Feature

Nitrogen Fertilizer Q&A

By Jason Lanier, University of Massachusetts

Q: What exactly is an “enhanced efficiency” nitrogen fertilizer? Are these essentially the same as “slow-release” sources of N?

A: A great question for this time of year, which is the most important time for fertilizing cool-season grasses. The short answer? No, not really.

Nitrogen (N) is the most important essential nutrient for most cultivated plants, including turf. One of the reasons we fertilize with N is that plant growth is very responsive to N levels. Higher N availability translates to more growth. While we like the idea of vigorous growth and deep green color, a lot of growth can be too much of a good thing. Overstimulation of growth can promote succulent, less resilient plants, stunted root systems, and lots of mowing, which is the most time consuming and costly input for most management programs.

Too much N can also cause problems beyond the turf system. Nitrogen is typically very transient and mobile in the environment, and N that is not quickly taken up by plants is subject to potential loss. Some forms of N readily move with water, either across the soil surface with runoff, or below the reach of the root system with gravitational drainage (leaching). This movement of N into the environment can impair both surface and drinking water.

Some N sources are also prone to conversion to a gaseous form after application, which is subsequently lost to the atmosphere; a process called volatilization. Volatilization losses can be significant under certain conditions (especially warm and damp). While concerns around N volatilization are not as acute as N loss involving water, it contributes to inefficiency and waste. There is also increasing evidence that some gaseous N compounds in the atmosphere can play a role in the greenhouse effect, along with more general impacts on air quality.

Slowly available or slow-release nitrogen (SRN) fertilizer materials (as opposed to readily available, water-soluble N sources, WSN) include water-insoluble N (WIN) materials and various engineered slow-release N technologies sometimes referred to as controlled release nitrogen (CRN). These slowly available sources, which “meter out” the N contained in them, offer valuable opportunities for tailoring N availability to plant needs and reducing “surge growth” and losses via runoff and leaching promoted by excess N availability. Read more about the utility of slow-release N for fertilizer programming in and around Table 12 in Chapter 7 of UMass Extension’s Lawn BMP document.

In contrast, the term enhanced efficiency fertilizer (EEF), by strict definition, most often refers to stabilized N sources. The stabilization process yields fertilizers that behave differently compared to SRN or WIN materials. There are two current stabilization technologies found among nitrogen EEFs for turf:

• Urease inhibitors – Prevent N conversion to the gaseous form, thereby reducing volatilization loss. While these sources can reduce N movement to the atmosphere and improve plant use efficiency of applied N, they don’t necessarily mitigate the potential for runoff or leaching losses of N as described above.

• Nitrification inhibitors – Work to keep more of the N in the fertilizer in the ammonium form. Ammonium is preferred by plants, and is less susceptible to loss routes like leaching relative to other forms of N.

Be aware that recommendations and regulations can sometimes conflate these different N sources and blur the lines between their distinct and varying attributes. Be sure to read labels and endeavor to gain a better understanding of the nuances, benefits, and potential drawbacks of these materials. The take home is that there is a range of fertilizer technologies available today, with bearing on how the N contained in them behaves relative to plant and soil health as well as human health and natural resource protection. It is important to have a sound management plan, and to have a good understanding of how these tools can enable maximum turf performance as well as good environmental stewardship.

Jason D. Lanier is Extension Turf Specialist and Group Leader in Commercial Horticulture at the University of Massachusetts-Amherst. Reprinted from Hort Notes September 2024.