It’s that time of year again. Time to apply sidedress N to corn and grain sorghum and apply preflood N in rice. Unfortunately the weather has not been as cooperative this year as many of us would have liked, but there is still time to make the best out of a bad situation. If the current forecast/weather pattern holds, the soil in many fields will be moist to muddy and a decision will have to be made to apply sidedress/preflood N onto wet soil or wait for the rain to stop and soil surface to crust before urea is applied. Ammonia volatilization losses from surface-applied urea can be significant and result in as much as 30-40% of the total N being volatilized and unavailable to the crop (Fig. 1). Losses of urea as ammonia gas increase as wind speed, soil moisture, humidity (>70%), soil pH, and temperature increase. Under warm (spring and summer) conditions, urea needs to be incorporated with tillage, rainfall (>0.5 inch rainfall), or irrigation immediately and at least within two days following application to significantly reduce gaseous ammonia loss. Ammonia volatilization loss of urea is much more rapid and extensive when urea is applied at high rates or to a muddy low cation exchange capacity soil compared to a dry soil (Fig. 1). The ammonia loss potential tends to be lower in clayey soils because of their higher cation exchange capacity compared to silt or sandy loam soils (Fig. 2). Research in Arkansas shows that under warm/summer conditions, ammonia loss from surface-applied urea occurs most rapidly the first five days after application to a silt loam (Fig. 1) and the first ten days when applied to a clay soil (Fig. 2). The use of NBPT (N-(n-butyl) thiophosphoric triamide), a urease inhibitor, can help minimize urea loss via ammonia volatilization when applied to a muddy soil; however, every effort should be made to apply urea to a dry soil even when NBPT is used.
A number of NBPT-containing products are now available for adding to urea and urea-ammonium-nitrate (UAN) fertilizers. These products may contain different amounts of the active ingredient NBPT. The concentration of each individual ingredient is typically given on the product label. However, some NBPT-containing products have elected to not show the specific concentration of NBPT on the label, which makes it difficult to compare the cost and potential effectiveness among products applied at equal NBPT rates. A limited amount of research has shown that the duration and magnitude of urease inhibition is dependent on NBPT rate, which makes knowledge of a product’s NBPT concentration critical. Application of the proper NBPT rate to urea or UAN fertilizer is important and assumes that the ability of NBPT to inhibit ammonia volatilization is not affected by the other ingredients that may be included in the actual product or other products (other inhibitors and drying agents) that may also be added to the fertilizer. Manufacturers of NBPT-containing products are encouraged to list the concentration (percentage by weight) of each individual ingredient including NBPT on the label. The most common recommended NBPT application rate ranges from 1.6 to 1.8 lb NBPT per ton of urea which is equivalent to the addition of 0.08 to 0.09% NBPT by weight. Note that the NBPT application rate for UAN fertilizer is different than that for granular urea.
Farmers have adopted the use of NBPT as a management tool for surface-applied urea, particularly in rice fields where establishing a flood in two days or less is difficult, or in upland crops where urea may remain on the soil surface for extended periods of time following application (e.g., not incorporated by tillage or irrigation). The high adoption rate of products containing NBPT has stimulated the marketing of numerous products with claims of similar benefits (i.e., reducing ammonia volatilization). Before the University of Arkansas Division of Agriculture recommends a product that claims to have inhibitory effects on the enzyme urease or simply “ammonia volatilization loss of urea” the products must be subjected to a laboratory test, which measures ammonia volatilization from urea amended with the commercial product of question compared to the volatilization losses of urea and NBPT-treated urea. At the time this article was written only the NBPT-containing products listed in Table 1 have been tested and shown to effectively inhibit ammonia volatilization from urea when applied at 0.08 to 0.09% NBPT.
Taking the time to manage sidedress/ preflood N correctly will pay dividends come harvest time. When choosing a urease inhibitor to protect your sidederess/preflood N investment a little due diligence in making sure you are applying a documented inhibitor and applying it at the correct rate will be money well spent in a year such as this. For more information of urease inhibitors please refer to University of Arkansas Division of Agriculture publication FSA 2169.
Table 1. List of tested and recommended NBPT-containing urease inhibitors (based on product labels available in May 2015) and suggested application rates for urea and urea-ammonium-nitrate (UAN).
|Product Name||Manufacturer||Weight per gallon||NBPT concentration||Label Recommended Volume†|
|Agrotain Ultra||Koch Fertilizer, LLC||8.84||26.7||3.0||1.5|
|Arborite AG||MicroSource Co||9.15||20.0||2.0 – 3.0||1.5|
|Arborite AG-NT||Weyerhauser NR Co. ‡||9.15||24.0||3.0||1.5|
|Factor X2||Rosen’s Inc.||9.1||50.0||1.63||0.815|
|N-Fixx||Helena Chemical||8.80||unknown¶||3.0 – 4.0||1.5 – 2.0|
|Nitrain||Loveland Products Inc.||8.93||26.7||3.0||1.5|
|NitroGain#||Arclin, Inc.||8.92||20.0||4.0||1.5 – 2.8|
|NitroGain#||Arclin, Inc.||9.00||26.7||3.0||1.5 – 2.8|
|N-Veil||Invictis Crop Care, LLC||8.92||26.7||3.0 – 5.0||1.5 – 2.8|
† One ton of fertilizer approximates 181 gallons of 32% UAN and 187 gallons of 28% UAN.
‡ Arborite AG-NT distributed by Gavilon Fertilizer
- Limus also contains 5.6% N-(n-propyl) thiophosphoric triamide [NPPT] for a total thiophosphoric triaminde concentration of 22.5%.
¶ Unknown, the product label does not specify the concentration of NBPT in the product.
# NitroGain will be sold containing a single trade name (NitroGain) with one of two concentrations. Anyone using this product should pay close attention to the NBPT concentration listed on the label to match the appropriate product use rate (3 or 4 qt/ton urea) which may vary with NBPT concentration