Nitrogen is consumed by crop plants in large quantities. However, N in nitrate form is also highly leachable, particularly in sandy soils, and can contaminate groundwater (Hartz, 2006; Jackson et al., 1994; Sanchez, 2000). Therefore, on soils with limited nutrient retention capacities, it is desirable to increase the number of split applications of N fertilizers to reduce leaching potential or to use products designed to release N over time. Nutrients from slow-release fertilizer (SRF) and controlled-release fertilizer (CRF; referred to collectively as S/CRF) release N, and in some cases, other fertilizer elements, at different rates and through different mechanisms (Sartain et al., 2004). These release mechanisms will be discussed below. Available since the 1950s, most S/CRF consumption was by nonfarm or specialty markets (i.e., nurseries, home lawns, recreational areas, and golf courses). The primary reason for the lack of use of S/CRF materials in agriculture has been the cost per unit of N (Simonne and Hutchinson, 2005; Trenkel, 1997).
Vegetable production in the United States often is located upstream and/or adjacent to large tracts of land set aside for water management, ecosystem restoration, or urban development. These lands are often located near densely populated urban areas with citizens highly engaged in water and nutrient management issues. Because vegetable growers are being asked to reduce potential impacts of agricultural production on water quality through implementation of best management practices (BMPs), there is a need to better manage fertilizer inputs. Despite their present cost, S/CRFs have the potential to increase fertilizer efficiency and reduce N loss to the environment. There are several manufacturers of S/CRFs, and each manufacturer has one or more formulations. Some S/CRF products have already been thoroughly tested, and targeted products have been developed for use in high-value perennial plantings such as citrus (Citrus spp.) (Obreza and Rouse, 1993, 2006). S/CRF technology is currently being widely investigated in vegetable crops, but it remains to be seen whether this technology is appropriate for short duration crops with lower per-unit value than citrus, landscape plants, or greenhouse-grown products.
S/CRF materials have been shown to increase nutrient use efficiencies and reduce environmental impact of agricultural production (Sartain et al., 2004). Increased pressure from environmental groups and state regulators for the adoption of BMPs have led to increased use of S/CRF. However, consumption of these fertilizers remains a relatively small portion of total agricultural use in the United States (Simonne and Hutchinson, 2005). This is particularly true for short-term crops such as vegetables. S/CRF technologies are classified by their release mechanism. Therefore, understanding these mechanisms in terms of nutrient availability to the target crop plant is critical to the choosing the proper material for the crop to be grown.
Most SRF are chemical compounds that are only slightly soluble in water or are slowly broken down by microbial action (Sartain et al., 2004). On the other hand, CRF are made of soluble fertilizers coated with materials that limit exposure of the soluble material to water and/or release of the resulting nutrient solution by diffusion. Thus, the rate of nutrient liberation from SRF is related to their water solubility, microbiological degradation, and chemical hydrolysis. Important factors affecting degradation and hydrolysis are particle size, soil temperature, and microbial activity. Particle size relates to increased surface area for chemical and biological degradation in reduced particle size. Release rates of CRF products, on the other hand, are a function of temperature and soil water content. The following discussion will concentrate on the release mechanisms of various categories of S/CRF materials and how their release mechanisms influence use in vegetable production.
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