With nitrogen (N) prices hovering in the $0.35 to $0.40 per unit range depending on source, losses from volatilization, denitrification, and leaching become more costly.  Not only do you lose the value of the lost nutrient, but the expense to purchase and reapply N is greater.  Choosing not to reapply lost N often results in significant corn yield reductions.  That valuable lesson was learned the hard way in 2004.  Further, the new UW nitrogen recommendations do not build-in a “just in case” insurance factor. 

        A growing corn plant doesn’t need significant amounts of N until it reaches the V8-V9 stage (mid to late June – see Figure 1).  This means that N applied pre-plant or early post must “hang around” for about 40 to 50 days before it is really needed.  Losses can occur if granular urea or liquid 28% is surface applied without being moved into the soil by a subsequent rainfall.  Further, any N source that has been converted to the nitrate form, which they all eventually are, is subject to denitrification or leaching if soils remain wet or waterlogged for a significant period of time.  The good news is that there are some products and management strategies that will help corn growers minimize the risk of N losses.

Figure 1.  N uptake by corn from planting to harvest.

Urease inhibitors

        As mentioned previously, urea containing fertilizers applied to the soil surface are subject to volatilization losses.  The process is initiated by an enzyme called urease.  Unless these fertilizers are moved into the soil by tillage or rainfall within several days, some of the urea is eventually converted into ammonia gas and lost to the atmosphere.  The problem is exacerbated by dry soil conditions and/or on high pH soils.  Research indicates that up to 20 percent of the N can be lost with surface-applied granular urea.  Losses will be less with liquid N sources because only about 50 percent of their total N is in the urea form. 

        Fortunately, there is a product called Agrotain that inhibits or delays the activity of the urease enzyme.  This product has been proven effective in multiple research trials.  The cost of using Agrotain is in the neighborhood of $6.00 per acre ($2.00 for every 100 lb./acre of urea applied).  In high risk conditions (dry, high pH soils) the additional cost is easily justified. 

Nitrification inhibitors

        Soils that become wet or waterlogged for extended periods can cause N loss from denitrification regardless of what fertilizer N source was used.  All N sources are eventually converted into the nitrate (NO₃) form by soil bacteria.  In anaerobic wet soils, nitrates undergo a conversion to nitrogen gas, which is lost to the atmosphere.  The loss is greatest in high pH soils that are warm (>50^oF).  Documented losses of up to 100 lb. N per acre have occurred where environmental conditions are favorable.  Nitrification inhibitors (NI) such as N-Serve delay the conversion of ammonium to nitrate. Many research trials have confirmed NI products are effective in delaying the conversion of ammonium to nitrate. These same trials show a wide range of results from the standpoint of cost-return benefits depending upon soil type, drainage, time of N application, and environmental conditions. Like urease inhibitors, the economics of use becomes more favorable as N price increases and N rates are reduced.  However, the greatest return will come from soils that are slowly or poorly drained.  The cost of using an NI is about $7.00 per acre. 

Sidedressing nitrogen

        Both urease and nitrification inhibitors are used to prevent the loss of early season applied N.  Another strategy is to simply delay N applications until a time that is closer to rapid plant uptake.  Some people have suggested that less sidedress N is needed compared to pre-plant or pre-emerge rates.  This simply is not the case unless N losses occur early in the growing season. Sidedressing N does have a considerable advantage over earlier applications when conditions for N loss exist. There is some risk associated with sidedressing from the standpoint that you must be able to get in the fields to apply the N.  Wet weather during the middle of June can make sidedressing difficult.  It’s best to start a little early than to wait too long.  Nevertheless, it remains an excellent strategy to avoid early season N losses.