Considerations for Use of Enhanced Efficiency Nitrogen Fertilizers
Farmers, the fertilizer industry, and researchers have been interested in nitrogen fertilizers that are slow to dissolve in soil so that the N release would be gradual and slowly available for crop uptake during its production cycle. Many formulations of urea fertilizers or chemicals to treat urea are on the market today. Only a few are effective at protecting the fertilizer from quickly losing N. Even then, farmers must consider the higher cost of these specialized fertilizers against the soil conditions at the time of application.
Enhanced Efficiency Fertilizers (EEFs)
Different types of EEFs exist on the market but many of them do NOT protect N from loss despite false claims. Proven EEFs are either controlled release N fertilizers or slow release N fertilizers. The term N stabilizers is sometimes wrongly used for these fertilizers when in fact, N stabilizer is the substance or mixture of substances (or additives) used to treat N fertilizer to protect it from the quick release of N.
How EENFs protect urea from N loss
Urease inhibitors reduce ammonia volatilization from surface applied urea: Urease is an enzyme that is widespread in soils, and produced by soil organisms and plants. With soil moisture it causes urea to break down to produce ammonia (NH3) gas, which is easily lost to the atmosphere through volatilization. When urea is treated with a urease inhibitor (a chemical compound), the inhibitor temporarily blocks the urease enzyme from converting urea into ammonia gas.
Conditions that favor urea breakdown and N loss from the action of urease near the soil surface include, wet soils, crop residue, soil pH > 7, length of time fertilizer is left on the soil surface.
When should you use urease inhibitor-treated urea? When application is made on soil surface that is not dry, or surface covered with plant residue with no impending rain in sight. If the soil is dry and rainfall is anticipated in a couple of days, plain urea can be used. Even though urease inhibitor will provide stability for close to 10 days, 0.5 inches of rain will dissolve the fertilizer and move it well below soil surface.
Common examples of urea fertilizers with proven urease inhibitors
N-(n-Butyl) thiophosphoric triamide (NBTPT or NBPT) is the urease inhibitor used in some of the most effective and commercially available EENFs. It can reduce N loss by as much as 50% or more compared to urea. Examples are various versions of Agrotain®, SuperU®, and ANVOL®.
Nitrification inhibitors reduce nitrate N leaching
Nitrification occurs when soil organisms convert ammonium or ammonia to nitrate-N (NO-3), which is the available form by which most plants take up N. Because nitrates have a negative charge, as opposed to ammonium which is positive, they cannot bind onto clay or organic matter surfaces which are also negatively charged. Rainwater can therefore easily move soil nitrate deeper in the soil below the roots (leaching).
Nitrification inhibitors cause reduction of a specific group of soil bacteria (nitrifying bacteria, e.g. Nitrosomonas and Nitrobacter), which are responsible for converting ammonium to nitrate.
Nitrification inhibitors are most valuable when NO3-N losses are expected to be high from leaching or denitrification, under the following situations:
- Wet or poorly drained soils
- Tile-drained soils
- Fall N application
Nitrification inhibitors reduce denitrification
Denitrification is when soil microbes, under very wet, anaerobic soil conditions, convert nitrate-N to the gas form of N (N2 or N2O) which is unavailable to non-nitrogen fixing plants. The gas is easily lost to the atmosphere. Nitrification inhibitors reduce nitrification by delaying the conversion of ammonium to nitrate.
Common examples of urea fertilizers with proven nitrification inhibitors
A number of fertilizer products with proven efficacy for inhibiting nitrification contain the chemical DCD (dicyandiamide) such as SuperU®, nitrapyrin (e.g N-Serve™ and Instinc™), and a recent addition is pronitradine with the trade name, Centuro™. Note that DCD is NOT a N stabilizer as per FIFRA (Federal Insecticide, Fungicide, and Rodenticide Act). Note that SuperU contains urease and nitrification inhibitors.
Slow Release N fertilizers
A known example of slow release urea fertilizer is ESN (Environmentally Smart Nitrogen). Even though very effective at slowly releasing N, the release can be too slow, and sometimes the N may not become fully available when the plant most needs it. Preplant application and incorporation of a blend with urea (3 or 4-parts urea to 1-part ESN) before planting in spring, has been suggested as beneficial to corn and wheat.
Lots of products on the market today are claimed to stabilize urea N, and increase yields or profit. North Dakota State University researchers do not recommend any products out there on the market that they have not studied and found to be effective. If farmers must use any of the specialized (EENFs) on their fields, they risk losing lots of money invested on the wrong products, and even when a proven product is used on a field where the risk of significant N loss is low. Of several field studies conducted at Carrington with some established EENFs yield differences from conventional urea have mostly not been different. That is because N losses have either been less than expected, or N from soil has often been high enough to compensate for any losses. Remember to only use N stabilizers for what it is proven to do, and to apply it only when it is most needed.
To Minimize N loss
To minimize N loss, delay application under conditions of high risk. Incorporate where applicable within a day or two by tillage (> 2 inches deep), or use subsurface banding (> 2 inches deep). Another option is to apply N fertilizer shortly before irrigation or anticipated rain (> 0.5 inch). If none of these choices are available and the conditions are likely to promote nitrogen loss, then use NBPT-treated urea for surface applied fertilizer, or a slow-release N fertilizer.
There is no endorsement of commercial products or companies even though reference may be made to trade names, trademarks or service names.
Jasper Teboh, Ph. D