Turfgrass often requires supplemental nitrogen (N) applications to maintain acceptable aesthetics and functionality. The average 18-hole golf course in the United States applies N at 100 kg·ha−1 per year (Environmental Institute for Golf, 2016). The N is required to increase production of amino acids and proteins, which leads to increased density and color as well as turfgrass recovery from traffic, drought, and other stresses. Most N sources used on turfgrasses are either urea or derived from urea (Association of American Plant Food Control Officials, 2017). Since 1998, the cost of urea has fluctuated between $70 and $845 per ton (Index Mundi, 2019). These costs increase when urea is manufactured into slow-release N (SRN) because SRN requires the addition of other components such as sulfur or polymer to impart the slow-release characteristics. However, if turfgrass response to SRN increases proportional to the cost, then the additional cost of SRN sources may be justified. Confirmation of this postulate would validate a consumer’s purchasing decisions and aid in justifying the use of SRN from an economic perspective.
Nitrogen sources may be broadly classified into four groups: soluble, natural organic, reacted, and coated (Association of American Plant Food Control Officials, 2017). Soluble N sources are the least expensive granular N sources because they do not require the additional manufacturing steps of SRN sources. Natural organics are commonly the least expensive N source per ton but contain less N than synthetic N sources (Carrow et al., 2001). This results in natural organics having the greatest cost per pound N. Examples of reacted N sources are urea formaldehyde and methylene urea. Because reacted ureas require additional processing, they are more expensive than urea. Coated SRN sources include sulfur-coated urea and polymer-coated urea. Sulfur-coated ureas are normally the least expensive coated N source because sulfur is less expensive than the polymers necessary to produce polymer-coated ureas (Christians et al., 2017).
It has been well established that N sources applied correctly consistently increase turfgrass quality. Telenko et al. (2015) applied N at 49 kg·ha−1 as urea, polymer-coated urea, and a natural organic on centipedegrass (Eremochloa ophiuroides) in Jay, FL, and reported turfgrass receiving urea resulted in quality greater than or equal to turfgrass receiving polymer-coated urea or natural organic during 3 of the 4 years. In addition, Telenko et al. (2015) documented that turf quality of treated plots remained acceptable during all years regardless of N source, which implies the cost associated with SRN sources was unnecessary. Shaddox et al. (2016) used centipedegrass to assess the influence of four rates of urea on turf quality and reported that N applied at 18 kg·ha−1 per year as urea resulted in acceptable turf quality during each fertilizer cycle over 2 years. Young et al. (1999) reported that N applied as urea at 49 kg·ha−1 every 60 d resulted in st. augustinegrass (Stenotaphrum secundatum) quality greater than or equal to N applied as polymer-coated urea or biosolid at 98 kg·ha−1 every 120 d or at 147 kg·ha−1 every 180 d. Similarly, Carrow (1997) reported that sulfur-coated urea, polymer-coated urea, reacted urea, and natural organics resulted in average annual turf quality less than or equal to urea during both years of the study. Reports of urea not resulting in acceptable turf quality are scarce and often conditional of N rate. Trenholm et al. (2012) reported that urea did not result in acceptable st. augustinegrass quality when applied at an N rate of 49 kg·ha−1 per year. However, Trenholm et al. (2012) noted that 49 kg·ha−1 per year was less than the recommended N rate for st. augustinegrass in central Florida (Trenholm et al., 2011).
Agronomic and environmental benefits of SRN sources have been well established but the cost to acquire these benefits has not been investigated. When properly applied, SRNs may result in reduced N leaching (Guillard and Kopp, 2004) and reduced applications (Soldat et al., 2008) without reducing turf quality. The use of SRNs has also resulted in as much as 70% reduction in N volatilization compared with urea (Del Moro et al., 2017). Advantages of SRN sources have been measured when N is applied at 98 kg·ha−1 or greater, which is not commonly recommended using soluble N sources due to the increased risk of N loss through runoff or leaching (Shaddox et al., 2016). Thus, in some cases, SRN sources provide additional value relative to urea. However, published studies that have investigated the cost of N sources applied to warm-season turfgrasses are scant. This is particularly true when the slow-release characteristics and their concomitant influence on turfgrass response are factored in. Determining the cost of N sources when their slow-release characteristics are taken into account would be useful information for turfgrass managers who desire to save money by applying the least expensive N source. It has been proposed that the cost “per day of response” based on turfgrass quality should be used to offset the additional cost of SRN. Therefore, the hypothesis of this study was that N source and rate influence N fertilizer cost compared with urea when turfgrass response is included in the cost.
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Cisar, J.L., Snyder, G.H., Haydu, J.J. & Williams, K.E. 2001 Turf response to coated-urea fertilizers. II. Nitrogen content in clippings, nitrogen uptake, and nitrogen retention from prills Intl. Turfgrass Soc. Res. J. 9 368 374
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Young, N.G., Cisar, J.L., Erickson, J.E., Snyder, G.H., Sartain, J.B. & Williams, K.E. 1999 St. augustinegrass response to nitrogen sources under contrasting application rates and frequency Intl. Turfgrass Soc. Res. J. 11 121 136