Mark Twain once wrote, “There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.” Oddly this is the best way to describe how the “dogma” of late season nitrogen and fertilization has evolved in turf.
For much of the last 40 years few have questioned the value of applying high rates or nitrogen to almost dormant turf just prior to the onset of winter. There appeared to be a significant benefit, fertilizer was relatively inexpensive when compared to other inputs, and it provided an additional service opportunity to most lawn and landscape firms.
However, during the last decade, growing concern for the effect of nitrogen on water quality and the overall effect of global fertilizer demand on price called the questions: How real is the benefit of late season applied nitrogen? And if there is a benefit, how much is enough?
History of N
Nitrogen is considered the most important macronutrient for turf growth. At between 3-4 percent of plant tissue, nitrogen is the most abundant in the plant after carbon, hydrogen and oxygen.
While it surrounds the plants in the atmosphere and is often abundant in various forms in the soil, it is the most limiting nutrient for turf growth. Consequently, it is the most common nutrient supplied by managers to maintain healthy turf systems.
When I began in the turfgrass industry in early 1970s, it was not uncommon to apply in excess of 8-10 pounds of actual nitrogen per 1,000 square feet. The goal was to keep the turf areas green and growing. If you were in the mowing management business you were happy to apply all that nitrogen, as it kept the mowers running throughout the season.
Over the years, as the concern for the effect of landscapes on water quality increased and fertilizer prices increased in response to rising fuel prices, nitrogen application rates slowly declined. While there is still some debate over the exact amount required for most lawns, there is agreement that there is no need to return to the old days of double-digit annual rates.
As the turfgrass industry has evolved, the number of nitrogen fertilizer options has increased. The old standard for nitrogen fertilizer was water soluble/quick release sources such as ammonium nitrate, ammonium sulfate or urea. An immediate response would be observed both in color and growth and then dissipate quickly as the nitrogen was taken up or leached past the root system.
Slow release products were available mostly as organic forms that would release based on soil temperature and moisture. These products were developed by coating traditional water-soluble sources with compounds that would slow their release or reacted with other chemicals to create products that released the nitrogen more slowly over time.
The goal of slow release nitrogen was to minimize the number of applications required by pulsing the nitrogen to the plant in a way that might mimic soil available nitrogen. This would provide a sustained response much longer in duration than the typical water soluble sources. It is interesting to note there has been cyclical interest in coating products for a “one application per year” strategy, theoretically to meet the plants needs over the season through sophisticated coatings.
Regardless of the technological developments fertilizer costs have increased and the more technology in the bag, vis-à-vis slow release formulations, the more the product cost. Of course the argument for slow release is that it lowers labor costs by requiring fewer applications.
Yet with all the energy directed towards fertilizer technology, very little effort has been exerted to add precision to the amount, timing and frequency of applications required to sustain a healthy turf. It seems the one-pound application rate four to five times per year is still sacred. That is until the questions began to increase over the impact of lawn fertilizer on water quality.
N and water quality
Eutrophication is the slow, natural nutrient enrichment of streams and lakes and is responsible for the "aging" of ponds, lakes, and reservoirs. Excessive amounts of nutrients, especially nitrogen and phosphorus, speed up the eutrophication process. As algae grow and then decompose they deplete the dissolved oxygen in the water. This condition usually results in fish kills, offensive odors, unsightliness, and reduced attractiveness of the water for recreation and other public uses.
Excessive nitrate in drinking water can cause human and animal health problems, particularly for small babies. The U.S. Public Health Service has established a specific standard of 10 milligrams of nitrate nitrogen per liter as the maximum concentration safe for human consumption. Problems in adults that drink water with excessive nitrate are essentially nonexistent and are rare in infants. Nevertheless, concern over the use of nitrogen in lawn and landscapes has led to a growing number of regulations. The regulations run the gamut from local timing and rate restrictions to larger scale watershed restrictions on total loading amounts.
Areas such as the Peconic Bay Estuary and the Chesapeake Bay Watershed are poised to enact large-scale restrictions on fertilizer use for lawns.
Recent research conducted under the direction of Professor Karl Guillard at the University of Connecticut has raised the question of the “sacred” one-pound per 1,000 square feet application rate and interval. Professor Guillard’s work suggests that at any one time maybe about a one-half pound rate might be adequate for the desired response. This research has been ground-breaking in many ways, questioning the long held dogma of many of our current fertilizer practices.
Additionally, research investigating water quality and lawn fertilization has concluded that slow release nitrogen sources do add a level of safety but still the overall loading rates remain a concern. Furthermore, while some precision is being added to in-season application of nitrogen, either by reduced rate or extended frequencies due to the age of the lawn, few have questioned the application of nitrogen in the late season when most top growth has slowed.
Some of the oldest turfgrass research has espoused the benefits of applying nitrogen at the end of the growing season prior to the onset of winter. The agronomic benefits of enhanced rooting, reduced spring clipping production, enhanced spring green-up, enhanced winter hardiness, etc., have been well established.
Still, as the discussion about nitrogen has evolved to include water quality, research has indicated that independent of the source of nitrogen the later in the season the application is made the more leaches into the groundwater. Clearly there is an environmental concern related to late season nitrogen use, in-spite of the well-established agronomic benefits. It then becomes a question of balancing the two needs.
Several studies have investigated sources and timing to reveal some interesting results. Oddly, while most of the research was conducted with various nitrogen sources, it was always applied at the “sacred” one pound of actual nitrogen per 100-square-feet rate.
More recent cool-season turfgrass research on Kentucky bluegrass and perennial ryegrass at the University of Wisconsin-Madison and here at Cornell University has begun investigating reduced rates using different nitrogen sources applied at different timings from September through December. To date, it appears the agronomic benefit from late season nitrogen can be achieved by applying inexpensive forms of water-soluble urea or ammonium sulfate at 0.3 to 0.5 pounds per 1,000 square feet in September or October.
These earlier application times combined with the lower application rates provide adequate agronomic benefits with reduced overall leaching problems. This is the kind of research we need to get out into the hands of practitioners and regulators to help them enact enforceable, science-based regulations. Not regulations based on conjecture.
As an industry, we need to be open to the evolving ideas that science brings to enhance our precision. In the end, it will lead to improved efficiencies. Heck if we get the same response with less nitrogen that was leaching anyway and the sources we use are less expensive, who’d argue with that?
Of course many scientific and logistical questions remain about late season nitrogen. There are questions concerning uptake mechanisms, evapotranspiration, disease issues and further refining application strategies before a complete picture can be drawn, but for now as Twain would say, let the conjecture begin.
The author is associate professor at Cornell University.