MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

The escalating prices of N fertilizers and concerns regarding N fertilizer supplies have many producers questioning the potential benefits of N fertilizer additives, particularly nitrification and urease inhibitors.  Extensive research on these types of products has been conducted throughout the North Central region for many years, with results generally indicating that effectiveness is reliant on many factors, including N source, timing, soil type and tillage.  In years past, a somewhat common practice was for growers to apply a little extra N as a relatively cheap form of insurance.  However, with high N fertilizer cost many growers are looking for alternative strategies to assure adequate N is provided to the growing crop.

The primary objective of urease inhibitors is to reduce the potential volatilization losses of urea-based fertilizers.  When urea is applied to soil, it must be converted through hydrolysis (addition of water) to the ammonium form before it becomes plant available.  This conversion is driven by an enzyme called urease, which is abundant within the soil.  When urea is applied and incorporated or rained into the soil, the breakdown of urea to the ammonium N form will occur in about 2 to 3 days.  If urea is surface applied and not incorporated within 2 to 3 days, urea can be converted first to ammonium and subsequently to ammonia, which is susceptible to volatilization loss.  Nitrogen losses due to volatilization can be in excess of 20% of the applied N, and are typically the greatest where there is a relative high surface soil pH, high amounts of surface residue, and in warm and windy weather conditions.  Urease inhibitors are applied to interfere with the urease enzyme, and essentially delay the hydrolysis of the urea molecule, typically for 10 to 14 days. 

Most university research to date has focused on the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT), which is sold under the trade name Agrotain.  Research results have indicated that NBPT can be effective at reducing volatilization losses and enhancing yields, though consistent responses should not be expected every year or on all fields.  Hendrickson (1992) summarized nationwide research regarding the effects of NBPT use for surface applied urea and UAN.  The summary included data from 78 experiments in 17 states (including Michigan), with 45% on no-till, 45% reduced till, and 10% on conventional tilled fields.  When averaged across all sites and years, the use of NBPT increased grain yields 4.3 bu/ac when applied with urea and 1.6 bu/ac when applied with UAN.  Application of NBPT resulted in yield reductions of 10 bu/ac or more in 7% of the trials.  Table 1 is a summary adapted from Laboski (2006).

Table 1.  Summary of corn yield increases from application of NBPT with surface applied urea and UAN (Hendrickson, 1992). Yield increases were significant (P<0.01).

*Sites where yield increased when N fertilizer was applied

So where are urease inhibitors most likely to pay off?  If urea or urea-based fertilizers are surface applied and not incorporated, a practice typical of no-till cropping systems, the chance for volatilization losses is most significant.  Cropping systems/tillage practices that result in relatively high surface residues will promote volatilization, as will recent unincorporated lime applications.  The amount of N loss through volatilization will depend on whether adequate rainfall (about 0.25”) or mechanical incorporation occurs within a few days of application.  If adequate rain comes, volatilization losses will be minimized and money spent on a urease inhibitor may not be recovered in N savings and yield increases.  But, if the urea is not moved into the soil soon after application, the effectiveness of the urease inhibitor will be enhanced.  At about $50/gal, applied at a rate of 5 qt/T of urea, the additional cost of a urease inhibitor would be about $0.07 per pound of actual N.  When deciding whether to use this type of product, growers should evaluate their cropping system and determine whether the risk of N loss is worth the extra input cost.  If conditions are favorable for N loss, use of NBPT can help prevent N (and yield) losses, but positive responses should not be expected every year.

Nitrification is a microbial soil process by which N in the ammonium form is converted to nitrate.  Soil N as ammonium is held on soil cation exchange sites and is relatively stable, in terms of loss, while nitrate-N is highly susceptible to losses by leaching or denitrification.  Leaching losses are especially pronounced in coarse textured soils that receive excess water while denitrification occurs in warm (>50◦F), wet conditions primarily on fine textured soils, though both these processes can occur on some level regardless of soil texture. 

When conditions are favorable for these loss mechanisms, it is desirable for N in the soil to remain in the ammonium form until the time of rapid crop demand.  Nitrification inhibitors delay the conversion of ammonium to nitrate by interfering with the metabolism of Nitrosomonas bacteria.  Common nitrification inhibitors include nitrapyrin (N-Serve) and dicyandiamide (DCD/Guardian), which are typically effective for 3 to 6 weeks depending on soil and climate conditions.  With fall N applications (not recommended for MI), nitrification inhibitors may delay conversion to nitrate until soil temperature drop low enough (below 40◦F) to limit denitrification risks.  With spring N applications, the inhibitors keep soil N in the ammonium form until peak crop demand (for corn, about the 6-8 leaf stage).  As with urease inhibitors, many years of research have been conducted evaluating the effects of nitrification inhibitors on various crops and cropping systems.  Work in Wisconsin with N-Serve on corn showed that when all the N was applied preplant, N-Serve resulted in increases in both yield and return.  When N was applied sidedress there was no benefit to using N-Serve.  The results of the study also showed improved yields and return for sidedress N applications compared with preplant application, regardless of the nitrification inhibitor.  Data from the trial are shown in Table 2 adapted from Laboski (2006):

Table 2. Four year average effect of N timing and N-Serve on corn yield at Hancock, WI (sandy soil) (Wolkowski, 1995; adapted from Laboski, 2006)

140 lb N/a applied spring preplant (PP) or sidedressed (SD).  N-serve applied at 2 pt/a. Calculations based on $3.50/bu corn, $0.45/lb N, and $32/gal of N-Serve.

A report by Hoeft, 1984, summarizing results of research in many states, showed yield and return benefits of N-Serve for fall-applied N, but no benefit when N was applied in the spring.  Results from that study also demonstrate a clear economic benefit to spring N applications compared with fall applications. (See Table 3)

Table 3.  Effect of time and rate of N application and N-Serve on corn yield in Illinois (Hoeft, 1984).

Calculations based on $3.50/bu corn, $0.45/lb N, and $32/gal of N-Serve

In general, research has shown that the use of a nitrification inhibitor is most likely to result in yield and economic returns when applied with N on coarse-textured soils or fine-textured poorly drained soils.  Even when using a nitrification inhibitor, there is continued risk of N loss from the soil depending on environmental conditions and crop N uptake.  But, in high loss environments the use of an inhibitor has a much greater probability of reducing N loss.  Recognizing these conditions is important when making N management decisions, and will ultimately impact net return for growers.

Additional information on nitrification inhibitors for corn production can be found at: http://www.extension.iastate.edu/Publications/NCH55.pdf

Dr. Carrie Laboski, soil scientist at the University of Wisconsin, recently wrote an article discussing the fundamental processes of, and economics of using, nitrification and urease inhibitors.  I encourage you to take a look at Dr. Laboski’s report from the 2006 Wisconsin Fertilizer, Aglime & Pest Management Conference, found at:
http://www.soils.wisc.edu/extension/wfapmc/2006/pap/Laboski1.pdf

Note: Where trade or brand names are used, no endorsement by Michigan State University is intended, nor criticism of similar products not named.

References

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

Hoeft, R.G. 1984. Current status of nitrification inhibitors use in U.S. agriculture. p. 561-570. In Hoeft, R.G. 1984. Current status of nitrification inhibitors use in U.S. agriculture. p. 561-570. In R.D. Hauck (ed.) Nitrogen in crop production. ASA, CSSA, and SSSA, Madison, WI.

Laboski, C.A.M. 2006. Does it pay to use nitrification and urease inhibitors? In Proc. 2006 Fert., Aglime, and Pest Mgmt. Conf. Vol. 45. Available at: http://www.soils.wisc.edu/extension/wfapmc/2006/pap/Laboski1.pdf

Wolkowski, R.P., K.A. Kelling, and L.G. Bundy. 1995. Nitrogen management on sandy soils. UWEX Bulletin A3634. Univ. of Wisconsin-Extension, Madison, WI