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-mm2 openings had greater N release compared with a weed-block material with 0.07-mm2 openings (Wilson et al., 2009).
Several factors influence nutrient release from CRFs including soil temperature, moisture content, osmotic potential (ψS), 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 90- to 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.
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