5 tips for fall fertilization

As growth slows down and dormancy approaches, fall presents a crucial time for fertilization. Here are five tips to ensure turf stays strong heading into winter.

Watch growth rates
A first step to making a fall fertilizer application is to ensure the turf is actively growing, says Dr. Cale Bigelow, professor of agronomy at Purdue University. “If you don’t have an actively growing plant, it’s not going to take up nutrients,” he says.

New stands of turf or turf that has thinned out because of summer stress will require different rates. Superintendents will have to water nutrients into the soil, perhaps more frequently and in smaller increments, Bigelow says. “Let’s just say your target number is three units, and if you were going to do three one-unit applications, maybe you break it into six half-unit applications,” he says. “That’s not uncommon on a putting green.”

Apply less nitrogen
With nitrogen, a little bit goes a long way, says Dr. Michael Goatley, professor and extension turfgrass specialist at Virginia Tech. “When you stimulate a lot of late-season growth, you get a very succulent, moisture-rich plant, and those tissues are going to be much more likely to be damaged by extreme cold,” he says. “So you’re always trying to play a balancing game.”

The standard nitrogen rate for warm-season grasses of one pound per thousand square feet should often be reduced as the days get shorter and temperatures cool, Goatley says. Somewhere between a quarter- to a half-a-pound per thousand square feet should keep the grass green and photosynthetically active.

Applications in the North should be smaller still, Goatley says. “In the cool-season world, you basically survived the summer and maybe you’ve been spoon-feeding that turf with a tenth or .15 pounds of N, and that’s highly variable depending on the superintendent and their site, but just a little bit to keep the plant active but not overactive, and fall is the preferred time to really get aggressive,” he says.

For cool-season grasses in the Transition Zone, superintendents often continue to apply a pound of nitrogen per thousand square feet into the first couple weeks of November, he says. “That fits our window of opportunity quite well,” he says.

Provide adequate potassium
Fall applications of potassium are beneficial for maintaining the health of both warm- and cool-season turf and protecting those grasses from winter damage, Goatley says. “Fall is probably an optimal time to get it into cool-season grasses, and before we enter frost, it still is a period in which you can get a late-season potassium uptake response in warm-season,” he says.

While many professors and superintendents advise referring to soil tests to help determine potassium application rates, many superintendents over the years have paired nitrogen applications with equal rates of potassium, Goatley says. “That has long been kind of the standard, and that’s almost a given in the world of bentgrass management, especially for putting greens,” he says. Superintendents with Bermudagrass greens also follow the same practice. Too much potassium is usually not an issue until deficiencies appear in magnesium, calcium and other cations.

Don’t wait on nitrogen
Research has shown that superintendents might want to consider applying nitrogen earlier than experts have recommended in the past, Bigelow says. “For our region we actually get pretty good results finishing up around Halloween,” he says. However, application dates depend on how many applications superintendents plan to make. “We’ve realized based on the research that we’ve done in this region, from here north, that two applications are fine the second, third week of September and then again five or six weeks later,” he says. That’s for cool-season, lawn-height turf.

For their second application between mid-October and the first week of November, some superintendents choose to use ammonium sulfate on fairway or putting green turf, Bigelow says. Ammonium sulfate is slightly more expensive than urea, he says. “I think you’re splitting hairs in terms of difference (in quality),” he says. “Some of that’s going to be weather-dependent.”

Conduct soil tests
Soil tests are a major factor in determining whether a superintendent should make a fall fertilizer application and what that application should be, Bigelow says. “If they haven’t been doing the soil testing, particularly on their sandy soils, that’s key in terms of developing their nutrient management plan,” he says.

Phosphorus applications should be entirely based off soil tests, Bigelow says. A critical element, phosphorus produces new roots and sustains plant health and energy, but phosphorus runoff can lead to environmental degradation and other issues, which have prompted many government regulators to limit phosphorus applications strictly to those instances where soil tests call for it.

If a superintendent conducts a soil test in March that yields medium values, and then a significant amount of rain falls throughout the year, a fall potassium application could provide additional benefit to the turf, Bigelow says.

Patrick Williams is a GCI contributing editor.