Release Mechanisms for Slow- and Controlled-release Fertilizers and Strategies for Their Use in Vegetable Production

Fertilizer material costs, particularly nitrogen (N), have increased substantially over the past 5 years. Increased costs, along with increased awareness of the impact of fertilizer leaching on the environment in humid regions, have increased interest in use of slow-release fertilizer (SRF) or controlled-release fertilizer (CRY) materials. The goals of SRT and CRF use are that no nutrient should be limiting for crop uptake, there should be improved nutrient uptake efficiency, and nutrient-leaching potential should be reduced. These considerations are particularly important for crops grown on sandy soils with relatively low nutrient and water holding capacities. Release rates of biodegradable, or slow-release materials, such urea formaldehyde, isobutylidene diurea, and methylene urea are proportional to soil microbial activity and are therefore soil temperature dependent. These materials are N sources and depend on soil biological activity, thus, soil temperature during specific crop growth phenology must be considered and release may be delayed by soil fumigation. Whereas CRFs depend on diffusion through coatings and not biodegradation, both are soil moisture and temperature dependent. Examples of coated materials are sulfur-coated urea, polymer-coated urea, and polymer/sulfur-coated urea. The advantage of these materials is that leachable fertilizer elements other than N can be incorporated within the coating. However, this comes at an increased cost. The use of any single or combination of these materials depends on time of year, the length of crop cycle and crop nutrient demand patterns, and the use of soil fumigants.