Rethinking fall soluble nitrogen fertilization of cool-season grasses
Doug Soldat, Ph.D., is a turfgrass scientist at the University of Wisconsin-Madison, where he directs his research efforts to better understanding soil- and fertility-related problems facing superintendents. Soldat can be reached at djsoldat@wisc.edu.
Q: What led you to conduct research on fall and late fall soluble nitrogen fertilization programs for cool-season grasses?
In graduate school I was taught to question the conventional wisdom. In terms of nitrogen fertilization of cool-season grasses, we learned that soluble nitrogen should be applied when turfgrass is actively growing. But we also learned cool-season grasses should be fertilized with soluble nitrogen in fall and late fall.
These two concepts seemed to be in opposition to each other, so the first research project I started when I was hired at the University of Wisconsin was to take a critical look at fall and late fall soluble nitrogen fertilization of cool-season turf.
Q: What were the findings of the research?
I want to emphasize that our research was with soluble nitrogen only. We found that the ability of cool-season turf to take up nitrogen decreased the later into fall we applied isotopically labeled nitrogen fertilizer. For soluble nitrogen applied on Sept. 15, Oct. 15 and Nov. 15 in Madison, Wis., nitrogen uptake efficiency was 80 percent, 20 percent and 10 percent, respectively. We tracked nitrogen uptake until June 1 the year following application, so we were able to measure the amount of applied nitrogen taken up in fall and spring.
Q: Why is the nitrogen not being taken up in mid-October and mid-November?
Nitrogen is taken up by plants primarily through a process called mass flow. As water moves through soil to roots to be taken up for transpiration, nitrogen is carried along with the water and taken up by the plants. The greater the rate of transpiration, the more nitrogen is being carried to the plant roots by mass flow.
In mid-September evapotranspiration (ET) is high, so nitrogen is being carried to the roots and the plants take up the nitrogen. In mid-October and mid-November ET is low, so only a small amount of mass flow is occurring, and therefore only a small amount of nitrogen is taken up by the turf plants.
Q: Based on the results of your research, what approach to nitrogen applications in the fall and late fall do you recommend?
For putting greens in the northern portion of the cool-season zone, if a superintendent is planning to apply more than 0.5 pounds of soluble nitrogen per 1,000 square feet in a single application in the fall, that application should be made prior to Sept. 15. Starting in early October and throughout the rest of the fall, nitrogen should be applied in small doses like 0.1 pounds of nitrogen per 1,000 square feet every two weeks until the end of the growing season. Spoon feeding nitrogen at the end of the growing season will provide good green color and have greater uptake efficiency than a large application.
For tees, fairways and rough, fertilizing prior to Sept. 15 with a combination of soluble and slow-release fertilizer will work well.
Q: What’s next in this line of research?
We think calendar systems aren’t accurate enough for nitrogen applications. Based on our research, we believe ET rate may be a good guideline to follow to determine when to apply soluble nitrogen in fall. Work on turfgrass growth models based on temperature done by Larry Stowell, Ph.D., of PACE Turf, also offers guidelines concerning when to fertilize turf that is an improvement over a calendar system. Preliminary work by Bill Kreuser, Ph.D., at the University of Nebraska using soil organic matter indicates it may also be useful in determining nitrogen application timing.
An approach that considers climate and soils data like ET, temperature and soil organic matter may turn out to be the best approach — certainly better than calendar-based approaches.
