Nitrogen fertilizers are the primary component of most turfgrass nutrition programs. The average 18-hole golf facility applies approximately 2 pounds of nitrogen/1,000 square feet (97.6 kilograms/hectare) annually (3). Thus, it is not surprising that a large portion of research funds has been directed toward improving nitrogen use.
Past studies have explored the influence of nitrogen fertilizers on turfgrass growth rate and quality, environmental concerns, etc. However, few studies have investigated the longevity of turfgrass response to nitrogen fertilizers. Soluble products such as urea and ammonium sulfate may release all of their nitrogen within only a few hours, whereas slow-release sources like polymer-coated ureas and natural organics may require several months to release their nitrogen.
A study conducted in New York found that some slow-release products resulted in the same turfgrass response longevity as urea (6), whereas a study conducted in Florida found that polymer-coated urea actually resulted in less turfgrass response longevity than urea (7). This is rather curious considering slow-release nitrogen fertilizers are documented as having a longer release than soluble sources (5). In any case, none of the previous studies considered the cost associated with nitrogen fertilizers.
It has been well established that a pound of nitrogen from urea will release faster than a pound of nitrogen from a slow-release nitrogen source. Thus, it is plausible that a more expensive slow-release nitrogen source may be less expensive than a soluble nitrogen source if the sources are compared based on longevity of response rather than on the cost of nitrogen. This concept has floated around the industry in various marketing claims for several years. However, studies designed to determine the validity of such claims have not been published. Therefore, the hypothesis of this study was that nitrogen sources influence the cost of nitrogen fertilizer compared with the cost of urea alone when turfgrass response is included in the cost.
The experiment
This study was conducted during 2018 at the University of Florida’s Research and Education centers in Jay, Fla., and in Fort Lauderdale, Fla. The experimental design was a split plot in a randomized complete block design using four replications. Treatments included non-treated turfgrass and 10 nitrogen sources (Table 1), which were applied at nitrogen rates of 1 and 2 pounds nitrogen/1,000 square feet (48.8 to 97.6 kilograms/hectare) every four months.
Click on table to enlarge:
Table 1. Days of turfgrass response, area under the curve (AUC), cost in dollars per day of response, and cost in dollars per AUC as influenced by nitrogen source on Celebration bermudagrass in 2018 in Jay and Fort Lauderdale, Fla. Data are pooled across the 1- and 2-pound nitrogen rates.
The interaction between nitrogen source and nitrogen rate was insignificant on an annual basis, and results were thus pooled across nitrogen rates. Nitrogen sources were assigned to whole plots (6.5 × 9.8 feet; 2 × 3 meters), and nitrogen rates were assigned to subplots (3.2 × 9.8 feet; 1 × 3 meters). Turf response longevity and area under the response curve (AUC) from non-treated turfgrass were subtracted from treated turfgrass.
Treatments were applied every four months to coincide with best management practices. Turfgrass was irrigated with 0.25 inch (0.635 centimeter) of water following each treatment application. Treatments were applied by hand to Celebration bermudagrass grown on a Hallandale fine sand in Fort Lauderdale and on a Fuquay loamy sand in Jay. The turfgrass in Fort Lauderdale had been established two years prior, whereas the turfgrass in Jay had been established for more than five years.
Turfgrass was cut at a height of 0.5 inch (1.27 centimeters) with reel mowers. Post- and pre-emergence herbicides, fungicides and insecticides were applied uniformly to all plots as needed to prevent pest damage. Irrigation supplied water equivalent to 80% of the previous week’s reference evapotranspiration and was automatically stopped for 24 hours when rainfall exceeded 0.25 inch during any 24-hour period. Following each treatment application, potassium and phosphorus were applied to turfgrass plots that did not receive those elements from the main treatment, to balance out any effect from non-nitrogen elements.
Turf quality was recorded weekly on a scale of 1 to 9, where 1 is dead/brown turf and 9 is optimally healthy/green turf. Turf quality ratings of 6 or higher were considered acceptable. Turfgrass response longevity was determined as the number of days that turfgrass quality was greater than or equal to 6.0. The AUC was calculated using graphing software to provide a measure of both response longevity and magnitude (Figure 1, below). The theoretical maximum AUC was 1,460 quality-days, which was equivalent to a turf quality rating of 9 for 365 days.
Turfgrass response curve
Figure 1. An example of a turfgrass response curve showing the magnitude and length of response to nitrogen fertilizers. The area under the curve (AUC) combines both length and magnitude of response and was found to better represent the differences between nitrogen fertilizers than using length of response alone.
Fertilizer sale prices were determined by taking the average price from four Florida fertilizer distributors, except for proprietary products sold by only one distributor. Fertilizer cost per day of response (dollars/acre/day) and cost per AUC (dollars/acre/quality-day) were determined by dividing the nitrogen source cost (dollars/pound nitrogen) by the response days or AUC, respectively.
Results and discussion
Response longevity
The total annual response longevity is the measurement of how long the turfgrass responded to each fertilizer and was not influenced by nitrogen source in each location (Jay or Fort Lauderdale) (Table 1). Total annual response longevity ranged from 122 to 152 days in Jay and from 191 to 245 days in Fort Lauderdale. Determining turfgrass response longevity by measuring the days turf quality was equal to or greater than 6 is a unique metric and has not been previously reported. Other researchers have documented the influence of nitrogen sources on turf quality over time, but these studies were not designed to determine the number of response days (1, 2). However, when previous research reported turf quality by months over a year, response longevity could be inferred.
These results compare favorably with those reported by Soldat et al. (6), who documented turfgrass quality in response to slow and soluble nitrogen sources on Kentucky bluegrass, a blend of creeping bentgrass with annual bluegrass, and a blend of Kentucky bluegrass and fine fescue in New York. They reported that stabilized urea, methylene urea and urea formaldehyde resulted in similar response longevity (days turf quality ≥ 6) as urea on each sward.
Differing results were reported by Young et al. (7), who measured the influence of nitrogen sources on St. Augustinegrass in Fort Lauderdale and reported that polymer-coated urea resulted in 32 fewer days than urea. The reduction in response days occurred during the 32 days following the initial treatment applications. Presumably, this delayed response was due to the time required for the polymer to begin releasing the nitrogen. This delay was not observed in the current study, and, nevertheless, nitrogen sources resulted in similar response longevities. This implies that although the release characteristics of nitrogen sources differ, the turfgrass response to nitrogen sources may not be concomitant.
Area under the response curve
Total AUC is the measurement of longevity (duration of greening) combined with magnitude of response (increase in greenness). Total AUC was influenced by nitrogen sources in both locations, varying from 202 to 279 in Jay and from 302 to 456 in Fort Lauderdale (Table 1).
In Jay, Duration CR resulted in a 28% increase in AUC compared with urea; all other nitrogen sources resulted in increases similar to that produced by urea. In Fort Lauderdale, no nitrogen source resulted in greater AUC than urea, and, in fact, compared with urea, methylene urea reduced AUC by 29%. In Fort Lauderdale, the greatest AUC was found to be from ammonium sulfate, which resulted in a 25% AUC increase compared with both polymer-coated ureas and a 32% AUC increase compared with methylene urea and urea-formaldehyde. Because ammonium sulfate did not increase response longevity compared with reacted ureas, the increased AUC was a result of ammonium sulfate increasing turf quality rather than longevity.
Reacted ureas contain water-insoluble nitrogen, which requires microbial degradation to become available to the plant. The microbial degradation process has been identified as a causal factor in reducing turfgrass quality, color, and nitrogen uptake from reacted ureas when compared with urea, especially during the initial weeks following application (4). However, continued applications of reacted ureas may allow the water-insoluble nitrogen from prior years to degrade and increase plant-available nitrogen. This accumulation effect has not been documented using AUC. Single-year studies, such as the current study, may not be capable of capturing this potential benefit of slow-release nitrogen sources. Therefore, multiyear studies that use AUC to measure turfgrass response to nitrogen fertilizers would be valuable.
Differences among nitrogen sources were documented using total annual AUC, whereas differences among nitrogen sources were not documented using annual cumulative days of response in either Jay or Fort Lauderdale. This indicates that measuring both turfgrass response longevity and the magnitude of response may be more useful than measuring only response longevity when comparing the agronomic value of nitrogen sources.
Cost of urea
The cost of urea was less than or equal to other nitrogen sources in Jay and Fort Lauderdale using dollars/acre/day and dollars/acre/quality-day (Table 1). Among slow-release nitrogen sources, the cost of sulfur-coated urea was less than or equal to other slow-release nitrogen sources. Both natural organics were approximately 4.6 times as expensive as urea under Jay, Fla., conditions. As in Jay, natural organics in Fort Lauderdale were approximately six times as expensive as urea when using either measurement method. Because natural organics and urea resulted in similar response days and AUC, the increased cost of natural organics was a result of their sale price (dollars/pound of nitrogen), which was also approximately six times that of urea.
Summary
The agronomic value of slow-release nitrogen sources includes reduction of environmental risk and the reduction of potential turfgrass burn. These benefits alone are arguably sufficient to warrant their inclusion in nutritional programs. However, the perceived notion that slow-release nitrogen fertilizers prolong turfgrass response may be overestimated.
This study, like others, has shown that extended turfgrass response from slow-release nitrogen sources does not always occur. On all but one occasion, both urea and ammonium sulfate were found to provide length and magnitude of response equivalent to those of the slow-release nitrogen sources used in this study. As a result, the cost of the slow-release nitrogen sources was not reduced when turfgrass response was factored in.
In the end, the cost of nitrogen sources based on dollars per pound of nitrogen was determined to be a valid assessment method. Therefore, superintendents wishing to fairly compare their nitrogen sources may be best served by simply using the cost per pound of nitrogen.
Further reading
The scientific article published about this research is available free: Determining nitrogen fertilizer cost using turfgrass response by T.W. Shaddox and J.B. Unruh in HortTechnology.
The research says ...
- The idea that slow-release nitrogen fertilizers prolong turfgrass response may be overestimated.
- Both urea and ammonium sulfate were found to provide length and magnitude of response equivalent to those of the slow-release nitrogen sources used in this study.
- Superintendents wishing to fairly compare their nitrogen sources may be best served by simply using the cost per pound of nitrogen.
Literature cited
- Carrow, R.N. 1997. Turfgrass response to slow-release nitrogen fertilizers. Agronomy Journal 89(3):491-496 (https://doi.org/10.2134/agronj1997.00021962008900030020x).
- Cisar, J.L., G.H. Snyder, J.J. Haydu and K.E. Williams. 2001. Turf response to coated-urea fertilizers. II. Nitrogen content in clippings, nitrogen uptake, and nitrogen retention from prills. International Turfgrass Society Research Journal 9:368-374.
- Golf Course Superintendents Association of America. 2016. Golf course environmental profile: Nutrient use and management on U.S. golf courses, Vol. III. Golf Course Superintendents Association of America, Lawrence, Kan. (https://www.gcsaa.org/uploadedfiles/Environment/Environmental-Profile/Nutrient/Golf-Course-Environmental-Profile–Nutrient-Management-Report.pdf). Accessed June 30, 2021.
- Landschoot, P.J., and D.V. Waddington. 1987. Response of turfgrass to various nitrogen sources. Soil Science Society of America Journal 51:225-230 (https://doi.org/10.2136/sssaj1987.03615995005100010046x).
- Medina, L.C., J.B. Sartain and T.A. Obreza. 2009. Estimation of release properties of slow-release fertilizer materials. HortTechnology 19(1):13-15 (https://doi.org/10.21273/HORTSCI.19.1.13).
- Soldat, D.J., A.M. Petrovic and J. Barlow. 2008. Turfgrass response to nitrogen sources with varying nitrogen release rates. Acta Horticulturae 783:453-462 (https://doi.org/10.17660/ActaHortic.2008.783.47).
- Young, N.G., J.L. Cisar, J.E. Erickson, G.H. Snyder, J.B. Sartain and K.E. Williams. 1999. St. Augustinegrass response to nitrogen sources under contrasting application rates and frequency. International Turfgrass Society Research Journal 11:121-136.
Travis Shaddox is a former assistant professor at the University of Florida and the University of Kentucky. J. Bryan Unruh is a professor and associate center director at the University of Florida, Jay, Fla.