DIVISION OF AGRICULTURE RESEARCH & EXTENSION Agriculture and Natural Resources Uh\--University of Arkansas System

FSA2169

Nitrogen Additives

Trent Roberts delay, but do not permanently prevent, Introduction a specific process of the cycle. Associate Professor Nitrogen is by far the most Fertility extensively applied fertilizer for agro- The emphasis on responsible and Extension Specialist nomic crop production in Arkansas. efficient nutrient management is now Urea [CO(NH2)2] and urea- greater than ever. The development of Rick Norman are the most com- conservation programs that offer monly used nitrogen fertilizers in partial payment for farmers to imple- Professor of Soil Fertility Arkansas because of their low price ment efficient nutrient management and high nitrogen analysis relative to practices and the rising cost of fer- Nathan Slaton most other nitrogen fertilizers. Urea- tilizers make products that stabilize Professor and Director of nitrogen is subject to volatilization loss fertilizer nutrients a lucrative market. Soil Testing as (NH3) gas if not properly Reducing soil and fertilizer nitrogen incorporated shortly after application. movement from agricultural fields into Once incorporated into the soil, urea the surrounding landscape has both Leo Espinoza and other ammonium-forming (NH4+) environmental and agronomic benefits Associate Professor and nitrogen fertilizers will eventually be and has led to the marketing of a Extension Soil Scientist converted to nitrate (NO3-), which is number of products that claim to prone to and/or denitrifica- ‘stabilize’ nitrogen. Many of these tion. For more information on the products are unproven in laboratory , refer to Extension fact and field research conducted by sheet FSA2148, The Nitrogen and unbiased and knowledgeable in . This fact researchers. Oftentimes, these prod- sheet will review the University of ucts are aggressively marketed with Arkansas System Division of Agri- well-designed advertisements, infor- culture’s current knowledge base on mational brochures and web sites but nitrification and (or ammonia lack specific information regarding volatilization) inhibition as it concerns their mechanisms or modes of action when nitrogen fertilizer additives that explain how they ‘stabilize’ should be used and their effectiveness. nitrogen (or other nutrients). Although some products provide a mode of What Are Nitrogen action with wording that includes scientific terms, the wording is usually Fertilizer Additives? vague and lacks scientific support. Nitrogen fertilizer additives are substances added to a water-soluble or The use of legitimate nitrogen readily available fertilizer that extend fertilizer additives does not guarantee the time a particular form of nitrogen that maximum crop yield can be Arkansas Is remains in the soil. Nitrogen fertilizer produced with lower nitrogen rates. additives are added to nitrogen fertil- While the use of lower nitrogen Our Campus izer by the manufacturer when the rates to produce maximum yield is a nitrogen fertilizer is made or by the possible outcome, the expected bene- fits of additives occur when conditions applicator shortly before the fertilizer exist for significant loss of the nitrogen Visit our web site at: is applied to the field. In most cases, in conventional fertilizers via ammonia https://www.uaex.uada.edu these products temporarily inhibit or

University of Arkansas, United States Department of Agriculture, and County Governments Cooperating volatilization and/or leaching, <40°F, and soil bacteria become soil microbial activity was runoff and of inactive when the soil becomes very negligible at the lowest tempera- nitrate. Unfortunately, the dry. The nitrification process ceases ture, and all of the nitrogen added frequency and magnitude of under anaerobic (saturated) soil as urea remained as ammonium for nitrogen losses cannot always be conditions, such as when the 28 days. For the Memphis soil, predicted since they are often the permanent flood is established on a nearly all of the added urea- result of weather-related events. rice field. nitrogen had been converted from ammonium to nitrate by 14 days at Laboratory incubation Nitrification Inhibition 77°F and 28 days at 59°F. experiments using Memphis (6.4 Nitrification was slightly more Nitrification is the two-step pH) and Calhoun (7.5 pH) silt rapid in the Calhoun soil with most microbial process that converts loams show the effect that three of the added urea-nitrogen constant temperatures had on the ammonium (NH4+) into converted from ammonium to nitrification rate of urea fertilizer (NO2-) and eventually into nitrate nitrate by 14 days at 77°F and when soil moisture was near (NO3-). The first step of the 21 days at 59°F. Average soil process is performed by specific optimal (20 percent moisture, temperatures in Arkansas are ammonia-oxidizing (Nitro- -85 kPa; Figure 1). In both soils, warm enough for nitrification from somonas spp. being the most common one) and the second step 120 o 120 o by nitrite-oxidizing bacteria Calhoun soil (41 F) Memphis soil (41 F) ( spp.). Ammonium and 100 0 100 - 0 0 0 0 0 0 nitrate are both plant-available 0 - nitrogen forms, but ammonium is 80 0 80 0 the more stable nitrogen form in 60 60 - the soil. Ammonium is a cation and Nitrate - can be retained by the soil’s cation 40 0• Ammonium 40 exchange properties, making it far less mobile than nitrate, an anion. 20 20 Nitrate is susceptible to loss by 0 0 runoff, leaching in permeable soils • • • • • • • • • • (sands and sandy loams) and 0 5 10 15 20 25 30 0 5 10 15 20 25 30 denitrification [reduction to nitrous 120 120 oxide (N O) and dinitrogen (N ) o o 2 2 Calhoun soil (59 F) Memphis soil (59 F) gasses] in poorly drained soils 100 100 when anaerobic soil conditions • 0 0 persist. Applying ammonium or an 80 0 • 80 0 • ammonium-forming nitrogen fer- 0 - tilizer is desirable since ammonium 60 60 is less susceptible to denitrification • 40 • 40 - 0 and leaching losses. Eventually, 0 however, the nitrification process 20 • 20 - • converts ammonium supplied from 0 0 0 - fertilizer or mineralized from 0 • 0 • • organic matter, manures or 0 5 10 15 20 25 30 0 5 10 15 20 25 30 biosolids into nitrate. 120 120 o o The requirements for Calhoun soil (77 F) Memphis soil (77 F) nitrification are ammonium, a 100 100 • population of nitrifying organisms, • • 80 • 80 0 and a suitable environment • • 0 (e.g., pH, temperature and mois- 60 0 • 60 ture). The nitrification process is affected by soil chemical and phys- 40 40 ical properties associated with • 20 0 20 • different soils. In general, nitrifica- • 0 tion increases as soil temperature, 0 0 0 0 0 0 0 moisture and pH increase. Near optimal soil conditions for nitrifica- 0 5 10 15 20 25 30 0 5 10 15 20 25 30 tion include a temperature range of

77-95°F, soil moisture near field Days after urea application Days after urea application capacity and pH near 8.0. The Figure 1. The effect of three temperatures (laboratory incubation) and time on nitrification process is very slow or the proportions of soil nitrate and ammonium (nitrification) for 28 days follow- negligible at soil temperatures ing urea fertilizer application on Calhoun (pH 7.5) and Memphis (pH 6.4) silt loam soils. March through November of most had little or no effect on the advance of planting would be years (Table 1). amount of ammonium remaining advantageous for many growers. after 10 days for the Calhoun soil University researchers will Nitrification inhibitors are but delayed nitrification in the continue to evaluate potential nitri- chemicals that temporarily delay or DeWitt and Henry soils. Additional fication inhibitors under laboratory slow nitrification by affecting the research is needed to identify the and field conditions and incorporate activity of the spp. soil characteristics that would legitimate nitrification inhibitors bacteria. Nitrification inhibitors allow us to accurately predict into nitrogen management recom- have been used mostly in the whether a nitrification inhibitor mendations when appropriate. Midwest for fall application of would be of significant benefit in The legitimacy of a product’s claim anhydrous ammonia. Generally, slowing the conversion of ammo- as a nitrification inhibitor can be nitrification inhibitors have not nium to nitrate and thus reducing quickly and efficiently examined been used extensively in the nitrate-nitrogen losses. in the laboratory before it is mid-South and southern United field tested. States because they have not been Nitrogen management highly effective. By ‘highly effective’ strategies for corn, cotton, grain Several products (active we simply mean that the length of sorghum, rice and wheat in the ingredients) are currently recog- time these products delay/slow mid-South usually involve split nized as legitimate and docu- nitrification has not offered a applications with the majority of mented nitrification inhibitors; significant nitrogen management the nitrogen fertilizer side-dressed however, our discussion here will advantage across a wide array of near the onset of rapid crop growth. concern dicyandiamide (DCD) and soils. Preliminary results from These nitrogen management nitrapyrin, the two nitrification recent laboratory experiments have strategies minimize the potential inhibitors being marketed in shown that dicyandiamide (DCD) benefits of a legitimate nitrification Arkansas and surrounding states. and nitrapyrin (Corteva Agri- inhibitor. However, there is still The active ingredients in these science, Indianapolis, Indiana) have interest and a place for nitrification products have proven to be effec- effectively delayed nitrification on inhibitors in our nitrogen manage- tive at inhibiting nitrification in some soils, but had little or no ment systems, especially on sandy- industry and university research, effect on the nitrification rate of textured soils and for nitrogen that which has been peer reviewed and other soils (Table 2). For example, is applied preplant. The ability to published in refereed journals. the DCD in Super U (Koch apply a greater proportion of nitro- The process (e.g., mode of action) Fertilizer, LLC, Wichita, Kansas) gen fertilizer preplant or weeks in by which a chemical inhibits

Table 1. Average monthly soil temperatures (4-inch depth of bare soil) during 2009 and 2010 for three sites in eastern Arkansas. Month Site Year Jan Feb Mar April May June July Aug Sept Oct Nov Dec Rohwer 2009 40.8 43.2 49.5 56.1 64.7 73.8 75.8 75.8 71.8 56.6 49.4 42.0 (Gallion sl) 2010 37.8 37.5 42.5 61.7 72.2 83.6 82.3 md† md md 52.1 42.0 Marianna 2009 42.0 46.6 53.1 51.7 72.0 83.5 83.6 84.9 79.5 63.3 57.1 43.9 (Calloway sl) 2010 40.8 40.1 51.5 66.2 76.1 87.7 88.8 90.8 82.2 70.4 56.1 42.9 Keiser 2009 38.3 44.1 50.1 58.4 69.7 78.3 81.6 81.0 75.9 60.6 53.8 41.3 (Sharkey cl) 2010 38.3 36.2 49.1 63.2 72.5 85.5 87.7 90.7 80.1 67.5 52.5 40.1 † Missing data.

Table 2. The net percentage of urea or Super U fertilizer remaining as ammonium after 10 and 20 days in three silt loam soils incubated at 77°F and 25% gravimetric soil moisture (Golden et al., 2009). Month Soil† Calhoun series DeWitt series Henry series N fertilizer Time (days) % of added fertilizer present as ammonium 10 <1 36 11 Urea 20 0 4 <1 10 8 77 54 Super U 20 0 65 49 † Soil properties: 1.0% total C, 0.10% total N, and soil pH values of 6.2 (DeWitt), 7.3 (Henry) and 7.6 Calhoun. nitrification can differ. For Ammonium sulfate NBPT-urea example, the inhibitor nitrapyrin 30 is a bactericide and has been Urea + ammonium sulfate Urea registered with the EPA as a pesti- 25 cide since 1974. Dicyandiamide, on ...·························· ~,.....--­ the other hand, is not a bactericide ..· and is not required to be registered 20 with the EPA. The mode of action of DCD is by inhibition of ammo- nium monooxygenase, an enzyme 15 required by ammonia-oxidizing __.--·-·-·-·- bacteria to metabolize ammonium 10 into nitrite. ,, Nitrapyrin (also called 5 OptinyteTM) is the active ingredient ,, in N-Serve and Instinct formula- . --. tions. Instinct formulations contain 0 -- an encapsulated form of nitrapyrin 2 4 6 8 10 12 14 16 18 20 Ammonia volatilization (% of applied N) applied of (% Ammonia volatilization used with UAN, urea or liquid Time after application (days) manure, whereas N-Serve is used exclusively with anhydrous Figure 2. Ammonia volatilization losses of urea, NBPT-treated urea, ammonium ammonia. Encapsulation allows sulfate and a urea/ammonium sulfate blend on a Calloway silt loam. Flood was

Instinct formulations to be impreg- applied on day 10. (Norman et al., 2009) nated onto the surface of granular reaction in the hydrolysis of as temperature declines, and thus (ammonium) fertilizers or to be urea is alkaline forming, urea- there should be less ammonia vola- spray applied to the field without containing fertilizers are more tilization loss of urea when applied immediate volatile loss of the prone to loss by this mechanism in the winter to wheat compared to active ingredient. Dicyandiamide is than ammonium sulfate in the spring or summer to corn, the active ingredient in Agrotain [(NH ) SO ] which is an acid- cotton, grain sorghum or rice. Plus, Super U (Koch Fertilizer, 4 2 4 forming nitrogen fertilizer. Urea Ammonia volatilization loss of urea LLC, Wichita, Kansas), and the nitrogen when applied to a soil is is much more rapid and extensive product sold as Guardian (Conklin hydrolyzed by the urease enzyme when urea is applied at high rates Company, Inc., Shakopee, (urea amidohydrolase) and or to a muddy silt loam soil Minnesota) and is applied directly converted first to ammonium compared to a dry silt loam soil onto or incorporated into the fertil- carbonate [(NH ) CO ] and then to (Figure 3). Also, the ammonia izer granule or solution. A number 4 2 3 ammonia gas. Urease is every- loss potential tends to be lower in of other products that have shown where in the environment and can clayey soils because of their higher to be effective nitrification inhibi- be found in soils, manures, on cation exchange capacity compared tors have been registered previ- plants and plant residues. The to silt or sandy loam soils ously or are available in other urease enzyme is believed to be (Figure 4). countries. The publication entitled released to the environment Nitrification Inhibitors for Corn One approach to reduce from living and disintegrating Production (http://www.extension. potential ammonia volatilization microbial cells. iastate.edu/publications/NCH55. loss of urea when immediate incor- pdf) provides a list of product trade Losses of urea as ammonia gas poration is not possible is to amend and chemical names and gives an increase as wind speed, soil mois- urea or urea-ammonium-nitrate excellent review on the use of ture, humidity (>70 percent), soil with a chemical that inhibits or nitrification inhibitors. pH and temperature increase. slows urease activity, or conversion Under warm (spring and summer) of urea to ammonia gas. A urease Ammonia Volatilization conditions, urea needs to be incor- inhibitor allows the urea prill time porated with tillage, rainfall to dissolve and move into the soil Inhibition (>0.5 inch rainfall) or irrigation before hydrolysis to ammonia Ammonia (NH3) volatilization within 2 days following application occurs or allows time to be incorpo- is a soil surface loss mechanism to significantly reduce gaseous rated either mechanically or by that may occur when an ammo- ammonia loss. Ammonia volatiliza- water (e.g., irrigation or rain). nium or ammonium-forming tion loss of surface-applied urea to Enabling the urea to hydrolyze to fertilizer is surface applied and a dry silt loam soil can reach ammonia after it has been not incorporated immediately. 20 percent to 30 percent in 5 days dissolved or moved into the soil All ammonium fertilizers can to when the temperature exceeds allows the ammonia produced from some degree be lost via ammonia 75-80°F (Figure 2). The speed of the urea to acquire a hydrogen volatilization, but since the first the urea-urease reaction decreases (H+) atom from the soil and become ammonium (NH4+), which inhibit urease activity. While all triamide (NBPT) is the only one is a salt that is retained by the the compounds listed in Table 3 that has seen widespread imple- soil’s cation exchange properties. can minimize ammonia volatiliza- mentation on a commercial scale Only a few chemical compounds tion of urea, the compound and has substantial published have been shown to effectively N-(n-butyl) thiophosphoric literature to support its efficacy. The patent on NBPT has expired 35 (circa 2002), allowing a number of

Urea-mud companies to develop and market 30 their own products that include -NBPT-dry NBPT. The urease inhibitor NBPT 25 - is the active ingredient in several NBPT-mud products that have proven to

-Urea-dry reduce ammonia loss of urea in 20 - unbiased research. BASF released 15 the product Limus which contains both NBPT and NPPT (N-(n- 10 propyl) thiophosphoric triamide). The Limus product has been 5 shown to effectively reduce

Ammonia volatilization (% volatilization of applied) Ammonia ammonia volatilization from 0 urea, but to our knowledge NPPT 0 2 5 7 9 12 has not been tested as a stand- Time after application (days) alone product. A good number of trials have Figure 3. Ammonia volatilization losses when urea and NBPT-treated urea were applied to a dry or muddy soil 5 days prior to flooding. (Norman et al., 2006) been conducted in Arkansas that have consistently shown the effectiveness of NBPT in reducing 30 ammonia volatilization losses from Ammonium Sulfate surface-applied urea when condi- 25 tions for ammonia volatilization Urea were present. Table 4 shows rice 20 I= grain yield as affected by flooding 15 1, 5 and 10 days after urea or NBPT-treated urea were applied to 10 the dry surface of a Calloway silt loam. Results from this study 5 underscore the importance of timely soil incorporation of the urea 0 fertilizer and utility of NBPT when Volatilization (% of applied N) Volatilization 0 2 4 6 8 10 12 14 16 18 20 22 timely incorporation is not possible. When flooding was conducted the Time after application (days) day after N fertilizer application, Figure 4. Ammonia volatilization loss of urea and ammonium sulfate when rice fertilized with urea produced a applied to a Perry clay. (Griggs et al. 2007).

Table 3. Documented Urease Inhibitors Table 4. Rice grain yield when urea, NBPT-treated urea, ammonium sulfate (AS) and a urea/AS blend (1:1 urea/AS blend on a N weight basis) were Name of Active Abbreviation applied 1, 5, and 10 days before flooding. (Norman et al., 2009) Ingredient Application Time N-(n-butyl) NBPT (days before flooding) thiophosphoric triamide N Source N rate 1 5 10 N-(n-propyl) NPPT thiophosphoric triamide pounds N/A bushels per acre phenylphosphoro- Urea 120 187 160 154 PPD diamidate Urea + NBPT 188 182 175 AS 181 178 171 TPT thiophosphoryl triamide Urea/AS blend 179 166 161 ATS ammonium thiosulfate LSD (0.05) 8.6 TU MU methyl urea grain yield similar to NBPT- ingredient NBPT. The concentra- application rate ranges from 1.3 to treated urea. However, rice grain tion of each individual ingredient is 1.8 pounds NBPT per ton of urea yield significantly decreased when typically given on the product which is equivalent to the addition the flood was delayed until 5 days label. However, some NBPT- of 0.065 to 0.09 percent NBPT by after application when urea was containing products have elected to weight. Note that the NBPT appli- the N source, but not when NBPT- not show the specific concentration cation rate for urea-ammonium- treated urea was the N source. Rice of NBPT on the label, which makes nitrate fertilizer is different than grain yield for all of the N fertilizer it difficult to compare the cost and that for granular urea. sources significantly decreased potential effectiveness among prod- ucts applied at equal NBPT rates. Farmers have adopted the use when the flood was further delayed of NBPT as a management tool for until 10 days compared to 1 day, A limited amount of research has shown that the duration and surface-applied urea, particularly but more so for urea compared to in rice fields where establishing a NBPT-treated urea. Research in magnitude of urease inhibition is dependent on NBPT rate, which flood in 2 days or less is difficult, or Arkansas shows that under warm/ in upland crops where urea may summer conditions, ammonia loss makes knowledge of a product’s NBPT concentration critical. remain on the soil surface for from surface-applied urea occurs extended periods of time following most rapidly the first 5 days after Application of the proper NBPT rate to urea or urea-ammonium- application (e.g., not incorporated application to a silt loam (Figure 2) by tillage or irrigation). The high and the first 10 days when applied nitrate fertilizer is important and assumes that the ability of NBPT adoption rate of products con- to a clay soil (Figure 4). The use of taining NBPT has stimulated the NBPT can help minimize urea loss to inhibit ammonia volatilization is not affected by the other ingredi- marketing of numerous products via ammonia volatilization when with claims of similar benefits (i.e., applied to a muddy soil; however, ents that may be included in the actual product or other products reducing ammonia volatilization). every effort should be made to Before the University of Arkansas apply urea to a dry soil even when (other inhibitors and drying agents) that may also be added to the System Division of Agriculture NBPT is used. fertilizer. Manufacturers of NBPT- recommends a product that claims A number of NBPT-containing containing products are encouraged to have inhibitory effects on the products are now available for to list the concentration of each enzyme urease or simply “ammonia adding to urea and UAN fertilizers. individual ingredient including volatilization loss of urea” the prod- These products may contain NBPT on the label. The most ucts must be subjected to a labora- different amounts of the active common recommended NBPT tory test, which measures ammonia

Table 5. List of tested and recommended NBPT-containing urease inhibitors (based on product labels available in March, 2019) and suggested application rates for urea and urea-ammonium-nitrate (UAN).

Recommended † Weight Per NBPT Volume Product Name Manufacturer Gallon Concentration Urea UAN lb/gallon % qt/ton fertilizer Agrotain Advanced Koch Fertilizer, LLC 8.87 30 2.0 1.0 Agrotain Ultra Koch Fertilizer, LLC 8.84 26.7 3.0 1.5 ANVOL¶ Koch Fertilizer, LLC 9.26 16 1.5 0.75 Arborite AG-NT Weyerhauser NR Co.‡ 9.15 24.0 3.0 1.5 ContaiN AgXplore 8.5 unknown§ 4.0 2.0 Factor Rosen’s, Inc. 9.09 24.5 3.25 1.625 Limus BASF 9.06 16.88# 3.0 1.5 N-Fixx PF Helena Chemical 8.50 unknown§ 3.0-4.0 1.5-2.0 Nitrain Loveland Products 8.93 26.7 3.0 1.5-2.8 Nitrain Express Loveland Products 8.99 24.8 3.0 1.5 N-Veil Invictis Crop Care, LLC 8.92 26.7 3.0-4.0 2.0-3.0 PinnitMax Corteva Agriscience 9.26 50 1.5 0.75 † One ton of fertilizer approximates 181 gallons of 32% UAN and 187 gallons of 28% UAN. ‡ Arborite AG-NT (Nitrolock Technology) distributed by Gavilon Fertilizer § Unknown, the product label does not specify the concentration of NBPT in the product. # Limus contains 16.88% NBPT and 5.63% NPPT, which is a proprietary urease inhibitor owned by BASF. ¶ ANVOL contains 16% NBPT and 27% duromide which has also been shown to reduce ammonia volatilization loss. volatilization from urea amended blending ammonium sulfate with During this process, nitrogen with the commercial product urea can lower total ammonia can be lost by volatilization of question compared to the volatilization loss (Figure 2), the of ammonia gas if the urea is volatilization losses of urea and decrease in ammonia loss from the not properly incorporated soon NBPT-treated urea. At the time of blend may not be enough to result after application. Ammonium the writing of this fact sheet, only in significantly greater grain sulfate is less prone to the NBPT-containing products yields compared to urea nor grain ammonia volatilization loss listed in Table 5 have been tested yields similar to ammonium compared to urea and is and shown to effectively inhibit sulfate or NBPT-treated urea an alternative to NBPT- ammonia volatilization from urea (Table 4). treated urea. when applied at 0.08 to 0.09 percent NBPT. • The only documented urease Summary inhibitors being marketed at An alternative to NBPT- • Nitrification inhibitors delay the time this information was treated urea for minimizing the conversion of ammonium prepared contain NBPT, the ammonia volatilization losses is to nitrate by temporarily active ingredient in the prod- the use of ammonium sulfate. inhibiting the activity of ucts listed in Table 5. The most Ammonium sulfate is much less Nitrosomonas bacteria and common recommended NBPT prone to ammonia volatilization have the potential to reduce application rates range from loss compared to urea due to nitrate leaching and denitrifi- 1.3 to 1.8 pounds NBPT per ton its slightly acidic properties cation losses. However, the of urea which is equivalent to (Figures 2 and 4). Fertilization longevity and effectiveness of the addition of 0.065 to 0.09 with ammonium sulfate can result nitrification inhibitors in the percent NBPT by weight. in grain yields similar to NBPT- mid-South are limited due to Caution should be exercised treated urea when conditions are the warmer climate (compared when using products that do conducive for ammonia volatiliza- with the Midwest). not list specific NBPT concen- tion loss (Table 4). Also, when trations on the label. is required in addition to • Research of nitrification nitrogen, supplying a portion of inhibitors will continue in the • NBPT-treated urea should be the fertilizer-N as ammonium hope of understanding the used if >2 days are required sulfate is logical. Shortcomings of soils, crops and situations for incorporation on a silt loam ammonium sulfate are that it typi- where and when they can be of soil and when >7 days on a cally costs considerably more than benefit in the mid-South. The clay soil. urea, on a N weight basis, and the use of best management prac- Any other purported N lower N analysis of ammonium tices, such as split applications • stabilization products that sulfate compared to urea may and specific application timings claim inhibition of ammonia increase application expenses. of nitrogen fertilizer, should be volatilization of urea cannot be Ammonium sulfate is typically not used until we better under- recommended for use because used solely as the preflood N stand when a nitrification we have tested them and they source for rice, but is often inhibitor is a viable option. do not inhibit this process, we blended with urea to offset some have not tested them or we are of the costs and still possibly gain • Urease inhibitors work on currently testing them. some of the beneficial effects of the enzyme urease, which the ammonium sulfate, such as is responsible for the conver- plant-available sulfur. Although sion of urea to ammonium. References and Literature of Interest Griggs, B.R., R.J. Norman, C.E. Wilson, Jr. and N.A. Slaton. 2007. Ammonia volatilization and nitrogen uptake for conventional and conservation tilled dry-seeded, delayed flood rice. Soil Sci. Soc. Am. J. 71:745-751. Golden, B.R., N.A. Slaton, R.J. Norman, E.E. Gbur and C.E. Wilson, Jr. (2011). Nitrogen release from environmentally smart nitrogen fertilizer as influenced by soil series, temperature, moisture and incubation method. Comm. Soil Sci. Plant Anal. 42(15): 1809-1824. Golden, B.R., N.A. Slaton, R.E. DeLong, R.J. Norman and E.T. Maschmann. 2009. Nitrification inhibitors influence on rice grain yield and soil inorganic nitrification fractions. p. 215-223. B.R. Wells Rice Res. Studies 2008. Ark. Rice Res. Ser. 571. Norman, R.J., C. E. Wilson, Jr., N.A. Slaton, B. R. Griggs, J.T. Bushong and E.E. Gbur. 2009. Nitrogen fertilizer sources and timing before flooding dry-seeded, delayed-flood rice. Soil Sci. Soc. Am. J. 73: 2184–2190. Norman, R.J., C. E. Wilson, Jr., N.A. Slaton, D.L. Frizzell, W.J. Ross, J.T. Bushong and A.L. Richards. 2006. Influence of urea and Agrotain applied to dry and muddy silt loam soils several days prior to flooding on ammonia volatilization and grain yields of delayed flood rice. p. 291-297. B.R. Wells Rice Res. Studies 2005. Ark. Rice Res. Ser. 540. Rawluk, C.D.L., C.A. Grant and G.J. Racz. 2001. Ammonia volatilization from soils fertilized with urea and varying rates of urease inhibitor NBPT. Can. J. Soil Sci. 81:236-246. Roth, C.R., T.L. Roberts and R.J. Norman. 2009. Variable rates of Agrotain® on ammonia volatilization loss of urea applied to a DeWitt silt loam. p. 240-245. B.R. Wells Rice Res. Studies 2008. Ark. Rice Res. Ser. 571.

The information given herein is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by the Arkansas Cooperative Extension Service is implied.

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DR. TRENT ROBERTS is associate professor and Extension Issued in furtherance of Cooperative Extension work, Acts of May 8 soil fertility specialist, DR. RICK NORMAN is professor and and June 30, 1914, in cooperation with the U.S. Department of DR. NATHAN SLATON is professor and director of soil testing with Agriculture, Director, Cooperative Extension Service, University of the Crop, Soil and Environmental Sciences Department located at the Arkansas. The University of Arkansas System Division of University of Arkansas in Fayetteville. DR. LEO ESPINOZA is Agriculture offers all its Extension and Research programs and associate professor and Extension soil scientist with the Crop, Soil and services without regard to race, color, sex, gender identity, sexual Environmental Sciences Department, University of Arkansas System orientation, national origin, religion, age, disability, marital or Division of Agriculture, located at the State Extension Headquarters in veteran status, genetic information, or any other legally protected Little Rock. FSA2169-PD-5-2019RV status, and is an Affirmative Action/Equal Opportunity Employer.