U of MN grounds manager Jonathan Spitzer applies a 50/50 blend of quick release and slow release nitrogen sources to the St. Paul campus turf in the late-September.
ST. PAUL, Minn. (10/3/2012) —University of Minnesota Extension has adjusted long-standing recommendations on fall nitrogen fertility to turfgrass and now advocates for applications to be made no later than mid-October. This recommendation aims to reduce the negative environmental impacts of nitrogen applications in the late fall, while maximizing the turfgrass growth benefits for the cost-conscious consumer or turfgrass manager.
The previous recommendation on late fall-applied nitrogen was one pound of quick-release nitrogen per 1000 square feet after the last mowing of the season.
"That meant late October or early November," said Extension turfgrass educator Sam Bauer, whose research helped lead to the new recommendation. "The idea was that the nitrogen is absorbed by the roots, promoting plant growth the following spring." However, Bauer's research showed that the uptake capacity for turf during cold temperatures is typically very low, meaning that much of the nitrogen in fertilizer would be lost to the environment.
When nitrogen (particularly nitrate) leaches into the ground water, it can affect the safety of drinking water supplies. While properly maintained landscapes have been proven to be very receptive of nitrogen applications, late fall is considered a susceptible time of year for nitrate leaching to occur.
Collaboration between the University of Minnesota and University of Wisconsin-Madison was important in determining the effects of nitrogen applications throughout the region. Controlled-environment research at Madison demonstrated that only 11 percent of the fertilizer nitrogen was taken up by the turf under a mid-November temperature regime. Mid-September applications resulted in greater than 75 percent nitrogen absorbed and utilized for growth.
During Bauer's putting green research in the fall of 2009, temperature played more of a role than the specific date of application. "We had several cases in which upper soil temperatures dropped to nearly 40 degrees in October," said Bauer. "When it rained following a nitrogen application, significantly less nitrogen was recovered in the plant under these low temperatures." Generally, the remaining nitrogen would be lost to the environment.
While the recommendation is now to complete any fertilizer applications by mid-October, it's also important to reduce fertilizer rates during the month or to use a fertilizer that contains at least half of its nitrogen in slow-release form.
"Considering that fertilizer prices have tripled in the last decade, applications extending into November also meant a loss to the pocketbook," said Bauer. "Unfortunately, there's little evidence to show these added costs guarantee a lush, green lawn the following spring."
The new Extension recommendations are not meant to downplay the importance of adequate nitrogen for winter survival of turf, but rather to shed light on the fact that homeowners and turfgrass managers should not consciously apply fertilizer in environments conducive to nutrient loss, such as the late fall when growth potential is low.
In order to maximize nitrogen absorption, Extension recommends that homeowners or turfgrass managers:
- Make all fertilizer applications before mid-October.
- Combine quick-release and slow-release nitrogen sources when applying rates above 0.5 lb. nitrogen per 1000 sq. ft.
- Be aware of temperature and precipitation impacts on your fertilizer applications.
Visit www.extension.umn.edu/garden/turfgrass for more turfgrass information.