Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
There has been considerable work on controlled release fertilizer over the years and many of the different technologies have shown potential for use with vegetable crops. Controlled released fertilizer is most useful with nutrients that are subject to leaching losses, particularly nitrogen.
Controlled release fertilizers are most commonly based on coatings (polymer or sulfur coated ureas for example) or having nutrients in chemical forms that slow their release (Ureaform and IBDU nitrogen for example). Polymer coatings can be used on most fertilizers and are common in the nursery and greenhouse industries with complete fertilizer products applied to potted plants. Coated product technologies have advanced over the years to give more precise release properties. However, release will still be dependent on the type of coating, the thickness of the coating, as well as temperature and moisture. Controlled release fertilizers are commonly rated as to how long they take to release nutrients in days (70 day, 90 day, 120 day formulations for example). They can also be mixed with a small amount of regular soluble fertilizer to give an initial nutrient charge.
It should be noted that manures have a component of their nitrogen (the organic fraction) that is available upon decomposition and mineralization so they can be considered a slow release fertilizer.
One main advantage of controlled release fertilizers is that only one application is necessary for a crop, thus reducing application and management costs. The other main advantage is reducing nitrogen leaching losses because not all nitrogen is available at once. In some trials, efficiency of nitrogen utilization is improved (by reducing losses) so that less nitrogen is needed.
For many years, the cost of controlled release fertilizers limited their use to high value horticulture applications. As costs are decreasing, especially with the polymer coated products, the economics is changing for vegetable and field crops.