With rising economic and environmental concerns regarding the efficient use of fertilizers in urban settings, it becomes important to understand the role that late-fall fertilization plays in our lawn care program. Long-standing recommendations for late-fall nitrogen fertility involved the use of quick release nitrogen sources (urea, ammonium sulfate, others) to be applied after the last mowing of the year. The theory was that the nitrogen would be absorbed by the turfgrass roots prior to winter, but would not be utilized for growth until the following spring. While this theory seems reasonable, and generally results in a healthier lawn, the predictability of quick release nitrogen applications at this time is low.
Photo 1: 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 late-September.
Collaborative research between the University of Minnesota and the University of Wisconsin-Madison has demonstrated that turfgrass absorption of nitrogen reduces as temperatures cool later into the fall. We refer to this as a reduction in turfgrass nitrogen use efficiency (TNUE). In climates that are conducive to a reduction in TNUE, as in the case with slow growth associated with late-fall temperatures, fertilizer applications have a greater potential to move off-site. This off-site loss is particularly concerning due to environmental and economic implications.
Consider the fact that the costs of producing nitrogen fertilizers have more than tripled in the last decade due to the rising price of fossil fuels used for nitrogen fertilizer production. Additionally, the Environmental Protection Agency has placed a 10 ppm nitrate standard on drink water. These are the driving factors for refining the late-fall nitrogen fertilizer recommendations, as we can no longer afford to make nitrogen fertilizer applications that have a high potential to move off-site.
The new recommendations can be summarized as follows:
- Make final nitrogen fertility applications no later than mid-October
- Combine quick release and slow release nitrogen sources when applying more than 0.5 pounds of nitrogen per 1000 square feet
- Only apply fertilizer to actively growing lawns, because TNUE reduces when growth is low
Phosphorus and potassium applications should always be based on soil test results. Soil testing information and submission forms can be found at the University of Minnesota Soil Testing Laboratory website:
For more detailed information regarding the use of home lawn fertilizers follow these links to a three part discussion from retired Extension Turfgrass Educator Bob Mugaas.
Understanding and Using Home Lawn Fertilizers- Part 1: The Basics
Understanding and Using Home Lawn Fertilizers- Part 2: Nitrogen
Understanding and Using Home Lawn Fertilizers- Part 3: Phosphorus and Potassium