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Determination of nutrient release duration from controlled-release fertilizers (CRFs) or soluble fertilizers encapsulated in polymer, resin, or sulfur covered fertilizer coated with a polymer differs among manufacturers, but may be determined as 75% to 80% nitrogen (N) release at a constant temperature (e.g., 20 to 25 8C). Increases or decreases in temperature compared with the manufacturer release determination temperature increase or decrease CRF N release; thus, coated fertilizer may release more rapidly than stated during the fall season when soil temperatures in seepage-irrigated tomato (Solanum lycopersicum) production can reach 40.1 8C. The objectives of this study were to evaluate N release duration of CRFs by measuring N release from CRFs incubated in pouches under polyethylene mulch-covered raised beds and to determine the CRF duration suitable for incorporation into a fall tomato fertility program. In 2011 and 2013, 12 and 14 CRFs from Agrium Advanced Technologies, Everris, Florikan, and ChissoAsahi Fertilizer were sealed in fiberglass mesh pouches (12.7 3 14 cm) that were buried 10 cm below the bed surface in a tomato crop grown using commercial production practices. A data logger collected soil temperature 10 cm below the bed surface. Pouches were collected and N content was measured eight times through two fall seasons. A nonlinear regression model was fit to the data to determine N release rate. During the 2011 and 2013 seasons, minimum, average, and maximum soil temperatures were 21.2 and 19.2, 25.7 and 23.5, and 32.2 and 27.7 8C, respectively. Seasonal total CRF N release was between 77.6% and 93.8% during 2011 and 58.3% and 94.3% in 2013. In 2011, PCU90 and in 2013, PCU90 and PCNPK120 had the highest seasonal total percentage N release (PNR) and FL180 had the lowest in both years. A nonlinear regression fit N release from CRF with R = 0.85 to 0.99 during 2011 and 0.49 to 0.99 during 2013. Nitrogen release from all CRFs was faster than the manufacturer’s stated release, probably as a result of high fall bed temperatures. A CRF or CRF mixture containing CRFs of 120to 180-day release duration may be recommended, but the CRFs must release greater than 75% N during the season. A series of best management practices, including use of CRF, has been implemented for vegetable and agronomic crops by the Florida Department of Agriculture and Consumer Services in response to the Federal Clean Water Act of 1972 and the Florida Restoration Act of 1999 (Bartnick et al., 2005). Controlled-release fertilizers are soluble fertilizer (SF) coated in polymer, resin, or sulfur-coated urea in a polymer coating (Trenkel, 2010). Field measurements of CRF N release have been made by researchers using the field pouch method, which consists of a known mass of CRF (e.g., 3.5 g N) sealed inside fiberglass mesh pouches that may be buried inside a polyethylene mulched or open bed vegetable systems and removed at prearranged dates (Carson and Ozores-Hampton, 2012). After collection, the N content remaining in the CRF prills is measured to determine the amount of N remaining and the N release rate. The mesh pouch allows for soil to CRF prill contact, which may affect CRF N release rate. For instance, pouches with 1.2-mm openings had greater N release compared with a weed-block material with 0.07-mm openings (Wilson et al., 2009). Several factors influence nutrient release from CRFs including soil temperature, moisture content, osmotic potential (yS), nutrient composition, coating thickness, and prill diameter (Carson and Ozores-Hampton, 2013). Manufacturers of CRF manipulate the nutrient release duration of resin-coated fertilizer, polymer-coated fertilizer (PCF), and polymer sulfur-coated urea (PSCU) by adjusting coating thickness and composition with thicker coatings having longer release durations (Carson and Ozores-Hampton, 2013). However, in irrigated vegetable production, soil temperature may be considered the most influential factor (Carson and Ozores-Hampton, 2013). Under laboratory conditions, Lamont et al. (1987) measured N release at nine temperatures from 5 to 45 C and found a quadratic release response with temperature and time. In a similar study, Gandeza et al. (1991) demonstrated a doubling of the percentage N release (PNR) with each 10 C rise in temperature from 10 to 30 C. Thus, a CRF release duration will increase or decrease inversely with soil temperatures that differ from CRF manufacturer label specification. Controlled-release fertilizer manufacturers determine nutrient release duration in water at a constant 20.0, 21.1, and 25.0 C, respectively (Agrium Advanced Technologies, 2010; Everris, 2013; Florikan ESA, 2012a, 2012b). However, the average daily soil temperatures under a polyethylene mulch-covered, raised vegetable bed in south Florida were greater than 23.9 C for 8 weeks after bedding and were as high as 40.1 C during the daytime in a fall tomato (Solanum lycopersicum) season (Carson et al., 2012a, 2013). Thus, high soil temperatures during the fall will affect CRF release duration used in tomato production. Therefore, selection of a CRF with too low of a nutrient release duration may result in slow nutrient release causing plant nutrient deficiencies, low plant growth, and reduced yield. In contrast, selection of a CRF with too high of a high nutrient release rate may result in increased soil electrical conductivity, plant toxicity and injury, and the loss of CRF benefits (Shaviv, 1996). Because CRF N release durations, independent of manufacturers’ labeling, have not been published for fall production in Florida, the objectives of this study were to evaluate N release duration of CRFs by measuring N release from 90to 180-d release CRFs incubated in pouches under polyethylene mulch-covered raised beds and to determine the CRF duration suitable for incorporation into a fall tomato fertility program. Material and Methods Two concurrent CRF field studies were conducted during each Fall 2011 and 2013 on a commercial tomato farm near Immokalee, FL (lat. 26 14#5$ N, long. 81 28#55$ W). The soil at the study location was Basinger fine sand (hyperthermic Spodic Psammaquents), which permitted seepage irrigation. The field configuration, bed preparation and fertilization, and fresh market tomato production practices are described in Carson et al. (2014a, 2014b), and dates relevant to these trials are in Table 1. The two concurrent CRF studies each contained six and seven CRFs in 2011 and 2013, respectively, for a total of 12 CRFs in 2011 and 14 CRFs in 2013 (Table 2). The CRF fall mixes (M112, M168, and M224) are CRF, SF, and filler mixes that when applied at 1493 kg·ha supply 112, 168, and 224 kg·ha CRF N; however, different SF and filler amounts in the pouch may have affected the PNR in 2011. Therefore, a CRF mix (FLmix) Received for publication 26 Aug. 2014. Accepted for publication 15 Oct. 2014. To whom reprint requests should be addressed; e-mail Ozores@ufl.edu. 1568 HORTSCIENCE VOL. 49(12) DECEMBER 2014 was added in 2013, which contained CRFs in a ratio equivalent to the fall mixes that were composed of FL100, FL140, and FL180. Fiberglass window screen (18 · 14-mesh or 39 pores/cm) rectangles (15.2 · 30.5 cm) were folded in half and sealed on two sides using a clothes iron (Model F210; Black & Decker, New Britain, CT). Controlled-release fertilizer samples containing 3.5 g N were placed inside the pouches and the last side was sealed, which resulted in internal dimensions of 12.7 · 14.0 cm (Carson et al., 2013). At bed formation in 2011 and 5 d after bedding in 2013 (to reduce personal protective equipment requirements), the pouches were placed level, 10 cm below the bed surface, in the center bed of a three-bed, 1.5-m-long plots. The studies were randomized complete block design with Table 1. Collection dates and days after placement (DAP) for pouches containing controlled-release fertilizer incubated in white polyethylene mulch-covered raised tomato beds during Fall 2011 and 2013 in Immokalee, FL.

Nitrogen Release Properties of Controlled-release Fertilizers during Tomato Production