists, Dept. of Crop Sciences, University of Illinois.
