Nitrogen needs

Features - Fertilizer/Soil Health

How higher prices and new science could change your application rates.

July 27, 2011
Frank S. Rossi

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 of 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?

Late-season nitrogen. Some of the oldest turfgrass research has espoused the benefits of applying nitrogen to turf at the end of the growing season prior to the onset of winter.

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” 1 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 - 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 an associate professor at Cornell University.