Illinois Fertilizer
Conference Proceedings
January 28-30, 1991
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E. C. VARSA1
The 1990s will bring continued changes to agriculture in the U.S. as the number of persons directly involved in the production of food, fiber and feed shrinks further. Concerns of the public over the safety of our food supplies and the quality of our water resources will continue to challenge all of us. Similarly, there are expressed concerns over the long-term sustainability of our soil resource, so productivity is maintained for future generations. Underlying many of the decisions regarding practices that will be recommended and used in agricultural production will be the availability and cost of energy, especially that of fossil fuels.
The likely changes in crop production in the 19909 include: larger farming units and increased size of farm equipment; further reductions in tillage and increased use of no-till; leveling-off of the amounts of fertilizers used; more "prescription" fertilization, such as the fine-tuning of placement, timing, and the match of varieties with nutrient-specific environments; innovative weed and feed methods; and increased reliance on soil testing and plant analysis.
Use of best management practices (BMPs) will be a necessity for producers to be economically successful in the decade ahead. Optimizing nutrient use by crops and reducing losses will be essential. Similarly, increased amounts of nutrients, such as nitrates in our water supplies, will be disallowed. The remaining paper is a summary of fertilizer management strategies that have proven to be successful in the past or will take on increased importance in the future.
The soil test remains the foundation of a successful crop production program. A recent survey by the Cooperative Extension Service found that less than 10 percent of the fields sampled for fertility status were collected at the desired intensity, which is about four acres per soil sample. Mapping the fertility status of a field will require even more intense sampling to take advantage of computerized spreader equipment that is capable of changing nutrient rates on the go. Such precision fertilization will allow soil productivity and profitability to be optimized as soil types change in the field.
Nitrate soil tests are currently being evaluated in several Midwestern states, including Illinois. Previously, such tests were thought to be too unreliable because of the transitory nature of nitrates in soil. However, because of the concerns over excessive fertilizer nitrogen use and the impact of residual nitrates on groundwater quality, a renewed look at soil nitrate tests to increase the accuracy of the nitrogen fertilizer recommendation, especially for corn, is ongoing. Initial evaluations of the nitrate test in Illinois suggests that it has potential, especially on farms where manure applications are a part of the soil fertility program.
Plant analyses will assume increased importance in the assessment of fertility programs as micronutrients and sulfur become more limiting in crop production. The reliability of most soil tests for micronutrients and sulfur are presently rather poor. Therefore, plant analyses will assume greater importance in diagnosing and correcting such problems.
It is essential that application rates of fertilizers, especially nitrogen, be applied in accordance with realistic yield goals, not hoped-for yield goals. Adding excessive nitrogen beyond crop utilization not only is wasteful and inefficient but also contributes significantly to residual nitrates that potentially could become a pollutant to groundwater.
Livestock wastes are a significant contributor to nutrients, especially nitrogen and phosphorus. Frequently, this source of nutrients is overlooked in a farm fertility plan. Confinement-generated wastes can be extremely variable in composition because of varying water contents. In such instances a manure analysis would be most useful in accurately crediting the nutrients contained in such wastes.
Residues of legume crops should be given a credit for the nitrogen they contain. As a rule of thumb soybean residues after harvest contain 30 to 40 pounds of nitrogen per acre and this amount should be considered in the nitrogen fertilizer recommendation for a succeeding crop of corn. Nitrogen credits for alfalfa or clover are more difficult to make because of stand variability. However, estimates usually range from 50 to 100 pounds of nitrogen per acre credit for a succeeding corn crop.
For phosphorus and potassium, the rule of thumb to follow is "fertilize the volume of soil where roots will tend to become most concentrated." For reduced tillage, this usually means that a portion of the soil volume should be fertilized, rather than complete mixing within the soil volume. For no-till, research suggests that broadcasting the fertilizer on the soil surface is more efficient with respect to yield than concentrating the phosphorus and potassium in narrow bands near the row. For ridge-till, there seems to be no clear-cut advantage for a specific site placement.
For nitrogen, apply the fertilizer immediately in advance of the time of greatest crop uptake and use. This usually implies that sidedressing is the preferred timing since chances for nitrogen losses are lower compared to preplant or an earlier time of nitrogen application. Split applications of nitrogen offer similar advantages. Shown in Figure 1 is the response of corn to increasing rates of ammonium nitrate applied at three different times. Fall application was the least effective, spring application was intermediate and sidedress was, the most effective. Only at the highest rate of nitrogen was the spring application as effective as sidedressing in the production of corn.
For no-till corn, surface application of urea or urea-containing fertilizers is not recommended because of the high potential for ammonia volatilization losses. In nearly all research conducted in the Midwest on no-till corn, results suggest that nitrogen should be injected below the mulch-covered soil surface to minimize nitrogen losses. The data below shows the effect of nitrogen source and placement on ear leaf nitrogen composition and no-till grain yields at the SIUC Belleville Research Center in 1989. Clearly, injection was superior to dribble placement of the nitrogen source even in the presence of a urease inhibitor. Granular urea that was broadcast resulted in the lowest leaf nitrogen and grain yields among the comparisons reported.
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Fertilizer Source and Placement
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Ear Leaf Nitrogen |
Grain Yield |
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%
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bu/ac
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UAN (dribble)
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2.23 c
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119 c
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UAN+NBPT (dribble)
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2.35 b
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125 b
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UAN (inject)
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2.57 a
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137 a
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Granular Urea (broadcast)
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1.96 d
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97 d
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Nitrification inhibitors are management tools available to producers that can enhance the stability of ammonium nitrogen fertilizers against loss. Benefits in terms of a yield increase from the use of a nitrogen stabilizer occur most often when the fertilizer is subject to loss conditions such as leaching or denitrification. In the absence of loss conditions, no yield benefit usually results from nitrification inhibitor use. In the years ahead, enhanced emphasis will be placed on the role of inhibitors in reducing leaching losses of nitrate because such products decrease the rate at which nitrates form in soil.
Make certain that products added for the specific purpose of increasing crop yields or improving the soil have a scientific basis for their activity. Without a recognized mode of action, such products should not be recommended for use.
In conclusion, the best nutrient management practices for the 1990s will be those that are agronomically sound, economically feasible and sociologically acceptable. Research holds the keys to future development of "new" and "improved" management practices.
