Fertilization is a critical step in maintaining a healthy, vigorous turf. Applying nitrogen fertilizer in fall promotes turf recovery, but the type of fertilizer used -- quick release vs. slow release -- should change based on when your applications are made.
Early fall fertilization
From late August to late September, evapotranspiration, or water loss through the grass leaf surface, is still high. This is due to continuing warm summer temperatures, high light conditions and relatively long days resulting in active grass growth. This causes the turf to “suck” up nitrogen as it becomes available to the roots, pulling it into the crown and leaves. This “sucking” movement is similar to drinking from a straw.
Since grass is actively growing and utilizing fertilizer in early fall, use a granular fertilizer with 50% quick release nitrogen (water-soluble nitrogen, WSN) and 50% slow release or controlled release ntrogen. This ratio provides an even release of nutrients for eight to 12 weeks.
To determine whether the product you are buying contains quick release nitrogen or slow release, check the active ingredient statement on the front of the bag.
• Water soluble nitrogen sources: urea, ammonium sulfate, ammonium nitrate, potassium nitrate, calcium nitrate, monammonium phosphate, diammonium phosphate
• Slow release nitrogen sources: sulfur coated urea, urea formaldehyde, IBDU, polymer coated fertilizer (PCF), Milorganite
Late fall fertilization
As we move further into fall, from early October to late November, lower light intensity, shorter day length and cooler temperatures reduce the "sucking" force of evapotranspiration and also reduce the grasses' ability to pull in nitrogen fertilizer.
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This reduces uptake efficiency and increases the risk of nitrogen loss to the environment during winter. If grass is not able to use up fertilizer applied in late fall, the best-case scenario is that the nitrogen sits in the soil and causes a large growth flush next spring. In the worst case scenario, nitrogen is moved off the landscape, either by leaching downward through the soil below the grass roots or through surface runoff into surface water (creeks, lakes and ponds) where it causes algae blooms next spring.
In addition, slow release fertilizers that require microbial degradation lose effectiveness in late fall as the soils cool and microbial activity slows.
For these reasons, use fertilizers containing more quick release nitrogen in early October and all quick release nitrogen in late October to maximize uptake prior to winter. Early October applications do increase turfgrass color and improve plant health going into the winter.
No fertilizer should be applied to turfgrass after October because uptake efficiency is low. Yes, grass roots can still take up small amounts of nitrogen in November, but the total amount is low.
This means ideally one fall turfgrass fertilizer application should be made from late August to late September. An October application is not absolutely necessary and can be skipped on most home lawns. But if an October application is made, use a quick release nitrogren source.
Lawn management calendars
There are many considerations when caring for a lawn. From scheduling mowing and irrigation to controlling weeds and white grub control, it can be difficult to stay on top of management practices at the proper time of year.
With this in mind, Nebraska Extension turf specialists Bill Kreuser has put together management calendars for cool- and warm-season Nebraska lawns summarizing common turf management practices. These new lawn management calendars should simplify your decision-making process and help you identify the best time of year for weed control and fertilization. They also list the total pounds of N recommended for each time of year.
Management Calendar for Cool-Season Lawns (Kentucky Bluegrass and Tall Fescue),http://turf.unl.edu/TurfAdvice/Cool-season%20lawn%20calendar.pdf
Management Calendar for Warm-Season Lawns (Buffalograss and Zoysiagrass),http://turf.unl.edu/TurfAdvice/Warm-season%20lawn%20calendar-.pdf
Sarah Browning is an extension educator with Nebraska Extension. To ask a question or reach her, call 402-441-7180 or write to her at firstname.lastname@example.org or 444 Cherrycreek Road, Lincoln, NE 68528.