Nitrogen fertilizer additives

Many producers question 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.

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. 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.