Illinois
Fertilizer Conference Proceedings
January 23-24, 1990
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R. Killorn1
There is concern that fertilizer nitrogen (N) that remains in the soil following harvest may be leaching beneath the root zone of crops and eventually ending up in groundwater. Such losses of N have economic as well as environmental implications. Agronomics throughout the Corn Belt are actively evaluating methods to manage fertilizer N more efficiently.
Nitrogen fertilizer recommendations are currently made by multiplying a farmer's expected yield goal by an empirically derived constant that is an estimate of the amount of N required to produce a bushel of corn. The factor used in Iowa is 1.22 (Voss and Killorn, 1988). This can be expressed mathematically by the equation:
Nf = 0 + 1.22 * YG
Nf is the N requirement (pounds/acre)
YG is the yield goal (bushels/acre)
Nf is then modified to account for N from other sources, such as a previous legume crop, an application of manure, or N-contained in an N-P-K fertilizer.
While the equation is mathematically consistent, it is not agronomically consistent unless two assumptions are made. The first is that the farmer has a realistic yield goal. This effectively places an upper limit on the equation. Secondly, the equation, as written, indicates if no N is applied, there will be no yield. Since this isn't true, we must assume that the N fertilizer requirements are larger than year-to-year variations in the N-supplying capacity of the soil (Olsen and Kurtz, 1982). This allows us to ignore the intercept of the line (which is defined as 0 in the equation above).
Table 1. Yield of continuous corn where no N was applied over several years at Beaconsfield, Iowa. (Data from G. Benson).
| 1980 | 1981 | 1982 | 1983 | 1984 | 1985 |
|---|---|---|---|---|---|
|
bushels/acre |
|||||
52 |
150 |
45 |
21 |
48 |
79 |
The data in Table 1 are average yields observed when no N was applied in a study conducted near Beaconsfield in southcentral Iowa. The range is from 21 bushels per acre in 1983 to 150 bushels per acre in 1981. Clearly, we cannot ignore year-to-year variation in yields when no N is applied. This variability is probably largely due to changes in the amount of residual and mineralized N supplied by the soil.
The search for suitable indices for estimating available N in soils has been in progress for some time (Magdoff, et al., 1984). There are three primary categories of N indices:
The biological studies are time consuming and can be difficult to run. Category 3 is an attempt to get the same information as the biological tests in a shorter time. There are several of these procedures that are correlated with one another, however, they tend to have little predictive value in terms of N fertilizer requirements in many situations.
Direct measurement of nitrate is being used in the low rainfall areas of the western U.S. However, because of the mobility of nitrate in soils, it has been difficult to obtain reliable calibrations in the more humid Corn Belt. This is due primarily to the unpredictability of rainfall and its effects on nitrates.
FOOTNOTES
Researchers in eastern Corn Belt states are currently investigating different ways to estimate how much N a soil will supply in a given year. There are two basic approaches, calibrating an index, or measuring nitrate nitrogen and reducing the fertilizer recommendation on a one-for-one basis.
There are also two general times to collect the samples, either in the spring before planting or in early June after the crop is planted and has emerged. The specifics differ from state to state. The information contained in the following discussion came from conversations with Extension soil fertility specialists in the various states and from published materials.
Illinois
Nitrogen fertilizer recommendations are based on the grower's yield goal multiplied by a standard factor.
Indiana
Nitrogen fertilizer recommendations are based on the grower's yield goal multiplied by a standard factor.
Iowa
Researchers in Iowa are currently evaluating several different indices. Correlation data developed by Blackmer, et al. (1989) indicates that the index developed by Magdoff, et al. (1984) shows promise in the state (Figure 1).
Nitrate-N concentration is determined on a 0-12 inch soil sample that is collected when the plants have reached the V3 to V4 growth stage (three to four leaf collars visible on the stalk), which normally occurs in early June. If the soil sample contains 21 ppm of nitrate-N or more, no fertilizer is required. If the sample tests less than 21 ppm nitrate-N, fertilizer is recommended on a pre-rated scale.
| ppm nitrate-N in soil | Recommendation (lb N/a) |
|---|---|
| <10 |
l.2 x yield goal |
| 12-Nov | 0.8 (1.2 x yield goal) |
| 13-15 |
0.6 (1.2 x yield goal) |
| 16-18 | 0.4 (1.2 x yield goal) |
| 19-20 |
0.2 (1.2 x yield goal) |
| >21 | 0 |
The test was offered for the first time this past spring. The Iowa State University Soil Testing Laboratory received over 800 soil samples. Nitrate-N concentrations ranged from less than 10 ppm to more than 100 ppm (Figure 2).
Sixty-five percent of them tested in excess of 21 ppm nitrate-N (Figure 3).
Information sheets received with the samples indicated that most of those that tested in excess of 21 ppm nitrate-N had already been fertilized with at least a portion of what the grower anticipated would be required. The correlation data currently applies only when no N fertilizer (except that in a low-analysis starter) or manure was applied prior to taking the samples, or where such applications were broadcast. Further research is required in fields where N fertilizer has been band-applied, (i.e. anhydrous ammonia or banded manure). Calibrations will be refined with the results of on-going research.
Michigan
Nitrogen fertilizer recommendations based on a 2-foot soil sample (0-1' plus 1-2') were offered this past spring. A one-for-one reduction in fertilizer N was used. The soil had to contain more than 30 pounds N/acre before any credit was given. Extension specialists in the state offered the test at 38 clinics around the state in 1989 using a mobile test unit. (Vitosh, et al., 1989)
Minnesota
Nitrogen fertilizer recommendations based on soil tests have been offered for some time for the western portion of the state. Recommendations based on soil tests are now available for the entire state.
Missouri
Nitrogen fertilizer recommendations based on soil tests were available this past year. The recommendations are based on a sample taken in 1-foot increments to 2 feet (or to a root restricting layer). The amount of nitrate-N in the sample is deducted on a one-for-one basis from the fertilizer recommendation. The samples must be collected after April.
Ohio
Nitrogen fertilizer recommendations based on soil tests are not currently available. Research is currently in progress to evaluate the 1-foot soil index (Magdoff, et al., 1984).
Wisconsin
Nitrogen fertilizer recommendations based on a soil test were available this past year. The recommendations are based on the amount of nitrate-N in the top 2 or 3 feet of soil before planting (Bundy and Malone, 1988). Correlation data are shown in Figure 4.
A background nitrate-N value (either 40 or 50 pounds N/acre) is subtracted from the results of the soil analysis. The remainder is subtracted from the standard corn N recommendation. If the initial profile nitrate-N is less than 200 pounds N/acre, a minimum N recommendation of 50 pounds N/acre is given. A zero recommendation is given if the initial test value is more than 200 pounds N/acre. Finally, the recommendation is adjusted for manure and legume credits.
Our current system for making N-fertilizer recommendations needs to be strengthened, so that we can estimate the amount of N that will be supplied by the soil in any year and alter the amount of N-fertilizer applied accordingly.
There are currently several options being evaluated in the Corn Belt. Some of the options involved non-traditional sampling times and times of N-fertilizer application. However, in the next few years, we will see the implementation of N-fertilizer management tools that will maintain agricultural profitability while minimizing the effect of agricultural production on the environment.
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. J. 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.
Killorn, R., R.D. Voss and J. Hornstein. 1987. Refined nutrient management for reduced energy consumption. In Summary report, Integrated Farm Management Demo. Program. Pm-1305. Iowa State Univ.
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.
Olsen, R.A. and L.T. Kurtz. 1982. Crop nitrogen requirements, utilization and fertilization. In Nitrogen in Agricultural Soils, F.J. Stevenson ed. Am. Soc. Agron. pub. pp 567-604.
Vitosh, M.L., B.P. Darling and D.B. Campbell. 1989. Nitrate testing clinics. Proc. 19th N.C. Ext.-Industry Soil Fert. Conf., Nov. 8-9, St. Louis, MO. Potash & Phosphate Institute, 2805 Claflin Rd., Suite 200. Manhattan, KS.
Voss, R.D. and R. Killorn. 1988. General guide for fertilizer recommendations in Iowa. AG-65 (Rev. 1988). Iowa State Univ.
1Randy Killorn is Associate Professor of Soil Fertility, Department of Agronomy, Iowa State University, Ames.
