Nitrogen Extenders and Additives for Field Crops

Nitrogen (N) management for crop production continues to be difficult due to transformations of N fertilizers when applied to soil and the uncertainties of weather (Cabrera et al., 2008).

Nitrogen fertilizer in the form of urea is subject to ammonia volatilization through the activity of the urease enzyme found ubiquitously in soil (Kissel et al., 2008). Nitrate fertilizer is subject to leaching (Randall et al., 2008) or denitrification (Coyne, 2008), depending on the water content of the soil and water movement through the soil. Ammonium forms of N can be fixed (Kissel et al., 2008) or transformed to nitrate through the activities of specific soil bacteria (Norton, 2008). Because of these and other processes, N use efficiency is low.

Nitrogen often is applied to crops in the north-central region of the U.S. before planting. During the first four to six weeks after planting, corn requires only about 5 percent of the N applied. The following two to four weeks of growth require a large proportion of the total seasonal N requirement because the rate of N uptake dramatically increases past the V6 growth stage (Bender et al., 2013).

In winter wheat, low levels of plant-available N are required for overwintering. However, once wheat breaks dormancy, a large proportion of N uptake occurs during the next few weeks. In spring wheat, a small of amount of N is required to establish the crop during the first two to four weeks after seeding; however, most of the remaining N is required during the next 30 days.

To address some of the delayed N requirement issues of winter wheat, much of the crop is top-dressed in the spring. In corn, some growers use side-dress applications; however, spring preplant application is most common, with fall application preferred by growers in some Northern and Midwestern states. In spring wheat in the northern Plains, some post-N applications are made.

Due to the variability in rainfall during growing seasons, postemergence N applications to spring wheat as a primary source of the major plant N requirement are discouraged (Franzen, 2009), especially in northern areas with compressed growing seasons, except under irrigation.

To increase N use efficiency, by increased yields or decreased N rates, a number of products have been developed to delay the N transformation process so that the period of time in which the N source remains available for uptake is close to the time of maximum N uptake. These products can be classified into the following groups: nitrification inhibitors, urease inhibitors and slow-release N products.

Nitrification is the process of oxidizing ammonium (NH4+) to nitrate (NO3-). The process usually proceeds in two steps:

Step 1:
(Nitrosomonas sp aerobic bacteria):
NH4+ + 1½ O2  NO2- + H2O + 2H+

Step 2:
(Nitrobacter sp aerobic bacteria):
NO2- + ½ O2  NO3-1

The nitrification process proceeds at temperatures above 32 F, with rate maximized near 75 F. Nitrification is an aerobic process and will not proceed in saturated soils, but it is maximized under moist, aerated soil conditions. Nitrification also is maximized between soil pH 6.5 and 8.

Specific nitrification inhibitors are required because inhibition of the second nitrification step should not occur. Otherwise, nitrite which is toxic to plants, could accumulate. Nitrification control is generally through inhibition of a specific enzyme pathway in bacteria involved in step one of nitrification.

Photo Credit:

PubChem

Figure 8. Urea 3-D image Gray is C, red is O, cyan are N, and light blue are H

Urea is a common fertilizer used by farmers in the north-central states. The chemical formula for urea is CO(NH2)2. A 3-D  image of urea is displayed in Figure 8.

When applied to the soil, the enzyme urease splits the urea into its component parts of ammonia (NH3) and carbon dioxide (CO2) through the following reaction (called hydrolysis), with a net increase in pH:

CO(NH2)2 + H2O - NH3 - + CO2 

The urease enzyme is a three-dimensional molecule made up of amino acid chains with a Ni ion as a part of the structure. The urease enzyme has a unique configuration that matches the triangular shape and size of the urea molecule (Figure 9).

Urease is utilized in plants and microorganisms, particularly bacteria, to manage the movement of ammonia through the plant and release the ammonia where it is required. When the plant or microorganism decays, the urease enzyme is very resistant to decay and continues to function after whatever organism that produced it is dead and largely decayed.

Photo Credit:

PubChem

Figure 9. Urease enzyme, oriented to show active site.

When urease comes into contact with a urea molecule, the urea is drawn to the active site and transformed into free ammonia  and CO2. If the ammonia released is at or near the surface, NH3 volatility to the air is likely.

Soils with greater residue at the soil surface have much greater concentrations of urease than those with little residue at the surface. Soils with pH greater than 7 have much greater ammonia volatilization rates than those in acid soils, with the main reason being a higher percentage of NH4 as NH3 as pH increases. Therefore, ammonia volatilization potential is greater with these conditions: surface-applied urea, high surface residue, high soil pH, low cation-capacity soils, moist to drying soils, high temperatures and no rainfall after application.

The compound that most consistently has decreased urea volatilization when mixed with urea or urea-ammonium nitrate solutions is NBPT (N-(n-butyl) thiophosphoric acid triamide) (Figure 7). NBPT originally was marketed as Agrotain (Agrotain International LLC), with the commonly used product today branded Agrotain Ultra (Koch Agronomic Services, Wichita, Kan.). The NBPT active ingredient also has been made available to other distributors and manufacturers, so NBPT products today have many brand names.

Table 4. Effect of seed-placed urea with and without Agrotain on stand density and grain yield of barley on a fine sandy loam soil, 1994-96. (from Grant, 2004)

The mechanism for NBPT is to lock onto the urease enzyme-binding sites, preventing the enzyme from reacting to the urea (Manunza et al., 1999). Agrotain has at least two possible uses in crop production: One is to protect against seed injury for growers, especially in the northern Plains, who apply urea with small-grain seed at planting.

The use of Agrotain has increased the rate of urea that can be applied safely with small-grain seed in some studies (Table 4), although the practice is rarely recommended due to the uncertainty of the length of time required for germination in cooler soils. The other use is to delay ammonia volatilization from surface applied, or near-surface applied urea, for about 10 days (Table 5).

Table 5. Cumulative ammonia volatilization losses for urea, ammonium sulfate and urea + NBPT from a Dewitt silt loam soil during a 15-day laboratory incubation at 25 C. (Norman data, University of Arkansas, Fayetteville, from Franzen et al., 2011)

†NBPT= N–(n-butyl) thiophosphoric triamide ‡LSD to compare means among N sources within the same sampling time.
§LSD to compare means among sampling times with the same N source.

Photo Credit:

PubChem

Figure 10. NBPT (N-(n-butyl) thiophosphoric acid triamide) structure. NPPT (n-propyl) also has the basic active-site inhibiting triangular structure, but the tail of the molecule has one more C group than NBPT.

A more recently introduced product is Limus (BASF Corp., Research Triangle Park, N.C.,). Limus contains NBPT and NPPT (N-(n-propyl) thiophosphoric acid triamide) (Figure 10). The same mode of action is utilized by NPPT to inhibit urease as that of NBPT.

Recent research from North Dakota has shown that Limus is at least as effective as an inhibitor as Agrotain Ultra and technical grade NBPT alone (Figure 11).

Agrotain decreased the rate of ammonia volatilization from urea applied to the surface as dry urea or urea-ammonium nitrate solutions (Brouder, 1996, Table 6; Varsa et al., 1999, Table 7), resulting in yield increases when the fertilizer was treated with NBPT. Ammonia volatilization losses from urea at Brandon, Manitoba, decreased from 40 milligrams (mg) to 2 mg and from 88 to 12 mg with Agrotain in two separate studies for a seven-day period after application (Grant, 2004).

Photo Credit:

Franzen 2014, unpublished data

Figure 11. Comparison of cumulative ammonia loss during an eight-day period near Gardner, N.D., on a conventionally tilled, low-residue, neutral-pH soil, compared with 14 days near Valley City, N.D., on a high-residue, long-term, no-till field with slightly acid surface pH. Urea N rate applied was 100 pounds N per acre.

In a Kansas study (Weber and Mengel, 2009), urea was applied in three site-years to the soil surface after corn emergence using a number of nitrogen-extending additives, including Agrotain. The Agrotain treatment was superior to urea alone by 25 bushels per acre in one of the three site-years. The two locations that received significant rainfall immediately following applications did not receive a yield benefit from the Agrotain treatment.

Table 6. Mean corn yield from Purdue Agronomy Farm, SEPAC, Pinney, Purdue and Kosciusko locations, with urea and UAN alone and treated with NBPT. (from Brouder, 1996, citing work by Phillips, Mengel and Walker, 1989, unpublished work, Purdue University)

In sorghum, urea + Agrotain and urea + SuperU were 11 and 12 bushels per acre, respectively, greater in yield than with urea broadcast alone (Weber et al., 2009a). At two drier locations, no yield differences occurred between urea + Agrotain and urea alone.

Table 7. No-till corn yield as affected by N fertilizer sources, Agrotain and placement in Illinois. (from Varsa et al., 1999)

A 14-year study in southern Illinois (Ebelhar et al., 2010) showed a 3-bushel corn yield advantage of urea + Agrotain, compared with urea broadcast in conventional till surface and incorporated during 12 years of treatments. In no-till, urea + Agrotain held an 11 bushel/acre advantage over urea surface applied during four years of treatments.

In Kentucky, 50 pounds of N/acre was applied preplant to all corn plots (Schwab and Murdock, 2009). Side-dress applications of urea and UAN with several additives or formulations were applied to the soil surface at the six-leaf stage. Higher yields were achieved with urea + Agrotain and SuperU than with urea alone. Higher yields were achieved with UAN + Agrotain and UAN + Agrotain Plus (combination of NBPT and DCD formulated for use with UAN) (Table 8) than with UAN alone.

Table 8. Yield for side-dressed no-till corn in Hardin County, Ky. (from Schwab and Murdock, 2009)

*Numbers followed by the same letter are not significantly different (5%)

Also notable: The ammonium nitrate treatment was the highest yielding treatment, indicating that some N loss was realized in the Agrotain treatments. The study also demonstrates that grower practices that limit N loss, particularly placement of urea or UAN below the soil surface may be required for maximum yield, even though urease inhibitors are effective in limiting but not eliminating ammonia volatilization.

Ammonium thiosulfate (ATS) and several additional commercial thiosulfates have nitrification- (Goos, 1985; Janzen and Bettany, 1986) and soil urease-inhibiting properties (Goos, 1985). Although ATS has measureable nitrification inhibition properties in some experiments, in others it has very little (Figure 4).

Thiosulfate readily breaks down rapidly when soils are warm. In a laboratory study at 15 C, ATS essentially was mineralized in about a week. Under cooler temperatures, however, significant thiosulfate remained after two weeks in two of three soils, with mineralization complete in all soils by week three.

When thiosulfate was placed in a band with aqua ammonia in the fall in North Dakota (Oct. 3, 1996), thiosulfate resulted in similar spring (May 12, 1997) ammonium and nitrate levels as aqua ammonia treated with nitrapyrin (Goos and Johnson, 1999). Spring wheat yields of aqua ammonia treated with thiosulfate and nitrapyrin were similar, and both were greater than aqua ammonia alone.

In the process of identification of thiosulfates as nitrification and soil urease inhibitors, researchers noted that the compounds would not be expected to perform as well as some other alternative nitrification and urease inhibitors due to the shorter decomposition period for ATS, compared with nitrapyrin (Goos, 1985).

Photo Credit:

Goos, 2013

Figure 12. Effect of treatment of UAN with ATS and other through time under greenhouse conditions (Goos, 2013). amendments on ammonia volatilization through time under
greenhouse conditions.

When ATS is put into solution with purified urease, no inhibition occurs. However, ATS reacts with Mn-oxides and forms Mn2+

and tetrathionate (S4O6, 2-). Tetrathionate and Mn2+ inhibit urease activity. The result of the oxidation reaction is some inhibition of urease with ATS, especially when the UAN is banded. The use of ATS sometimes has been implicated in the increase of nitrites in some experiments.

The second step of nitrification (nitrite to nitrate) is usually instantaneous due to the presence of Mn-oxides (Bartlett et al., 2008). The reactions of ATS with Mn-oxides may be the reason for its nitrification inhibition and the increase of nitrite. Adding ATS to UAN has been shown to reduce ammonia volatilization from surface-applied UAN under greenhouse (Goos, 2013, Figure 12) and field (Grant et al., 1996, Figure 13) conditions.

Goos and Johnson (1992) compared the nitrification rates of banded UAN amended with emulsified nitrapyrin (N-Serve 24E), DCD, ATS and ATS+DCD at three field sites. The results are summarized in Figure 14. The nitrapyrin was most effective at slowing nitrification and ATS was least effective.

Photo Credit:

Grant et al., 1996

Figure 13. Ammonia volatilization from UAN and UAN with ATS and NBPT through time under field conditions.

One study was unable to duplicate urease inhibition results, but it used different methods than originally presented at rates of

ATS from 3.3 to 33 times the rates of Goos, 1985 (McCarty et al., 1990). Thiosulfate activity is regulated by its concentration (effective at S rates of 25 mg kg-1, Goos and Johnson, 2001).

Janzen and Bettany (1986) expressed cautions on high rates of banded ATS (in excess of 100 parts per million, or ppm) due to nitrite accumulation from ATS inhibition of not only the ammonium to nitrite process but the nitrite to nitrate process. The rate used by Goos (1985) was about 43 ppm if expressed as a band with a radius of 2 inches, which did not accumulate nitrite at the use rate in the Janzen/Bettany (1986) study.

The use of thiosulfate was re-examined in Kansas research. Application in the spring of a 5 and 10 percent calcium thiosulfate by volume solution with UAN had similar yield as urea broadcast in no-till (Tucker and Mengel, 2007).

Photo Credit:

Goos and Johnson, 1992

Figure 14. Mean of three field sites, Incubation of soil with beginning concentration of 120 ppm ammonium-N as UAN through time. Soil treatments were UAN (urea-ammonium nitrate fertilizer), ATS is ammonium thiosulfate, DCD is discyandiamide, nitrapyrin is the formulation of anhydrous ammonia.

Poly-coated urea is used to physically maintain urea as a urea molecule and therefore extend N availability during a longer period of time. ESN (Environmentally Smart Nitrogen, Agrium Inc., Calgary, Alberta, Canada) is the most widely distributed and applied poly-coated urea available to crop producers in the north-central region. It does not perform as a nitrification or urease inhibitor, but ample evidence suggests that its use extends the availability of N into the growing season and protects N from loss until urea is released into the soil.

In ESN studies in North Dakota (ESN placed into the soil), fall-applied ESN had more than half of the urea within the pellets remaining throughout the winter, and about one-third were remaining in late May to early June (Table 9). The rate of urea release from the coating depends on temperature and soil moisture. While urea remains inside the poly coating, it is not affected by the urease enzyme, but neither is it available for crop uptake. Dry weather delays release, while moist soil conditions increases the rate of release.

Table 9. Urea in ESN remaining after field burial, Casselton, N.D., fall 2013. Ransom, unpublished data, North Dakota State University.

In Michigan during three years (Table 10), few corn yield differences were observed when examining multiple blending ratios of ESN and urea. Preplant application favored a 75 ESN: 25 urea blend when June or July rainfall was well above average for the region (2010 and 2013). Nitrogen at-planting application favored a 50 ESN:50 urea blend in years with a normal planting date for the region, but this ratio shifted toward a 25 ESN: 75 urea blend when spring planting was delayed beyond normal ranges. While side-dress applications of ESN did not synchronize well with peak corn N demand in this study, adding ESN extended the activity of N when dry weather conditions were encountered soon after side-dress application (2011).

Table 10. Impact of N application timing and ESN:urea blend ratio on corn grain yield. Preplant incorporated N applications were conducted two to four weeks prior to planting, while side-dress N applications were surface banded at V4-6. Grain yields averaged across 75 and 150 lb. N/acre application rates. (Steinke and Chomas, 2014)

In North Dakota, preplant N as Instinct-treated urea or ESN did not increase yield or protein content of spring wheat, compared with urea alone (Table 11). In Kansas (Table 12), a 50 ESN:50 urea blend spring applied for corn resulted in greater yield than winter-applied urea or ESN, and spring-applied UAN and spring-applied UAN with Nutrisphere. The 50:50 blend was similar in yield to spring-applied urea, spring-applied 100 percent ESN, spring-applied UAN using coulter injection, spring-applied Agrotain treated urea and spring-applied SuperU.

A three-year study in Nebraska (2009-2011) on a coarse-textured soil with irrigation showed ESN applied shortly after corn planting consistently performed better in corn N response, use efficiency and grain yield than UAN applied at the same time or a UAN split application (Maharjan et al, 2016).

Table 11. Spring wheat protein and yield at three sites in the Red River Valley of North Dakota and Minnesota as affected by source of N (ESN and urea) and Instinct, 200 lb. N/acre applied, spring, 2015. (Ransom, unpublished data, North Dakota State University)

A three-year study (2003-2007) at two locations in Iowa (Moore et al., 2008) showed a corn yield advantage for ESN, compared with urea, in only one of nine site-years. The soil ammonium-N concentrations usually were higher with ESN (V6 and V15 sampling), but postharvest soil nitrate-N and ammonium-N concentrations also were higher from ESN, which could lead to increased post-season nitrate loss.

An additional Iowa two-year study (2013-2014) at two locations with ESN and other N additives (Barker and Sawyer, unpublished) found no agronomic or N use efficiency benefit from ESN or other additives, compared with urea, when broadcast with or without incorporation.

Table 12. Corn grain yield as affected by 80 lb. N/acre applied as N source, timing, method of application and additive. (Manhattan, Kan. Weber and Mengel 2009)

In Ohio, a spring season was cooler than normal and ESN treatment did not perform as well as some other treatments (Table 13). The Ohio work supports the use of ESN in a blend in case the environment is unfavorable for urea release through the polymer coating of ESN.

Table 13. Soft red winter wheat yield response to N source and timing from 80 lb. N/acre, Custer, Ohio. (Lentz 2012)

2010 yields followed by the same letter are not significantly from each other at P < 0.05. 

Nitrapyrin and DCD are well-documented nitrification inhibitors, and NBPT and NPPT are well-documented urease inhibitors. Agronomic and environmental benefits may not be realized in every year because rainfall patterns and rainfall intensity dictate whether N will be leached, volatilized or denitrified.

Ammonium thiosulfate has measureable nitrification and urease inhibitors but is not nearly as effective as the well-documented chemistries. Polymer-coated ureas have the greatest benefit in sandy or water-logged soils, or any conditions with a high probability for N loss.

Using N extenders and additives has two potential economic benefits: increase yield or decrease the optimum fertilizer N required. Before deciding whether to use an alternative N product or amendment, producers must consider the major N loss mechanism in a particular soil and field.

These products add additional cost to a farming operation, so producers need to give careful thought to selecting the product appropriate for a specific field. In addition, when considering products not explained in this publication, look for independent field trials, preferably conducted by land-grant university researchers, to verify effectiveness and mode of action.

Barker, D.W., J.E. Sawyer and M.J. Castellano. 2014. Evaluation of fertilizer additives for enhanced nitrogen efficiency in corn. Iowa State University Digital Repository. http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1000&context=agro….

Bartlett, R., R.J.G. Mortimer and K. Morris. 2008. Anoxic nitrification: Evidence from Humber Estuary sediments (UK). Chem. Geol. 250:29-39.

Below, F.E., M.L. Ruffo and L.E. Paul. 2009. Enhanced urea sources for nitrogen fertilization of corn. Illinois Fertilizer Conference Proceedings. 2009.

Bender, R.R., J.W. Haegele, M.L. Ruffo and F.E. Below. 2013. Modern corn hybrids’ nutrient uptake patterns. Better Crops 97:7-10.

Brouder, S. 1996. No-till corn, broadcast urea and urease inhibitors. Citing research by Philips, Mengel and Walker. 1989. Unpublished results, Purdue University. In Purdue Pest Management & Crop Production Newsletter, April 5, 1996.

Cabrera, M., J. Molina and M. Vigil. 2008. Modeling the nitrogen cycle. p. 695-730. In Nitrogen in Agricultural Systems. Agronomy Monograph No. 49. J.S. Schepers and W.R. Raun, eds. ASA-CSSA-SSSA, Madison, Wis.

Chien, S.H., D. Edmeades, R. McBride and K.L. Sahrawat. 2014. Review of maleic-itaconic acid copolymer purported as urease inhibitor and phosphorus enhancer in soils. Agron. J. 106:423-430.

Cowie, G.A. 1918. Decomposition of cyanamide and dicyandiamide. J. Agric. Sci. 9:113-136.

Coyne, M.S. 2008. Biological denitrification. p. 201-254. In Nitrogen in Agricultural Systems. Agronomy Monograph No. 49. J.S. Schepers and W.R. Raun, eds. ASA-CSSA-SSSA, Madison, Wis.

Eberhar, S.A., C.D. Hart, J.D. Hernandez, L.E. Paul, J.J. Warren and F. Fernandez. 2009. Evaluation of new nitrogen fertilizer technologies for corn. Illinois Fertilizer Conference Proceedings, 2009.

Eberhar, S.A., C.D. Hart, J.D. Hernandez, L.E. Paul, J.J. Warren and F. Fernandez. 2010. Evaluation of new nitrogen fertilizer technologies for corn. Illinois Fertilizer Conference Proceedings, 2010.

Ferguson, R.B., G.P. Slater and D.H. Krull. 2008. Evaluation of Dow GF-2017 with urea-ammonium nitrate solution for irrigated corn. University of Nebraska Report.

Ferguson, R., G. Slater and D. Krull. 2009. Encapsulated nitrapyrin study, 2009. University of Nebraska Report.

Fernandez, F. 2010. Report on the use of nitrification and urease inhibitors on corn in Illinois. University of Illinois Report.

Franzen, D.W. Fertilizing hard red spring wheat and durum. NDSU Extension publication SF712. North Dakota State University Extension Service, Fargo, N.D. www.ndsu.edu/fileadmin/soils/pdfs/sf712_2.pdf. Accessed Oct. 24, 2016.

Franzen, D.W., R.J. Goos, R.J. Norman, T.W. Walker, T.L. Roberts, N.A. Slaton, G. Endres, R. Ashley, J. Staricka and J. Lukach. 2011. Field and laboratory studies comparing Nutrisphere®-N with urea in North Dakota, Arkansas and Mississippi. Journal of Plant Nutrition. 34:1198-1222.

Franzen, D.W. 2013. Volatilization losses from urea. p. 139-155. In Proceedings of the 2013 Wisconsin Crop Management Conference, Vol. 52. Madison, Wis. https://soilsextension.triforce.cals.wisc.edu/wp-content/uploads/sites/…. Accessed June 2016.

Goos, R.J. 1985. Identification of ammonium thiosulfate as a nitrification and urease inhibitor. Soil Science Society of America Journal 49:232-235.

Goos, R.J., and B.E. Johnson. 1999. Performance of two nitrification inhibitors over a winter with exceptionally heavy snowfall. Agronomy Journal 91:1046-1049.803-806.

Goos, R.J., and B.E. Johnson. 2001. Thiosulfate oxidation by three soils as influenced by temperature. Communications in Soil Science and Plant Analysis. 32:2841-2849.

Grant, C.A. 2004. Potential uses for Agrotain and polymer-coated products. p. 76-86. Canada Soil Science Conference Proceedings, 2004.

Grant, C.A., S. Jia, K.R. Brown and L.D. Bailey. 1996. Volatile losses of NH3 from surface-applied urea and urea ammonium nitrate with and without the urease inhibitors NBPT or ammonium thiosulphate. Canadian Journal of Soil Science 76:417-419.

Hendrickson, L.L., L.M. Walsh and D.R. Keeney. 1978. Effectiveness of nitrapyrin in controlling nitrification of fall and spring-applied anhydrous ammonia. Agronomy Journal 70: 704-708.

Hergert, G.W., and R.A. Wiese. 1980. Performance of nitrification inhibitors in the Midwest (west). p. 89-105. In Nitrification Inhibitors-Potentials and Limitations. ASA-SSSA, Madison, Wis.

Janzen, H.H., and J.R. Bettany. 1986. Influence of thiosulfate on nitrification of ammonium in soil. Soil Science Society of America Journal 50:803-806.

Kissel, D.E., M.L. Cabrera and S. Paramasivam. 2008. Ammonium, ammonia and urea reactions in soils. p. 101-156. In Nitrogen in Agricultural Systems. Agronomy Monograph No. 49. J.S. Schepers and W.R. Raun, eds. ASA-CSSA-SSSA, Madison, Wis.

Lenz, E.M. 2012. Blending ESN with urea to improve nitrogen efficiency in soft red winter wheat. 2012 Agronomy Abstracts. American Society of Agronomy Madison, Wis.

Maharjan, B., R.B. Ferguson and G.P. Slater. 2016. Polymer-coated urea improved corn response compared to urea-ammonium-nitrate when applied on a coarse-textured soil. Agron. J. 108:509-518.

Malzer, G.L., K.A. Kelling, M.A. Schmitt, R.G. Hoeft and G.W. Randall. 1989. Performance of dicyanadiamide in the North Central States. Communications in Soil Science and Plant Analysis. 20:2001-2022.

Manunza, B., S. Deiana, M. Pintore and C. Gessa. 1999. The binding mechanism of urea, hydroxamic acid and N-(N-butyl)-phosphoric triamide to the urease active site. A comparative molecular dynamics study. Soil Bioloby and Biochemistry 31:789-796.

McCarty, G.W., J.M. Bremner and M.J. Krogmeier. 1990. Evaluation of ammonium thiosulfate as a soil urease inhibitor. Fertilizer Research 24:135-139.

Menelas, B. 2014. Nitrogen fertilizer and nitrification inhibitor impacts on soil N dynamics. Purdue University, ProQuest Dissertations Publishing, 2014. 3636423. http://search.proquest.com/docview/1619180393.

Moore, J.A., R. Killorn and M. Gonzalez. 2008. Comparison of spring applied ESN and urea as sources of nitrogen for corn production. p. 237-252. In Proc. 20th Annual Integrated Crop Management Conference, Ames, Iowa. Dec. 10-11, 2008. Iowa State University, Ames.

Nelson, K.A., P.C. Scharf, L.G. Bundy and P. Tracy. 2008. Agricultural management of enhanced-efficiency fertilizers in the north-central United States. Online. Crop Management doi:10.1094/CM-2008-0730-03-RV. Plant Management Network.

Norton, J.M. 2008. Biological denitrification. p. 173-200. In Nitrogen in Agricultural Systems. Agronomy Monograph No. 49. J.S. Schepers and W.R. Raun, eds. ASA-CSSA-SSSA, Madison, Wis.

Randall, G.W., and J. Vetsch. 2005. Corn production on a subsurface-drained mollisol as affected by fall versus spring application of nitrogen and nitrapyrin. Agronomy Journal 97:472-478.

Randall, G., and J. Vetsch. 2009. Fall and spring-applied nitrogen sources for corn in southern Minnesota. University of Minnesota Southern Research and Outreach Center Report. Waseca, Minn.

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

Randall, G.W., J.A. Delgado and J.S. Schepers. 2008a. Nitrogen management to protect water resources. p. 911-946. In Nitrogen in Agricultural Systems. Agronomy Monograph No. 49. J.S. Schepers and W.R. Raun, eds. ASA-CSSA-SSSA, Madison, Wis.

Randall, G.W., G. Rehm, J. Lamb and C. Rosen. 2008b. Best management practices for nitrogen use in south-central Minnesota (Revised, 2008). University of Minnesota Extension Publication # 8554. University of Minnesota, St. Paul, Minn.

Reeves, D.W., and J.T. Touchton. 1986. Relative toxicity of dicyandiamide and availability of its nitrogen to cotton, corn and grain sorghum. Soil Science Society of America Journal 50:1353-1357.

Sassman, A.M. 2014. Corn production with Instinct nitrification inhibitor applied with urea-ammonium nitrate solution and liquid swine manure. MS Thesis, Iowa State University Digital Repository. http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=5283&context=etd.

Sawyer, J.E. 1985. Nitrification of ammonium nitrogen as affected by time of application, location, temperature, and nitrification inhibitors. M.S. Thesis. University of Illinois, Urbana, Ill.

Sawyer, J.E., M.A. Schmitt and R.G. Hoeft. 1990. Inorganic nitrogen distribution and soil chemical transformations associated with injected liquid beef manure. Agronomy Journal 82:963-969.

Sawyer, J.E., M.A. Schmitt, R.G. Hoeft, J.C. Siemens and D.H. Vanderholm. 1991. Corn production associated with liquid beef manure application methods. Journal of Production Agriculture 4:335-344.

Schmidt, E.L. 1982. Nitrification in soil. p. 253-288. In Nitrogen in Agricultural Soils- Agronomy Monograph No. 22. F.J. Stevenson, ed. ASA-CSSA-SSSA, Madison, Wis.

Schwab, G.J., and L.W. Murdock. 2009. Nitrogen transformation inhibitors and controlled release urea. University of Kentucky Cooperative Extension Service circular AGR-185.

Steinke, K., and A. Chomas. 2014. Corn response to polymer-coated urea application timing, nitrogen rate, and blending ratio. In 2014 Agronomy abstracts. ASA, Madison, Wis.

Touchton, J.T., R.G. Hoeft and L.F. Welch. 1978a. Nitrapyrin degradation and movement in soil. Agronomy Journal 70:811-816.

Touchton, J.T., R.G. Hoeft and L.F. Welch. 1978b. Effect of nitrapyrin on nitrification of fall and spring-applied anhydrous ammonia. Agronomy Journal 70: 805-810.

Touchton, J.T., R.G. Hoeft and L.F. Welch. 1979a. Effect of nitrapyrin on nitrification of broadcast-applied urea, plant nutrient concentrations, and corn yield. Agronomy Journal 71:787-791.

Touchton, J.T., R.G. Hoeft, L.F. Welch, D.L. Mulvaney, M.G. Oldham and F.E. Zajicek. 1979b. N uptake and corn yield as affected by applications of nitrapyrin with anhydrous ammonia. Agronomy Journal 71:238-242.

Tucker, A.N., and D.B. Mengel. 2007. Use of thiosulfates in UAN to reduce nitrogen loss and enhance nitrogen use efficiency in no-till corn and sorghum. p. 14-15. Kansas Fertilizer Research 2007.

Tucker, A.N., and D.B. Mengel. 2008. Nitrogen management of grain sorghum. p. 16-18. Kansas Fertilizer Research 2008.

Varsa, E.C., S.A. Ebelhar, P.R. Eberle, E. Gerhard and T. Wyciskalla. 1999. An evaluation of urease inhibitor technology as a nitrogen management tool in no-till corn and wheat production: Agronomics and economics. Illinois Fertilizer Conference Proceedings, 1999.

Weber, H.S., and D.B. Mengel. 2009. Use of nitrogen management products and practices to enhance yield and nitrogen use efficiency in no-till corn. pp 113-118. In Proceedings of the North Central Extension-Industry Soil Fertility Conference. 18-19 November, 2009, Des Moines, Iowa. International Plant Nutrition Institute, Brookings, S.D.

Weber, H.S., A.N. Tucker and D.B. Mengel. 2009a. Use of nitrogen management products and practices to enhance yield and nitrogen uptake in no-till grain sorghum. p. 6-8. Kansas Fertilizer Research, 2009.

Weber, H.S., A.N. Tucker and D.B. Mengel. 2009b. Use of nitrogen management products and practices to enhance yield and nitrogen uptake in no-till corn. p. 9-11. Kansas Fertilizer Research. 2009.

Weber, H.S., and D.B. Mengel. 2010. Use of nitrogen management products and practices to enhance yield and nitrogen uptake in no-till grain sorghum. p 19-24. Kansas Fertilizer Research, 2010.

Wolt, J.D. 2004. A meta-evaluation of nitrapyrin agronomic and environmental effectiveness with emphasis on corn production in the Midwestern USA. Nutrient Cycling in Agroecosystems 69:23-41.

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