Linda Benedict, Fluitt, Jacob, Regan, Ronald P., Leonards, James P., Harrell, Dustin L.
Dustin L. Harrell, James Leonards, Ron Regan and Jacob Fluitt
In drill-seeded, delayed flood rice production, the most important nitrogen fertilizer application is the application applied just before permanent flood establishment. It is important because the largest amount of nitrogen is applied at this time, and it has the largest potential for loss. The nitrate form of nitrogen is unstable under flooded, anaerobic (no oxygen) conditions and will be lost through a process called denitrification. Because of this loss potential, only fertilizers that contain nitrogen in the ammonium form, such as ammonium sulfate, or fertilizers that break down into the ammonium form, such as urea, are recommended in rice production.
Urea is the most commonly used nitrogen fertilizer source because it is least expensive per pound of nitrogen. The problem with urea, however, is the potential for it to turn into ammonia gas and float off the field if it is left exposed on the soil surface for an extended time. This process is called ammonia volatilization.
Studies conducted at the Rice Research Station over the past several years have shown that when urea is left on the soil surface for a 10-day period prior to permanent flood establishment, volatilization losses can range between 17 percent and 30 percent. That’s a potential for 30 percent of a rice producer’s fertilizer investment to be lost before he ever gets the water on the field.
Unfortunately in commercial rice production in Louisiana, it may take 10 or more days for a flood to be established on some larger fields. In these situations, a urease inhibitor containing the active ingredient N-(n-butyl) thiophosphoric triamide, or NBPT for short, is recommended. Urease inhibitors come in a liquid form and are applied on urea by a local fertilizer distributor. The urease inhibitor basically slows the breakdown of urea to the ammonium form that is available to plants. Because it temporarily delays the breakdown of urea, NBPT also temporarily delays the potential for ammonia volatilization losses.
Many other types of products claim to improve nitrogen efficiency and reduce ammonia volatilization losses when applied to nitrogen fertilizers like urea. To test the efficacy of new fertilizers and fertilizer additives, trials are conducted every year at the Rice Research Station in Crowley, La. In these field ammonia-volatilization trials, ammonia gas is captured as it floats off the field. To do this, semi-open volatilization chambers are installed in the rice field (Photo 1).
The volatilization chambers consist of a Plexiglas tube installed in the field (Photo 2). Nitrogen fertilizer or a treated urea product under investigation is applied to the rice inside the tube at a known application rate (Photo 3). Next, two round sponges that are treated with a weak acid are installed in the tube (Photo 4). As the nitrogen fertilizer turns into ammonia gas and rises, it will come into contact with the acid contained in the lower sponge, which will convert the ammonia gas back into the ammonium-nitrogen (solid) form. The top sponge captures ammonia gas from outside the cylinder to prevent contamination of the sample from outside sources.
Once the sponges are installed, a spacer (to allow air flow) and bucket (to protect the experiment from rainfall) are installed on top of the tubes. The sponges are changed approximately eight times over a 15-day period. Generally, a flood is applied on day 10 of the experiment.
Once the 15-day period is complete, the acid in the sponges is removed using an extraction solution, and the amount of ammonium in the sponge is then measured.
In 2012, Helena Chemical Company released the urease inhibitor N-Fixx containing the active ingredient NBPT. This product was tested at two application rates – 3 and 4 quarts of product per ton of urea. The two rates were evaluated because the label states that a rate of 3 or 4 quarts per ton of urea can be used.
The N-Fixx-treated urea was compared to urea treated with Agrotain Ultra at a rate of 3 quarts per ton of urea. Agrotain Ultra is an established and proven NBPT-containing urease inhibitor that has been used in commercial rice production for several years. The findings of the experiment are presented in Figure 1. Approximately 17 percent of the applied nitrogen from untreated urea was lost in the 10-day period prior to permanent flood establishment. This 17 percent of the applied nitrogen never had a chance to be taken up and used by the rice plants. However, when urea treated with Agrotain Ultra or N-Fixx (3 or 4 quarts) was used, less than 5 percent of the applied nitrogen was lost over the same 10-day period prior to permanent flood establishment.
During the 2011 season, an in-field volatilization trial was conducted evaluating urea and urea treated with Agrotain Ultra, Arborite and NZONE. Arborite Ag is an NBPT-containing urease inhibitor manufactured by the Weyerhaeuser Company and distributed by Gavilon. NZONE is a urea treatment chemical manufactured by AgXplorer and is marketed as a product which can improve nitrogen availability and reduce nitrogen loss. All products were applied 10 days prior to flooding, and ammonia volatilization losses were evaluated for a 15-day period similar to the 2012 trial described above.
Results from this trial are presented in Figure 2. Untreated urea lost approximately 22 percent of the nitrogen 10 days after application. The NZONE-treated urea lost approximately 18 percent of the nitrogen 10 days after application and was not statistically different from the untreated urea. The Arborite Ag-treated urea lost 4 percent of the nitrogen and Agrotain Ultra-treated urea lost 3 percent of the nitrogen during the same period. The Aborite Ag and the Agrotain Ultra treatments were not statistically different from each other. The NZONE-treated urea was not as effective in reducing ammonia volatilization and may not be the best urea treatment when the ammonia volatilization loss potential is high.
Losses of nitrogen by ammonia volatilization not only cause financial losses from the purchase of the fertilizer but also can cause yield losses. To quantify and correlate yield losses with nitrogen losses from ammonia volatilization, separate and paired yield trials are conducted along with the volatilization trials each year.
In the 2011 trial, the yield with Aborite Ag-treated urea was 8,481 pounds per acre, and the yield with Agrotain Ultra- treated urea was 8,640 pounds per acre. When untreated urea was used, yield was 7,217 pounds per acre, and when NZONEtreated urea was used, the yield was 7,073 pounds per acre. This field experiment clearly demonstrates that nitrogen losses from ammonia volatilzation can result in financial losses not only from nitrogen lost early in the season but also from lost yield at the end of the season.
Many times producers ask, “When is it economical to use a urease inhibitor and when is it not economical?”
The AgCenter recommendation is that it is economical to use a urease inhibitor when flooding a rice field takes more than three to five days. Three to five days is recommended instead of five days because the economical breakeven point changes from year to year and environment to environment. Currently, urea costs approximately 61 cents per pound of nitrogen, and the urease inhibitor costs approximately 6 cents per pound of applied nitrogen. Therefore, a nitrogen loss must be approximately 10 percent or more to break even when using a urease inhibitor. The results from the field evaluations show that untreated urea lost about 10 percent of nitrogen to ammonia volatilization somewhere between day five and day six during the 2012 trial and approximately at day four during the 2011 trial.
It is important to make sure that all preflood urea applications are only applied on dry ground and then flooded. When urea is applied to damp ground, the rate of volatilization increases. This shortens the two- to three-day delay in volatilization shown in Figure 1 (or the four-day delay in Figure 2), and the steep incline in the volatilization rate occurs sooner. Using a urease inhibitor will help in this situation; however, it is only half as effective compared to dry-ground applications. A urease inhibitor will not be beneficial if the treated urea is applied into the flood water.
In Louisiana rice production, nitrogen fertilizer represents one of the largest expenditures to produce a crop. It is, therefore, imperative that nitrogen be used in a way to maximize efficiency to keep nitrogen losses to a minimum and grain yield at a maximum. If there is a high potential for ammonia volatilization losses, a proven urease inhibitor containing the active ingredient NBPT should be used. As new nitrogen fertilizers and fertilizer treatments are released, the LSU AgCenter will promptly test these materials and base all recommendations on multi-year trial results.
Dustin L. Harrell is an associate professor and Mosaic Endowed Professor at the Rice Research Station in Crowley. James Leonards, Ron Regan and Jacob Fluitt are research associates at the Rice Research Station in Crowley.
(This article was published in the spring 2013 issue of Louisiana Agriculture magazine.)