by: Beatrix Haggard and Josh Lofton

The drying conditions in recent days have resulted in a percentage of corn finally being planted. While most producers are focused on planting, these intense planting conditions will result in a short window for N fertilization. With this narrowed window between planting and N fertilization, it is time to start thinking about N management. In recent years, conditions at or near fertilization have resulted in high potential loss of applied N. These losses not only are detrimental to the surrounding environments but also to the production system, resulting in insufficient available N supply to the crop. The use of N inhibitors has been an increasingly common production practice in an attempt to minimize in-season losses. While the use of N inhibitors is a valuable tool for potentially decreasing N loss, proper management and proper selection are critical to decrease losses successfully.

Selection of the proper inhibitor is potentially the most critical aspect and can be challenging because of the numerous options available. Determining the right inhibitor varies, depending on N source, N application method, and field/environment.

Chemical names to ask for:

• Urease inhibitors – NBPT (N-(n-butyl)thiophosphoric triamide), or NPPT (N-(n-propyl)thiophosphoric triamide)
• Nitrification Inhibitors – Nitrapyrin, or DCD (dicyandiamide)

Figure 1. Untreated urea at 240 lbs N/acre.

Figure 2. Super U urea at 240 lbs N/acre (NBPT and DCD).

Urease inhibitors:

Urease inhibitors function by inhibiting the urease enzyme, a natural enzyme in the soil system that hydrolyzes urea into ammonium. By doing this, urease inhibitors attempt to minimize volatilization losses during periods favorable to volatilization (dry, windy, urea on surface). When environmental conditions exist, namely moisture, urease inhibitor activity is diminished, allowing the urease enzyme to break down the urea. Urease inhibitors work very well with granular fertilizers because these are frequently surface-applied with limited to no incorporation. With surface application of granular fertilizers, volatilization is the primary potential loss, at least during early season when adequate moisture does not exist. However, these inhibitors are not only beneficial on granular fertilizers. Substantial volatilization can occur with liquid fertilizers that are surface-applied in warm, dry conditions with little soil moisture. When these same fertilizers are incorporated, such as being knifed-in in lieu of surface-applied, as little as 1-5% of applied N is typically lost through volatilization. Therefore, urease inhibitors are most effective when urea-containing fertilizers are surface-applied, whether liquid or granular, without incorporation.

Nitrification inhibitors:

Nitrification inhibitors function by inhibiting the soil bacteria, which are required for nitrification to occur. Inhibiting nitrification keeps more N as ammonium for longer periods of time. This is beneficial because it can limit or minimize both leaching and volatilization losses, both of which occur as nitrate compared to ammonium. These inhibitors have the potential to be the most universally beneficial in high rainfall and irrigated systems especially. However, the benefit of these products has not been widespread in research trials conducted across the Mid-South. A two-year study conducted at the LSU AgCenter, however, did show that these inhibitors can be beneficial in Louisiana systems. The best management of these inhibitors is to know their limitations. Research conducted at the LSU AgCenter as well as other research around the US indicate that these products last only 10-30 days. Therefore, these inhibitors provide early season “protection” of the N fertilizer, but N can still be lost after the nitrification inhibitor has degraded.

Coated-fertilizer:

Unlike the other inhibitors, coated-fertilizers do not inhibit any process within the soils system but slowly release N from the coated source throughout the season. By slowly releasing N into the soil system, these inhibitors minimize only the amount of N applied that can be lost through individual loss mechanisms. These N sources are promising at minimizing both volatilization, denitrification and leaching. However, few if any coated liquid sources are currently or will be commercially available in the near future. Therefore, these must be used solely on granular fertilizer sources. Additionally, because the fertilizer is slowly available, N available from the applied fertilizer during early season growth is minimal. Therefore, high amounts of residual soil N need to be available, or supplemental N fertilizer needs to be supplied.

Nitrogen inhibitor products have the potential to be very beneficial tools at managing N fertilizer additions in Louisiana production systems. However, if mismanaged, not only do you lose the benefit of applied inhibitors but also narrow the economic potential of the production system. Therefore, time needs to be given to selection of a particular inhibitor to fit the system as well as proper management within the production system.