Most Florida citrus is grown on extremely sandy soils inherently low in fertility, cation exchange capacity, and ability to retain applied plant nutrients. Traditionally, the main way of providing nitrogen (N) to Florida citrus trees has been broadcast applications of ammonium nitrate fertilizer. However, the high water-solubility of this material in combination with Florida's sandy soils and humid conditions can cause poor nutrient use efficiency and leaching of nitrate N (NO3-N) below the root zone. A circa 1990 Florida groundwater quality study revealed nitrate pollution of some central Florida drinking water wells located close to mature commercial citrus orchards (Lamb et al., 1999). Efficient nutrient management practices, including the use of controlled-release fertilizer technology, may improve N use efficiency and decrease the potential for N loss to the environment.
Controlled-release fertilizer is formulated to gradually deliver nutrients to plants at a rate that matches the physiological requirement (Oertli, 1980). Recent studies have shown that controlled-release fertilizer (CRF) applied as part of a citrus fertilization program could reduce N leaching on Florida sandy soils (Alva and Tucker, 1993; Dou and Alva, 1998; Wang and Alva, 1996). Reports have also indicated that comparable or improved citrus yield and quality was obtained from trees fertilized with CRF compared with water-soluble forms (Alva and Paramasivam, 1998; Koo, 1986; Obreza et al., 1999; Zekri and Koo, 1992). Obreza et al. (2006) found that fruit yield of mature ‘Hamlin’ orange (Citrus sinensis) trees was greater with a resin/Poly-S® (Scotts Co., Marysville, OH) mixture applied once annually at 90 lb/acre N when compared with water-soluble fertilizer applied at 180 lb/acre N per year in three equal doses. The use of CRF in the Florida citrus industry has been limited because of higher cost compared with water-soluble fertilizer and the lack of familiarity about its field performance. However, government regulatory agencies are now investigating if using CRF to fertilize citrus could be considered a best management practice (BMP), mainly in the central Florida ridge area where groundwater pollution is a concern.
Even if CRF use becomes economical, widespread acceptance by citrus growers will likely be limited as a result of grower concern about field performance. For example, nutrient release patterns determined by the manufacturer in the laboratory or greenhouse sometimes have not correlated well with release in the field. A study by Meadows and Fuller (1983) revealed that nutrient release periods of several polymer-coated fertilizers were shorter than those claimed by the manufacturers. Efforts have been made during the last decade to develop models describing nutrient release from coated fertilizers. Most of these models assumed that nutrient release from coated fertilizers is controlled either by the rate of solute diffusion from the fertilizer or by the rate of water vapor penetration into the granule through the coating (Shaviv, 2001). According to Sharma (1979), the most direct and widely used technique to evaluate N release characteristics is CRF incubation in soil where some or all of the mineral N released with time is determined. Information regarding the release periods and characteristics of individual CRFs is needed to expand commercial use of CRF for citrus production.
The objectives of this study were: 1) to characterize the nature of N release from a CRF blend designed for mature Florida citrus trees and its individual CRF components using a standard laboratory lysimeter incubation; 2) to measure the N release characteristics of the fertilizers in a 1-year field evaluation; and 3) to develop N release curves for the selected CRF. The results will help determine the efficacy of the evaluated CRF materials to provide N in a citrus nutrient BMP program.
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