A two-year field study was conducted in summer 2020 and 2021 at the William H. Daniel Turfgrass Research and Diagnostic Center in West Lafayette, Ind. Photos by Jada Powlen
Nitrogen is required for plant growth and development and impacts turfgrass vigor, canopy growth, green color and density (10). Seasonal nitrogen requirements and overall nitrogen needs during turfgrass establishment can vary by geographic location, soil
type and turfgrass species. One of the primary warm-season turfgrass species used for golf courses throughout the temperate regions is bermudagrass (Cynodon dactylon). Although bermudagrass is commonly established using sod or sprigs, seeded cultivars
of bermudagrass are gaining in popularity due to flexibility in establishment scheduling, increased genetic diversity, improved winter hardiness and overall reductions in establishment costs compared to other establishment methods (3, 6). Although
annual nitrogen requirements of established bermudagrass have been well documented, there is limited information regarding the optimal establishment nitrogen requirements for seeded bermudagrasses. General recommendations are to apply 1 pound nitrogen
per 1,000 square feet (49 kilograms nitrogen per hectare) at seeding with repeat applications every four weeks after seedling emergence using the same nitrogen rate (7, 9).
While the application of 1 pound nitrogen per 1,000 square feet every four weeks after emergence has been recommended, many turfgrass managers often apply much more than this recommendation during establishment, with some managers applying up to 1 pound
nitrogen per 1,000 square feet every week with the hope of speeding canopy coverage (4). Frequent high-nitrogen applications using readily soluble nitrogen sources during establishment could result in poor rooting, excess shoot growth, and reductions
in rhizome and stolon mass, and may lead to non-target-water-quality issues due to exceeding the capacity of the plant to absorb nitrogen (runoff and leaching) (2). Further, excess nitrogen applications are economically wasteful due to unnecessary
fertilizer and labor costs. Reducing application frequency and/or utilizing controlled-release fertilizer products could potentially reduce total nitrogen requirements during establishment and minimize off-target nitrogen movement (1, 5). Thus, the
objective of this two-year field study was to determine the optimal grow-in nitrogen establishment program for a seeded bermudagrass cultivar on a native soil using two nitrogen sources and variable application rates and frequencies.
Table 1. Nitrogen application rates and application frequencies using ammonium sulfate (AMS) or polymer coated urea (PCU) applied to seeded bermudagrass among various days after planting (DAP).
Materials and methods
A two-year field study was conducted in summer 2020 and 2021 at the William H. Daniel Turfgrass Research and Diagnostic Center in West Lafayette, Ind. Rio bermudagrass was seeded on June 3, 2020, and June 9, 2021, at 1 pound pure live seed per 1,000 square
feet using a 20-inch-wide (50.8-centimeter-wide) drop spreader. The study area was covered with a vegetative establishment blanket to minimize potential seed movement. Irrigation was applied lightly and frequently to keep the seedbed moist until visible
germination and reduced thereafter to maintain optimal moisture for seedling development.
Eight grow-in granular nitrogen fertilization programs, which varied by nitrogen source, application rate and frequency, were evaluated in this study (Table 1). Fertilizer sources were either ammonium sulfate (AMS; 21% nitrogen-0% phosphorus-0% potassium)
at planting and after emergence, or a polymer-coated urea (PCU; 44-0-0) at planting only. Total nitrogen applied over the potential six applications ranged from 0.0 to 6.0 pounds nitrogen per 1,000 square feet (0 to 293 kilograms per hectare), with
applications occurring only at planting (all treatments) or every seven or 14 days after initial visible germination (approximately 21 days after planting) (AMS only). In Year One, applications were initiated on June 3, 2020, and repeat applications
of AMS programs occurred on June 26, July 6, 14, 20 and 27, 2020. Fertilization treatments in Year Two began on June 9, 2021, with repeat applications on July 2, 9, 16, 23 and 31, 2021. Irrigation (approximately 0.2 inches/0.5 centimeters) was applied
immediately following nitrogen applications. There were four replications of each fertility program, and each plot measured 3 feet by 8 feet (0.9 meters by 2.4 meters). Mowing began approximately 30 days after planting using a 20-inch walk behind
reel mower at a height of 0.5 inches (1.27 centimeters) with clippings collected. Mowing took place every five to seven days thereafter. In 2021, canopy growth rate (dry matter yield) was measured 47 and 54 days after planting (July 27 and Aug. 3,
respectively) to gain an understanding about how the nitrogen program might affect potential mowing requirements. After seven days from the previous mowing event, turf was mowed, with leaf tissue collected from each plot. Clippings were transferred
to a paper bag, placed in a drying oven at 155 F (68 C) for five days and weighed.
Bermudagrass green coverage was determined using digital image analysis, with images taken approximately every five days after germination. A light box was used to take images, and percent green coverage was calculated using TurfAnalyzer software program.
During each study year, establishment rate [days to 50% green coverage (GC50), 75% green coverage (GC75) and 90% green coverage (GC90)] for each fertility program was determined using green coverage calculated from digital image analysis. The relationship
between total nitrogen applied and days to 90% green coverage was also modeled.
Table 2. Days to achieve 50% green coverage (GC50), 75% green coverage (GC75), and 90% green coverage (GC90) for various nitrogen rates and frequencies with ammonium sulfate (AMS) or polymer-coated urea (PCU) applied to seeded bermudagrass in West Lafayette, Ind., during 2020 and 2021. Means in the same column followed by a common letter were not significantly different according to Fisher’s protected LSD (P<0.05).
Results
Bermudagrass establishment varied by study year, total nitrogen rate and application frequency (Table 2). Establishment rates were slower in Year Two (2021) compared to Year One (2020). This was potentially due to reduced growing degree days (104 fewer)
during the first 60 days of establishment. During Year One, the nonfertilized control had the slowest establishment rate when compared to all nitrogen programs (Table 2). Time to reach GC50 and GC75 in Year One ranged from 20.1 days to 24.1 days and
23.6 days to 27.9 days, respectively. Among the nitrogen programs, PCU_2.0_@plant (PCU applied at 2 pounds nitrogen per 1,000 square feet/146 kilograms nitrogen per hectare at planting) reduced days to GC50 compared to AMS_0.25_7d, AMS_0.5_7d, and
AMS_0.5_14d (Figure 1). The time to reach GC90 ranged from 26.5 days to 32.6 days among treatments. AMS_1.0_7d and AMS_1.0_14d reduced time to GC90 by 5.7 days compared to the nonfertilized control. Fertilization programs using AMS totaling 2, 3 and
6 pounds nitrogen per 1,000 square feet (98, 146 and 293 kilograms nitrogen per hectare) and 2 pounds nitrogen per 1,000 square feet applied at planting with PCU reached the GC90 within approximately the same amount of time when modeling the relationship
between total nitrogen and time to GC90.
Time to GC50 in Year Two ranged from 23.1 days to 30.0 days for AMS_1.0_7d and nonfertilized control, respectively. Further, fertilization programs with higher rates of total nitrogen reduced GC90 in Year Two. Among the fertilization programs, days to
GC90 ranged from 32.7 days to 41.4 days for AMS_0.5_7d and nonfertilized control, respectively. Differences in establishment time between PCU treatments were not observed in Year Two. In Year Two, AMS_1.0_7d (6 pounds nitrogen per 1,000 square feet
total) increased canopy growth rate greater than 94% compared to the nonfertilized control (data not shown).
Among the fertilization programs, canopy growth rate increased by 0.4 pounds per square feet (19.5 kilograms per hectare) per day as total nitrogen increased by 1 pound nitrogen per 1,000 square feet. Additionally, the PCU programs generally had the least
canopy growth rate compared to AMS programs 54 days after planting.
Figure 1. Bermudagrass coverage as determined by digital image analysis among nitrogen fertilization programs using ammonium sulfate (AMS) applied every seven or 14 days after planting with application rates ranging from 0.25 to 1 pound nitrogen per 1,000 square feet, or polymer-coated urea (PCU) applied only at seeding at 1 or 2 pounds nitrogen per 1,000 square feet in 2020 (A) and 2021 (B).
Conclusion
Nitrogen is essential for seedling development, but there is a temptation for turf managers to apply nitrogen (e.g., 1 pound nitrogen per 1,000 square feet per week) more than plant need or uptake capacity with the hope of substantially speeding time
to 100% green coverage. Applying AMS at 1 pound nitrogen per 1,000 square feet per week during the first 60 days did not increase time to GC90 but did increase canopy growth rate, which may necessitate excess mowing. Based on the results of this study,
a single application of PCU at planting (2 pounds nitrogen per 1,000 square feet), or the application of AMS at planting (1 pound nitrogen per 1,000 square feet) followed by a repeat application after bermudagrass germination would be sufficient to
effectively reach 90% green coverage when planting during the optimal seeding time in the northern transition zone. When turf managers are selecting a fertilization approach during seeding, they should consider the overall cost of these fertilization
programs, including the labor costs of increased application frequency and the specific cost per pound of nitrogen of the fertilizer source (water-soluble versus controlled-release fertilizer sources). Additionally, factors such as climate, soil type,
bermudagrass cultivar and schedule for intended use should be taken into consideration as well (7). In summary, when bermudagrass is seeded in June (in the northern transition zone) and where rapid establishment is desired, the application of 2 to
3 pounds nitrogen per 1,000 square feet during the first 60 days after planting was sufficient to maximize seedling establishment on a silt-loam soil. Further, this amount of nitrogen did not result in excessive canopy growth rate, which would necessitate
increased mowing requirements and increased establishment costs and potentially cause injurious mechanical damage to young seedling plants.
Overview of various fertilization programs using ammonium sulfate (AMS) and polymer coated urea (PCU), applying 0.25, 0.5, 1 or 2 pounds nitrogen per 1,000 square feet every seven days, 14 days or at planting only to establish seeded bermudagrass. Images taken on July 9, 2021, 30 days after planting.
Acknowledgements
The authors would like to thank Max Schimmel for assisting in data collection for this study. This research was funded by the Midwest Regional Turfgrass Foundation and the Purdue College of Agriculture.
The research says
- A single application of PCU at planting (2 pounds nitrogen per 1,000 square feet) or the application of AMS at planting (1 pound nitrogen per 1,000 square feet) followed by a repeat application after bermudagrass germination would be sufficient to
effectively reach 90% green coverage when planting during the optimal seeding time in the northern transition zone.
- When selecting a fertilization approach during seeding, consider the overall cost of these fertilization programs, including the labor costs of increased application frequency and the specific cost per pound of nitrogen of the fertilizer source.
- When bermudagrass is seeded in June (in the northern transition zone) and where rapid establishment is desired, the application of 2 to 3 pounds nitrogen per 1,000 square feet during the first 60 days after planting was sufficient to maximize seedling
establishment on a silt-loam soil.
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This article is intended for educational purposes, and further comprehensive information can be accessed in our original publication in Crop, Forage & Turfgrass Management (https://doi.org/10.1002/cft2.20250).
Jada Powlen, Ph.D., (jpowlen@purdue.edu) is lead research scholar, and Cale Bigelow, Ph.D., is a professor of turf science, management and ecology in the Department of Horticulture and Landscape Architecture at Purdue University, West Lafayette, Ind.