(a) No fertilizer label shall bear a statement that connotes or implies that certain plant nutrients contained in a fertilizer are released slowly over a period of time, unless the slow release components are identified and guaranteed at a level of at least 15% of the total guarantee for that nutrient(s). (Official 1991)

(b) Types of products with slow release properties recognized are (1) water insoluble, such as natural organics, ureaform materials, urea-formaldehyde products, isobutylidene diurea, oxamide, etc., (2) coated slow release, such as sulfur coated urea and other encapsulated soluble fertilizers, (3) occluded slow release, where fertilizers or fertilizer materials are mixed with waxes, resins, or other inert materials and formed into particles and (4) products containing water soluble nitrogen such as ureaform materials, urea-formaldehyde products, methylenediurea (MDU), dimethylenetriurea (DMTU), dicyandiamide (DCD), etc. The terms, “water insoluble”, “coated slow release”, “slow release”, “controlled release”, “slowly available water soluble”, and “occluded slow release” are accepted as descriptive of these products, provided the manufacturer can show a testing program substantiating the claim (testing under guidance of the Experiment Station personnel or a recognized reputable researcher acceptable to the ________). A laboratory procedure, acceptable to the ________ for evaluating the release characteristics of the product(s) must also be provided by the manufacturer. (Official 1991)

(c)Until more appropriate methods are developed, AOAC International Method 970.04 (15th Edition) is to be used to confirm the coated slow release and occluded slow release nutrients and others whose slow release characteristics depend on particle size. AOAC International Method 945.01 (15th Edition) shall be used to determine the water insoluble nitrogen of organic materials. (Official 1994)

Source

Association of American Plant Food Control Officials (AAPFCO). 2026. Official Publication No. 79. West Lafayette, IN: Association of American Plant Food Control Officials, Inc., p. 58.