Enhanced efficiency fertilizers (EEF) are a group of fertilizers that reduce the risk of nutrient loss to the environment and subsequently increase fertilizer use efficiency (Slater, 2010). This increase may be accomplished through maintaining nutrients in the root zone by physical barriers (coating), reduced solubility, or retaining nutrients in a less leachable form (Trenkel, 2010). There are three subgroups of EEFs with different characteristics for horticultural production systems. Slow-release fertilizers (SRFs) contain N in a less-soluble, plant-unavailable form that usually need to be microbially degraded into plant available N. Stabilized fertilizers are a group of fertilizers that have a chemical inhibitor to either stop the oxidation of ammonium (NH4+) to nitrate (NO3−) by bacteria or to slow the enzymatic transformation of urea to NH4+ (Trenkel, 1997). Controlled-release fertilizers, the last subgroup of EEFs, are urea, ammonium nitrate, potassium nitrate, or other soluble fertilizer materials coated with a polymer (polyethylene and ethylene-vinyl-acetate or thermoplastics), resin (a subgroup of polymers and refers as alkyd-type resins and polyurethane-like coatings), sulfur, or a hybrid of sulfur-coated urea (SCU) coated with a polymer or resin. These coated materials release nutrients in water at a predictable rate when used at the manufacturer specified temperature (e.g., 25 °C) (Trenkel, 2010). The European Committee for Standardization's (2002) method determines nutrient release time based on 75% nutrient release from CRFs.
The European Union has developed both standard and accelerated laboratory procedures for measuring N release from CRFs; however, researchers in the United States are still developing a universal test for CRFs and SRFs for commerce purposes (European Committee for Standardization, 2002; Sartain et al., 2004). Growth chamber and greenhouse methods are used to evaluate or compare how CRFs will act in a particular controlled environment (Broschat and Moore, 2007; Huett and Gogel, 2000). Lastly, field methods are used to measure N release in commercial vegetable field conditions (Simonne and Hutchinson, 2005). Each research method has its own advantages and disadvantages (Engelsjord et al., 1996; Sartain et al., 2004; Simonne and Hutchinson, 2005). Therefore, the objective of this publication is to describe and summarize laboratory, growth chamber, greenhouse, and field methods currently used to measure CRF-N release on vegetable production using the research literature.
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