Agricultural Management of Enhanced-Efficiency Fertilizers: North Central

Search PMN

Agricultural Management of Enhanced-Efficiency Fertilizers in the North-Central United States

K. A. Nelson, Associate Professor, Division of Plant Sciences, University of Missouri, Novelty 63460; P. C. Scharf, Associate Professor, Division of Plant Sciences, University of Missouri, Columbia 65211; L. G. Bundy, Professor, Department of Soil Science, University of Wisconsin, Madison 53706; and P. Tracy, Agronomist and Adjunct Associate Professor, MFA Inc., Columbia, MO 65201

Abstract

The north-central United States harvests over 160 million acres of corn, wheat, soybean, and grain sorghum annually as grain or silage. Enhanced-efficiency fertilizer use in this region may help increase economic returns and reduce negative environmental risks. The objective of this paper is to summarize research in the north-central US with a focus on management of new technology and fertilizer products such as polymer-coated urea (PCU) that improve nutrient-use efficiency. Preplant applications of PCU had median corn grain yields similar to anhydrous ammonia, ammonium nitrate, urea + NBPT, and grain yields greater than urea or urea ammonium nitrate. Research indicates that poorly-drained soils subject to denitrification and soils with leaching potential may benefit greatest from enhanced-efficiency fertilizers. Early applications of PCU are needed in wheat. Median wheat grain yield increase over urea was similar for PCU, urea + NBPT, and ammonium nitrate. Medium and high yield environments had the most consistent grain yield responses while variable crop performance with PCU has been related to dry conditions, no-till production systems, and fertilizer placement. Crop production systems, individual management decisions, economic returns, and availability of government cost-share will affect the utility and adoption of enhanced-efficiency fertilizers in this region.

Introduction

There are over 160 million acres of corn (Zea mays L.), wheat (Triticum aestivum L.), soybean [Glycine max (L.) Merr], and grain sorghum [Sorghum bicolor (L.) Moench] with about 40% of this land area devoted to corn (36) in the north-central United States (Fig. 1). Rainfall and available moisture throughout the region affects the crop selection, crop production potential, and cropping system. Corn is fertilized with an average of 137, 58, and 83 lb/acre of N, P₂O₅, and K₂O, respectively (15). Improved fertilizer-use efficiency in crop production benefits productivity, profitability, crop quality, and the environment. Slow- and controlled-release fertilizers as well as stabilized fertilizers have been proposed as tools to improve fertilizer-use efficiency in the north-central region by improving crop safety due to a low salt index, promoting no-till production systems, reducing leaching loss of nutrients, and reducing gas emissions (51).

Fig. 1. Research sites in the north-central US evaluating polymer-coated fertilizers.

Previous summaries or reviews have been reported on enhanced-efficiency fertilizer research in the Midwestern US including n-(n-butyl) thiophosphoric triamide (NBPT, Agrotain International, St. Louis, MO) (21), nitrapyrin (N-Serve, Dow AgroSciences, Indianapolis, IN) (40,59), and application timings (9). Other research in the region has evaluated ammonium thiosulfate as a nitrification and urease inhibitor (18), dicyandiamide (DCD) as a nitrification inhibitor (11,16), 2-ethynylpyridine, and other nitrification inhibitors (32).

Refining rates of fertilizers is the first step to improve fertilizer-use efficiency. Second, synchronizing the release or supply of nutrients with the plant’s demand should optimize fertilizer-use efficiency. This may be accomplished by adjusting application timing, or using stabilized or slow-release fertilizers. Several factors including soils and environmental conditions affect the behavior of enhanced-efficiency fertilizers or nitrification inhibitors in this region (11,32,59).

The utility of enhanced-efficiency fertilizers is affected by practical and economic considerations. The objective of this paper is to summarize research in the north-central US (Fig. 1) with a focus on the management of new technology and fertilizer products such as polymer-coated urea (PCU) that improve nutrient-use efficiency.

This review presents data in a manner that will not compromise future publication of results from individual research trials. Individual means were analyzed with box and whisker plots using PROC UNIVARIATE (SAS Institute Inc., Cary, NC). The box and whiskers represent 50% and 95 to 99% of the observations, respectively. The horizontal line in each box represents the median.

Agricultural Management

Corn. The performance and limitations of nitrification inhibitors in the Midwest was reviewed in the 1980s (40). Bundy (9) reviewed the effects of N application source, application method, and timing on corn production. Nitrification inhibitors have targeted improved N-use efficiency of NH₄⁺ forms in Iowa (10), Illinois (23), Indiana (52), Minnesota (44), and Ohio (31). An increase in grain yield and protein concentration with nitapyrin depended upon the selected hybrid and N application rate over three years of research in Indiana (52). Convenience, favorable soil conditions at the time of application, reduced equipment and labor demand, and reduced cost of fertilizer sources have favored fall applied N applications for corn in the north-central US. The addition of nitrification inhibitors is particularly useful in fine- to medium-textured soils (9). However, fertilizer applications in the fall may increase risk of leaching under certain soil and weather conditions. Best management practices based on economic returns and N loss via subsurface drainage included fall N with nitrapyrin, spring preplant and split applications of anhydrous ammonia (45). Manure-use efficiency using nitrapyrin increased soil nitrate-N concentration and yield in some occasions (46), and reduced denitrification which resulted in lower nitrous oxide emissions (12).

DCD is a nitrification inhibitor that has been used to maintain N in the NH₄⁺ form which may be beneficial in starter fertilizers. Limited research has reported increases in grain yield with DCD in this region (4,16).

Several researchers have evaluated the use of the urease inhibitor NBPT with surface applied urea or urea ammonium nitrate (UAN) solutions. Grain yield increased 4.3 bu/acre when combined with urea and 1.5 bu/acre when added to UAN (21). N use could be reduced over 70 lb/acre when NBPT was included with surface applied urea (21). Increased yields up to 57 bu/acre have been reported with NBPT; however, response to urea or UAN solution containing NBPT was highly variable, depending on climatic conditions following fertilization (35).

Controlled-release fertilizer sources. Controlled-release fertilizer sources have been limited to use primarily in horticultural crops because of high cost. Fertilizer is coated with chemicals that limit release of N from applied fertilizer and prolong supply to the plant. Recently, controlled release fertilizers were introduced for agronomic crops to avoid periods of vulnerability for N loss (6). The supply or release of fertilizer coordinated with plant demand should optimize fertilizer-use efficiency.

Several studies in the north-central US have compared PCU such as ESN (Agrium Inc., Calgary, Alberta) and Polyon (formerly Purcell Technologies, Inc., Sylacauga, AL) to other N sources that are commonly available or used in that state or region. Preplant fertilizer sources were compared with PCU over various management systems, environments, and soils. Means from research in Iowa, Kansas, Nebraska, Minnesota, Missouri, and Wisconsin were combined over locations and reported as grain yield increase or decrease compared to PCU in Figure 2. Median grain yield for anhydrous ammonia and urea + NBPT was similar to PCU. Yields ranged from -12 to 22 (n = 17) bu/acre and -16 to 9 (n = 7) bu/acre for anhydrous ammonia and urea + NBPT, respectively. PCU had median grain yields that were 1, 2, and 10 bu/acre greater than ammonium nitrate, urea, and UAN, respectively. Broadcast applications of UAN probably contributed to reduced grain yields due to increased volatilization in some of these experiments. Variability in response to enhanced-efficiency fertilizer sources has been related to rainfall timing following fertilizer application, rainfall amounts, rainfall frequency, fertilizer incorporation, and soil texture.

Fig. 2. Increase or decrease in corn grain yield of preplant applied anhydrous ammonia, ammonium nitrate, urea, urea + NBPT, and urea ammonium nitrate for locations in the north-central US (Iowa, Kansas, Nebraska, Minnesota, Missouri, and Wisconsin) compared with polymer-coated urea (ESN, Agrium). Values greater than zero had grain yields greater than polymer-coated urea while values less than zero had grain yields less than polymer-coated urea. The dashed horizontal line represents the median grain yield of the N source, boxes represent 50% of the observations, and whiskers represent 99% of the observations. The number of means (n) is reported below the N source.

Corn grain yield increased 13 bu/acre with incorporated PCU compared to incorporated urea alone in non-drained soils with adequate moisture which may be related to increased denitrification on fine- to medium-textured soils (37). The following year grain yield decreased 10 bu/acre in drought situations when PCU was compared to urea alone. Availability of polymer-coated fertilizer in dry conditions may limit crop yield. PCU increased grain yield 35 bu/acre compared to ammonium sulfate when rainfall patterns favored leaching and all of the N was applied preplant in a sandy soil with irrigation (L. G. Bundy, personal communication). Sidedress or split N applications were superior to preplant applications in instances where heavy rainfall encouraged leaching loss.

Urea management in no-till corn production systems can be difficult due to volatilization loss. Medeiros (22) evaluated urea plus urease inhibitor NBPT, urea plus NBPT + DCD, polymer- and gel-coated urea (N-urea, Muscle Shoals), incorporated urea, and knife injected urea compared with broadcast urea in central Missouri no-till corn. None of these treatments resulted in significant (95% confidence) yield increases relative to urea in either of the two years. However, in both years, substantial rain occurred shortly after treatment application and would be expected to limit volatilization losses. Despite the expectation of low volatilization loss, ammonium nitrate and anhydrous ammonia had significantly higher yields than urea and all management options for urea in the first year. In that same year, there was weak evidence (70% probability) that NBPT may have given an 8 bu/acre yield increase. Over the two years, only NBPT may have given a small economic return as a tool for enhancing urea efficiency in no-till corn.

PCU may serve as an effective fall applied N source option in some regions of the north-central US (26). Fall applications of PCU and anhydrous yielded 17 and 20 bu/acre greater than urea, respectively (Fig. 3), while fall applied PCU yielded 11 bu/acre greater than urea in northern Iowa (27). No difference among N sources was detected when urea, PCU, and anhydrous were applied preplant (Fig. 3). Deep placement of fall applied PCU increased yield 16 bu/acre more than deep banded urea, 28 bu/acre greater than broadcast applied PCU, and 8 bu/acre greater than ammonia plus N-serve in 2005 (Randall, personal communication). However, a 2- by 2-inch placement of PCU as a starter fertilizer or an in-furrow placement compared with a 2- by 2-inch placement of other fertilizer sources did not increase grain yield in Ohio (2,3). Incorporation, banding, or injection of fertilizers can reduce volatility losses and minimize the impact of high salt concentration near the seed of some fertilizer sources.

Fig. 3. Effect of fertilizer fall and preplant application timings of polymer-coated urea (ESN) and anhydrous ammonia on grain yield compared with urea alone in MN and MO. At the MN site, ESN was deep banded and anhydrous was applied with nitrapyrin. The MO site was no-till surface applied ESN and anhydrous alone. Positive values had grain yields greater than urea and negative values had grain yields less than urea. Abbreviation: NS = not significant.

Variable rate and source applications. In-season applications of fertilizer have less time for fertilizer loss. Sidedress applications of N may improve water quality (45), yield (44,54), and maximize N uptake when compared with a preplant timing. Corn is responsive to late applications of N (13,49). Diagnosis of N deficiency at V10 allowed yield responses in Minnesota (54) while in Missouri applications of N as late as V11 indicated no evidence of yield loss even under extreme N stress (49). Adjusted yield goals based on early environmental conditions have helped estimate fertilizer applications for optimal yields to corn over 6 ft tall (13). However, optimal application timing of rescue N applications was when corn was 0.9 and 1.1 ft tall for fertilizer sources applied broadcast and between-row, respectively (38). Using N rich reference strips allow farmers to adjust N application rates mid-season with precision sensors that utilize economically optimal algorithms to adjust application rates (47). Research has demonstrated that most fields have highly variable fertility needs (Fig. 4). In general, sidedress N applications nearest the time needed by the crop is recommended to reduce losses due to denitrification and leaching.

Fig. 4. Nitrogen needs vary across many production fields. Correct diagnosis and management of this variability for optimal N rates (N_opt) will enhance efficiency of N.

Variable source preplant fertilizer applications in which two or more N fertilizer sources may be applied to specific areas of a field depending on mapped areas of a field that may be vulnerable to N loss have worked well in recent research (34). Wet, low-lying areas of a field treated with anhydrous ammonia or PCU increased yield over 16% compared to urea, but these differences were not as consistent at the other landscape positions where the yield potential was less and there was increased drainage. Targeting areas of fields that are prone to fertilizer loss can help a manager improve fertilizer-use efficiency.

Wheat. Limited research in the north-central region has evaluated enhanced-efficiency fertilizers for wheat. Anhydrous ammonia with and without nitrapyrin was evaluated on soft red wither wheat in Missouri (25). Early season accumulation of NO₃^- was greater in treatments without nitrapyrin than with the nitrification inhibitor; however, few wheat acres are raised using anhydrous as an N source. Split applications of N in the fall and spring are commonly used to improve fertilizer-use efficiency in this region (8). Timely applications in the spring may be delayed due to wet soil conditions. Recently, research has focused on comparing PCU and nitrification inhibitors, timings, and placement.

In Kansas, urea as a seed placed starter caused stand loss and reduced yield when compared to PCU; however, there was no effect on seed germination when PCU was placed with the seed (56,58). No increase in grain yield was observed when gel coated urea was compared with urea alone from 2004-2006 in central Missouri (33,48). Fertilizer applications (N = 9 to 13) of PCU and urea + NBPT from November to February in Missouri had median grain yields that were 4 bu/acre greater than urea alone while ammonium nitrate median yield was 5 bu/acre higher (Fig. 5). Urea was superior over broadcast UAN when applied from November to February. Over 95% of the yields with PCU, urea + NBPT, and ammonium nitrate ranged from -2 to 22 bu/acre, -4 to 9 bu/acre, and -2 to 12 bu/acre, respectively, when compared to urea alone.

Fig. 5. Increase or decrease in wheat grain yield due to broadcast applied urea plus NBPT, polymer-coated urea (ESN), urea ammonium nitrate (UAN), or ammonium nitrate (AN) for locations in central (January and February application timings from 2005 and 2006) and northeast (November application timings from 2004 to 2006) Missouri compared with urea. Values greater than zero had grain yields greater than urea while values less than zero had grain yields less than urea. The horizontal line represents the median grain yield of the N source, boxes represent 50% of the observations, and whiskers represent 99% of the observations. The number of means was 9 to 13 with an average N rate of 73 lb/acre.

Applications of slow-release fertilizers from fall applied to February are desirable to allow timely and adequate release of N while reducing application cost. January and February PCU applications increased grain yield 15 and 10 bu/acre, respectively, when compared to urea alone (33). However, heavy rainfall on frozen ground during this time may result in off-site movement of polymer-coated fertilizer sources in transitional, temperate weather zones. No known research has evaluated movement of polymer-coated fertilizer sources under these conditions.

Proper application timing and release rate are critical for successful use of enhanced-efficiency fertilizer sources for wheat. Fertilizer release rate of PCU may be too slow when applied later than February; therefore, additional research needs to evaluate blends of slow- and fast-release N sources for applications from March or later in this region. Finally, variable rate N applications using sensor technology has been evaluated in the southern US (47) as a method to integrate proper fertilizer timing and application rate. This technology may provide additional opportunities for enhanced fertilizer use-efficiency in the north-central US.

Forages. The amount of N loss from pastures due to denitrification, ammonia volatilization, and leaching averages from 2 to 20 lb N/acre/year (30). Loss of N to the environment from the system has been estimated at 30 to 40% of the total N entering the system (14). Forage research in the north-central US using slow release, polymer coatings, and nitrification inhibitors is limited. Tall fescue (Festuca arundiacea Schreb.) forage yields for mid-March or mid-August applications in Missouri were ranked urea + NBPT ≥ urea = PCU in 2005 (24). Slow release fertilizers were expected to increase forage yields during the summer months. However, low rainfall during these months limited N utilization by tall fescue.

Soybean. Additional N for soybean during reproductive development may have merit since a reduction in N₂-fixing capacity after R5 coincides with a peak N demand for seed development (1) and should limit lodging risks associated with excessive vegetative growth. However, PCU did not affect grain yield when compared to urea alone in Kansas (57), Minnesota (50), or Iowa (5). No difference in soybean grain yield between urea and PCU was reported over four locations in Iowa (41).

Foliar fertilizer applications have reported variable and inconsistent results unless plants were experiencing deficiency (39). Tank mixture combinations with pesticides would improve practical applications of fertilizers to soybeans. Several studies have evaluated soybean response to foliar fertilizer applied at early vegetative growth stages (19,20) or during late reproductive growth stages (17,42,53). These studies evaluated mixed nitrogen, phosphorus, potassium, and sometimes sulfur fertilizer sources (17,19,42,43) and several studies were conducted under optimal soil test fertility levels (19,20,42). Most of the reported responses to foliar fertilizer applications did not justify the application expense (7,19,42,43). In-season N applications are generally not recommended for dryland soybean production in the north-central US except as needed for pest management additives.

Grain sorghum. Limited research has evaluated grain sorghum response in the north-central US. Seedling injury with PCU was observed when N rates exceeded 24 lb/acre (58). In Kansas, grain yields were similar among ammonium nitrate, urea, and PCU in 2000 and 2002 (28,29). Additional research over several environments is needed to evaluate crop safety to new technology such as PCU when placed in close proximity of the seed.

Utility of Enhanced-Efficiency Fertilizers

The utilization of enhanced-efficiency fertilizers is affected by profitablity, availability, handling, and storage. Cost of manufacturing coated or encapsulated controlled release fertilizers is higher than conventional fertilizers. The cost of enhanced-efficiency fertilizers may be limiting in some instances; however, several government programs that promote conservation and nutrient management may offset these costs through incentive programs.

Several government cost-share programs such as EQIP (Environmental Quality Incentive Program) and CSP (Conservation Security Program) may provide incentive payments to farmers for using N nutrient management options such as nitrification inhibitors or N application technology that enhance fertilizer efficiency. Farmers may receive cost-share of $7 to 10/acre for various N management options including use of NBPT, PCU, or nitrapyrin and pay a Technical Service Provider (TSP) fees for developing a basic nutrient management plan in states like Missouri (Glenn Davis, personal communication) and Wisconsin (L. G. Bundy, personal communication). Cost-share programs for sensor based technology to manage N on corn was $20/acre in Missouri in 2007. State programs such as the Wisconsin Dep. of Ag., Trade and Consumer Protection paid $7/acre/year for nutrient management planning which required an updated plan each year for four years. However, Wisconsin Dep. of Natural Resources has a limited amount of money for cost sharing in their priority watershed program which is about $6/acre for nutrient management planning based on availability. Available programs may vary depending on the state.

Distributors and retailers may utilize enhanced-efficiency fertilizer products such as PCU at some locations due to limited supply of ammonium nitrate and regulation concerns. Similarly, methamphetamine regulations, application safety, and storage issues are affecting the desire to handle anhydrous ammonia. Polymer-coated fertilizers have several beneficial attributes such as high quality and consistent analysis, an elimination of a fertilizer impregnation stage, utility in cost-share programs, and decreased need, time and effort for sidedress N. However, concerns including limited bin storage space, crop response in dry and no-till conditions, and cost have affected wide-spread adoption of this technology. Other crop management uses such as integrated pest and fertility management systems, enhanced grain quality, and organic fertilizer sources need to be explored when adopting new crop production technology such as slow-release fertilizer sources.

Summary

Research in the north-central region has indicated that PCU sources may have minimal effects on wheat germination but may reduce stands of grain sorghum when in contact with the seed. PCU may be an effective fertilizer source for wheat when compared to urea and applied from November to February. Later application timings may need a faster-release N source. Limited research in the north-central region has evaluated performance of enhanced-efficiency fertilizers on forages and pastures. Additional research is needed for forage and pasture systems due to a high risk of N volatilization.

No-till corn production systems have been shown to benefit from enhanced-efficiency fertilizer use including PCU, NBPT, and nitrapyrin as well as split application timings, variable rate applications using sensor technology, and variable source applications to enhance fertilizer-use efficiency. However, reduced fertilizer-use rates of enhanced-efficiency fertilizers in areas prone to excessive denitrification have not been successful in providing consistent crop performance in fine- to medium-textured soils. Placement of fertilizer such as a deep band or knife-injection in no-till corn may also improve fertilizer-use efficiency. Differences in crop performance among enhanced-efficiency fertilizer sources were less pronounced when fertilizers were incorporated. In sandy soils with high leaching potential, slow-release fertilizers look promising. Limited research has evaluated fall applied polymer-coated research, but this application timing looks promising as well especially when deep banded in some environments. Dry conditions may limit fertilizer availability of polymer-coated fertilizer sources and has resulted in reduced yields in some instances. Desirable and optimal application timing, possible reduced rate applications, cost-effectiveness, equipment availability, handling, storage, and convenience need to be considered when utilizing this technology.

Participation in incentive programs may offset adoption costs of new technology that improves fertilizer-use efficiency. Performance of enhanced-efficiency fertilizers during dry conditions through the growing season and off-site movement when soils are frozen or there is low reside cover in no-till conditions needs to be considered.

A host of interacting factors including fertilizer source (urea, ammonia, nitrate), permeability of fertilizer coating as related to thickness and uniformity for polymer-coated fertilizers (60), climate (rainfall intensity and frequency, temperature) (55), and soil (type, texture, pH, moisture content, organic matter, and CEC) (11,22,32) has been studied to improve fertilizer-use efficiency. Continued research focused on understanding these effects of interacting factors so management decisions can be modified accordingly will improve the success of enhanced-efficiency fertilizer sources for increasing yields and minimizing environmental impacts. The ultimate goals of enhanced efficiency are synchronizing the release or supply of nutrients with the plant’s demand and reducing N loss. Technology that improves fertilizer-use efficiency needs to be thoroughly evaluated with the goal of targeting uses with long-term uniform response. Agriculture management decisions should consider use of enhanced-efficiency fertilizers, variable rate, source, and polymer-coated fertilizers especially on soils prone to leaching losses and a high denitrification potential.

Literature Cited

1. Afza, R., Hardarson, G., Zapata, F., and Danso, S. K. A. 1987. Effects of delayed soil and foliar N fertilization on yield and N2 fixation of soybean. Plant Soil 97:361-368.

2. Baker, D. 2000. Starter fertilizer comparison for corn. Online. Agronomic Crops Team On-Farm Research Projects 2000. Ext. Spec. Circ. 179-01, Ohio State Univ., Greenville, OH.

3. Baker, D. 1999. Polymer-coated fertilizer comparisons. Agronomic Crops Team On-Farm Research Projects 1999. Ext. Spec. Circ. 176-00, Ohio State Univ., Greenville, OH.

4. Barber, K. L., Maddux, L. D., Kissel, D. E., Pierzynski, G. M., and Bock, B. R. 1992. Corn responses to ammonium- and nitrate-nitrogen fertilization. Soil Sci. Soc. Am. J. 56:1166-1171.

5. Barker, D. W., and Sawyer, J. E. 2005. Nitrogen application to soybean at early reproductive development. Agron. J. 97:615-619.

6. Blaylock, A. 2005. What fertilizer materials are available to improve nitrogen efficiency? CD-ROM. Agronomy Abstracts. Am. Soc. of Agron., Madison, WI.

7. Boote, K. J., Gallaher, R. N., Robertson, W. K., Hinson, K., and Hammond, L. C. 1978. Effect of foliar fertilization on photosynthesis, leaf nutrition, and yield of soybeans. Agron. J. 70:787-791.

8. Buchholz, D. D., and Schaffer, J. A. 1990. Nitrogen rates and timing for winter wheat in Missouri. Missouri Soil Fertility Res. Update Rep. 90-01, Univ. of Missouri, Columbia, MO.

9. Bundy, L. G. 1986. Timing nitrogen applications to maximize fertilizer efficiency and crop response in conventional corn production. J. Fertilizer Issues 3:99-106.

10. Cerrato, M. E., and Blackmer, A. M. 1990. Effects of nitrapyrin on corn yields and recovery of ammonium-N at 18 site-years in Iowa. J. Prod. Agric. 3:513-521.

11. Clay, D. E., Malzer, G. L., and Anderson, J. L. 1990. Ammonia volatilization from urea as influenced by soil temperature, soil water content, and nitrification and hydrolysis inhibitors. Soil Sci. Soc. Am. J. 54:263-266

12. Comfort, S. D., Kelling, K. A., Keeney, D. R., and Converse, J. C. 1990. Nitrous oxide production from injected liquid dairy manure. Soil Sci. Soc. Am. J. 54:421-427.

13. Derby, N. E., Casey, F. X. M., Knighton, R. E., and Steele, D. D. 2004. Midseason nitrogen fertility management for corn based on weather and yield prediction. Agron. J. 96:494-501.

14. Edmeades, D. C. 2004. Nitrification and urease inhibitors. A review of the National and International literature on their effects on nitrate leaching, greenhouse gas emissions and ammonia volatilization from temperate legume-based pastoral system. Environ. Waikato Tech. Rep. 2004/22.

15. ERS-USDA. 2003. Agricultural chemicals and production technology: Nutrient management. Online. ERS-USDA, Washington, DC.

16. Francis, D. D., Doran, J. W., and Lohry, R. D. 1993. Immobilization and uptake of nitrogen applied to corn as starter fertilizer. Soil Sci. Am. J. 57:1023-1026.

17. Garcia, R. L., and Hanway, J. J. 1976. Foliar fertilization of soybeans during the seed-filling period. Agron. J. 68:653-657.

18. Goos, R. J. 1985. Identification of ammonium thiosulfate as a nitrification and urease inhibitor. Soil Sci. Soc. Am. J. 49:232-235.

19. Haq, M. U., and Mallarino, A. P. 1998. Foliar fertilization of soybean at early vegetative stages. Agron. J. 90:763-769.

20. Haq, M. U., and Mallarino, A. P. 2000. Soybean yield and nutrient composition as affected by early season foliar fertilization. Agron. J. 92:16-24.

21. Hendrickson, L. L. 1992. Corn yield response to the urease inhibitor NBPT: Five-year summary. J. Prod. Agric. 5:131-137.

22. Hendrickson, L. L., and Douglass, E. A. 1993. Metabolism of the urease inhibitor N-(n-butyl)thiophosphoric trimaide (NBPT) in soils. Soil Biol. Biochem. 25:1613-1618.

23. Hughes, T. D., and Welch, L. F. 1970. 2-Cholor-(trichloromethyl) pyridine as a nitrification inhibitor for anhydrous ammonia applied in different seasons. Agron. J. 62:821-824.

24. Kallenbach, R. L., and Massie, M. D. 2006. Alternative nitrogen sources on tall fescue pastures to reduce costs and improve forage quality in Missouri. Univ. of Missouri, Southwest Center Ruminations 12:2-3.

25. Kidwaro, F. M., and Kephart, K. D. 1998. Retention of nitrogen from stabilized anhydrous ammonia in the soil profile during winter wheat production in Missouri. Comm. Soil Sci. Plant Anal. 29:481-499.

26. Killorn, R., Gonzalez, M., Moore, J., and Haden, D. 2004. Effect of a slow-release N fertilizer on corn yield. Online. Annual Progress Report, N. Res. and Demonstration Farm Publ. no. ISRF04-22, Iowa State Univ., Ames, IA.

27. Killorn, R., Queseda, J. P., and Gonzalez, M. 2004. Product evaluations for improving N management. Online. 2004 Wisconsin Fertilizer, Aglime, & Pest Manage. Conf. Proc., UW Extension, Univ. of Wsiconsin, Madison, WI.

28. Lamond, R. E., Whitney, D. A., Pierzynski, G. M., and Godsey, C. B. 2003. Effects of nitrogen management and tillage on grain sorghum. Pages 82-83 in: Kansas Fertilizer Res. Rep. of Prog. no. 921. Kansas State Univ. Res. and Ext., Manhattan, KS.

29. Lamond, R. E., Whitney, D. A., Pierzynski, G. M., and Olsen, C. J. 2000. Effects of nitrogen management and tillage on grain sorghum. Pages 83-84 in: Kansas Fertilizer Res. Rep. of Prog. no. 868. Kansas State Univ. Res. and Ext., Manhattan, KS.

30. Ledgard, S. F. 2001. Nitrogen cycling in low input legume-based agriculture with emphasis on legume/grass pasutures. Plant Soil. 228:43-59.

31. Lueking, M. A., Johnson, J. W., and Himes, F. L. 1983. Effects of increasing the rates of potassium and nitrapyrin on nitrogen uptake by corn. Agron. J. 75:247-249

32. McCarty, G. W., and Bremner, J. M. 1990. Evaluation of 3-methylpyrazole-1-carboxamide as a soil nitrification inhibitor. Biol. Fertil. Soils 9:252-256.

33. Medeiros, J. A. S. 2006. Management alternatives for urea use in corn and wheat production. MS thesis, Univ. of Missouri, Columbia, MO.

34. Motavalli, P., Nelson, K., Kitchen, N., Anderson, S., Scharf, P., and Tracy, P. 2005. Variable source N fertilizer applications to optimize crop N use efficiency. CD-ROM. Agronomy Abstracts. Am. Soc. of Agron., Madison, WI.

35. Murphy, T. L., and Ferguson, R. B. 1997. Ridge-till corn and urea hydrolysis response to NCPT. J. Prod. Agric. 10:271-282.

36. NASS. 2006. National Agriculture Statistics Service. USDA, Washington, DC.

37. Nelson, K. A., Jones., M., and Smoot, R. 2006. MU dainage and subirrigation (MUDS) research update. Online. Missouri Agric. Exp. Station, Univ. of Missouri, Columbia, MO.

38. Nelson, K. A., Scharf, P., Stevens, W., and Burdick, B. A. 2006. Rescue nitrogen applications for corn. CD-ROM. Agronomy Abstracts. Am. Soc. of Agron., Madison, WI.

39. Nelson, K., Motavalli, P., Jones, M., and Smoot, R. 2005. Effect of slow-release N fertilizer rate and timing on wheat grain yield compared to other N sources. Greenley Memorial Research Center Field Day Report. pp. 78.

40. Nelson, D. W., and Huber, D. M. 1980. Performance of nitrification inhibitors in the Midwest (east). Pages 75-88 in: Nitrification Inhibitors: Potentials and Limitations. J. J. Meisinger, M. Stelly, G. W. Randall, M. L. Vitosh, and D. M. Kral, eds. ASA Spec. Publ. 38. ASA and SSSA, Madison, WI.

41. Ostermeier, G., Van De Woestyne, B., and Blackmer, A. 2004. Evaluation of polymer-coated urea as a fertilizer for corn. Online. ISA 2006 On-Farm Network Con. Poster Summaries. Iowa Soybean Assoc., Urbandale, IA.

42. Parker, M. B., and Boswell, F. C. 1980. Foliage injury, nutrient intake, and yield of soybean as influenced by foliar fertilization. Agron. J. 72:110-113.

43. Poole, W. D., Randall, G. W., and Ham, G. E. 1983. Foliar fertilization of soybeans. I. Effect of fertilizer sources, rates, and frequency of application. Agron. J. 75:195-200.

44. Randall, G. W., Vetsch, J. A., and Huffman, J. R. 2003. Corn production on a subsurface-drained mollisol as affected by time of nitrogen application and nitrapyrin. Agron. J. 95:1213-1219.

45. Randall, G. W., Vetsch, J. A., and Huffman, J. R. 2003. Nitrate losses in subsurface drainage from a corn-soybean rotation as affected by time of nitrogen application and use of nitrapyrin. J. Environ. Qual. 32:1764-1772.

46. Randall, G. W., Schmitt, M. A., and Schmidt, J. P. 1999. Corn production as affected by time and rate of manure application and nitrapyrin. J. Prod. Agric. 12:317-323.

47. Raun, W. R., and Schepers, J. 2005. Symposium. What new equipment is available to improve N-use efficiency? CD-ROM. Agronomy Abstracts. Am. Soc. of Agron., Madison, WI.

48. Scharf, P., Mueller, L., and Medeiros, J. 2005. Making urea work in no-till. Pages 110-113 in: Missouri Soil Fertility and Fertilizers Res. Update 2004. Agron. Misc. Publ. #05-01. Univ. of Missouri, Columbia, MO.

49. Scharf, P. C., Wiebold, W. J., and Lory, J. A. 2002. Corn yield response to nitrogen fertilizer timing and deficiency level. Agron. J. 94:435-441.

50. Schmitt, M. A., Lamb, J. A., Randall, G. W., Orf, J. H., and Rehm, G. W. 2001. In-season fertilizer nitrogen applications for soybean in Minnesota. Agron. J. 93:983-988.

51. Trenkel, M. E. 1997. Controlled-release and stabilized fertilizers in agriculture. Int'l. Fertilizer Industry Assoc. (IFA), Paris, France.

52. Tsai, C. Y., Dweikat, I., Huber, D. M., and Warren, H. L. 1992. Interrelationship of nitrogen nutrition with maize (Zea mays) grain yield, nitrogen use efficiency and grain quality. J. Sci. Food Agric. 58:1-8.

53. Vasilas, B. L., Legg, J. O., and Wolf, D. C. 1980. Foliar fertilization of soybeans: absorption and translocation of ¹⁵N-labeled urea. Agron. J. 72:271-275.

54. Vetsch, J. A., and Randall, G. W. 2004. Corn production as affected by nitrogen application timing and tillage. Agron. J. 96:502-509.

55. Welch, L. F., Mulvaney, D. L., Oldham, M. G., Boone, L. V., and Pendleton, J. W. 1971. Corn yields with fall, spring, and sidedress nitrogen. Agron. J. 63:119-123.

56. Whitney, D. A., and Gordon, W. B. 1998. Evaluation of polymer-coated urea as a starter nitrogen source for wheat. Pages 11-12 in: Kansas Fertilizer Res. Rep. of Prog. no. 829. Kansas State Univ. Res. and Ext., Manhattan, KS.

57. Whitney, D. A., and Gordon, W. B. 1998. Nitrogen source, rate, and application time for soybean. Pages 120-121 in: Kansas Fertilizer Res. Rep. of Prog. no. 829. Kansas State Univ. Res. and Ext., Manhattan, KS.

58. Whitney, D. A., Gordon, W. B., and Schiegel, A. J. 2000. Controlled-release nitrogen fertilizer in starter for sorghum production. Pages 89-91 in: Kansas Fertilizer Res. Rep. of Prog. no. 868. Kansas State Univ. Res. and Ext., Manhattan, KS.

59. Wolt, J. D. 2000. Nitrapyrin behavior in soils and environmental considerations. J. Environ. Qual. 29:367-379.

60. Zhang, M., Nyborg, M., and Ryan, J. T. 1994. Determining permeability of coatings of polymer-coated urea. Nutrient Cycling Agroecosys. 38:47-51.