Enhancing Efficiency

Nitrogen influences turf health and quality more than any other nutrient. For this reason, nitrogen efficiency is a key component of every turf management program — and a top concern among turf professionals.

Various forms of nitrogen are used in fertilizers. Water-soluble nitrogen fertilizers, such as ammonium nitrate, ammonium sulfate and urea, supply a form of nitrogen that is immediately available for the plant to use. The turf's response is a quick greening.

This form of nitrogen is generally used for small areas where frequent applications are possible or even desirable when used in combination with control products or growth regulators. However, quick release nitrogen has its drawbacks. It readily leaches, it can cause lush growth and its response length is limited. It also has a high potential to burn the turf if applied in hot or dry conditions.

The consequences of quick release nitrogen can be avoided, and the benefits of nitrogen prolonged, with an enhanced efficiency fertilizer. Most turf professionals, especially those responsible for large areas like golf courses and corporate grounds, are familiar with fertilizer products commonly referred to as “slow” or “controlled” release. But did you know that there is another category of enhanced efficiency fertilizer?


In 1994, the Association of American Plant Food Control Officials (AAPFCO) and The Fertilizer Institute established the Slow Release Fertilizer Task Force. The committee, composed of industry professionals and fertilizer regulatory control officials, was formed to develop a standard set of guidelines that the industry could use to manufacture — and control officials could use to regulate — slow-release fertilizers. In addition to clarifying the slow-release category, the task force acknowledged a similar, but separate, category of enhanced efficiency nitrogen: stabilized nitrogen.

“The committee recognized that while stabilized nitrogen and slow-release nitrogen are similar, they function in very different ways,” says Wilbur Frye, professor emeritus of agronomy at the University of Kentucky and former chairman of the Slow Release Fertilizer Task Force.

Stabilized nitrogen is a urea product that performs much like comparable slow-release nitrogen sources. The difference between the two categories is a matter of process. Under normal circumstances, nitrogen is released while it undergoes hydrolysis and nitrification as part of the soil nitrogen cycle. Stabilized nitrogen suspends the two transformations, minimizing potential loss from volatility, denitrification and leaching.


The term “slow release” is used to describe fertilizer products that release, or convert to a plant-available form, their plant nutrients at a slower rate than soluble nitrogen does. These nitrogen sources include polymer- or sulfur-coated urea; synthetic organic nitrogen sources like isobutylidene diurea, methylene urea and urea-formaldehyde; and natural organic sources like sewage sludge and animal by-products.

Examples of slow-release products are water insoluble, slowly available water soluble and coated or occluded to control the release of soluble nutrients. (Refer to the cover story in the April 2002 issue of Grounds Maintenance for detailed descriptions of the various types of slow-release nitrogen fertilizers.)

While a number of slow-release fertilizer materials exist, they all typically work through a slow breakdown of the nitrogen source or the coating material. Temperature, humidity and moisture impact the timing and duration of this process. Unless the nitrogen is coated, its form and the bacterial activity in the soil determine the length of its response after entering the soil.


AAPFCO defines stabilized nitrogen products as those that have been amended with an additive to reduce the rate of transformation of fertilizer compounds. As a result, the ammonium-nitrogen is available in the soil for an extended period of time. Stabilized nitrogen fertilizer is not a coated product, nor is it a slow-release product. Rather, its additives are incorporated during the production of urea.

Examples of stabilizing amendments are urease inhibitors, such as N-(butyl) thiophosphoric triamide (NBPT), and nitrification inhibitors, such as Dicyandiamide. NBPT slows the rate of urea hydrolysis, the process by which urea nitrogen is converted into ammonia and carbon dioxide, two products of the reaction that can be quickly lost into the atmosphere. Dicyandiamide slows the nitrification process, or the conversion of ammonium-nitrogen to nitrate-nitrogen in the soil.

“The result is a longer lasting urea product that improves nitrogen efficiency and environmental performance,” says John Street, associate professor of horticulture and crop science at Ohio State University.


Both nitrate and ammonium are readily taken up by plants and are beneficial for plant growth. However, there are major benefits related to keeping nitrogen in its ammonium form for a longer period of time.

Once nitrogen is released into its nitrate form, it is subject to leaching by water moving through the soil profile. However, Dicyandiamide holds nitrogen in its stable and immobile ammonium form longer. For example, the ammoniacal residual lasts up to 16 weeks with stabilized fertilizer products.

Unlike nitrate, ammonium is a cation that can be held in soil on negatively charged exchange sites. Because stabilized nitrogen spends more time as ammonium, it has less opportunity to leach. As ammonium, not only is nitrogen more usable for the plant, less nitrogen is released into the ground to potentially contaminate groundwater.

Research shows that on average, 30 percent of ammonium nitrogen can be lost within 72 hours of application through volatilization, unless it is moved into the soil. The NBPT in stabilized nitrogen minimizes the loss of nitrogen to the atmosphere, while allowing sufficient time for rainfall or irrigation to move stabilized nitrogen into the soil.


By enhancing the efficiency of nitrogen, the turf gets more of the nutrients it needs and responds accordingly. The results of field trials conducted at the University of Wisconsin (Madison) indicate that products with a stabilized fertilizer, in comparison to urea, result in more robust turfgrass root growth, depending on the turfgrass and site.

Additionally, the high level of performance is complemented by a lower cost per use. “Slow-release and stabilized nitrogen have similar advantages, but stabilized nitrogen has the added advantage of being cost effective,” says Wayne Kussow, professor of soil science at the University of Wisconsin.

Stabilized nitrogen uses urea, an economical but relatively short-lived nitrogen source. The inhibitors in stabilized nitrogen extend the life of urea nitrogen, which means fewer applications are necessary.

Stabilized nitrogen is available at a lower cost per unit than leading organic, coated and complex chain products. The cost of incorporating additives into the production process of stabilized nitrogen is typically less than that of standard slow release fertilizers.

Your nitrogen investment also is better protected as stabilized nitrogen. For one thing, there is less potential nitrogen waste or loss with stabilized nitrogen. According to the field research conducted by Kussow, stabilized nitrogen achieved 42 percent greater nitrogen recovery than sulfur-coated urea. And with stabilized nitrogen, you don't have to worry about handling, mowing or moisture damaging an expensive coating.


Another attribute of stabilized nitrogen is its versatility. Stabilized nitrogen is equally effective as a component in dry granular fertilizer blends or as a soluble nitrogen source for liquid application.

Nitrogen efficiency, value and versatility are all qualities of stabilized nitrogen that superintendents and turf professionals can appreciate. “My course handles 70,000 rounds of golf a year, sometimes 250 to 300 a day,” says Jason Harsh, superintendent of Memorial Park in Houston, Texas. He adds that stabilized fertilizer offers the versatility he needs for optimal results with minimal applications.

More information on stabilized nitrogen is available at www.stabilizednitrogen.com.

Alan Nees is vice president of AGROTAIN International (Mequon, Wis.).

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