There is a difference between the common definition of an organic fertilizer and the legal definition of organic fertilizer used by the United States Department of Agriculture (USDA) National Organic Program (NOP). The common definition is much simpler. The legal definition allows for consistent fertilization levels over time and helps to facilitate commercial production of food that can be legally labeled organic.
When many home gardeners think of organic fertilizer, compost and manure are two fertilizers that come to mind. However, from a legal standpoint, compost, manures and composted manures cannot be called fertilizer because their N-P-K, or nitrogen, phosphorous and potassium, content can vary and are not consistent. A fertilizer's N-P-K numbers indicate the percentage of a particular nutrient. For example, 10-10-10 fertilizer contains 10 percent of each nutrient. In fact, blood meal, bone meal, seed meal and various emulsions, like kelp and fish, cannot legally be called fertilizers. However, all of these items provide plant nutrition that helps to maintain soil health and quality.
National Organic Program
The National Organic Program analyzes both natural and chemical substances that can be produced with consistent fertilization results. For example, fish oil is an approved fertilizer component because it can be produced with consistent composition. Flax seed, however, is listed as not approved for use in organic fertilizers.
Approved Chemicals in Organic Fertilizer
Although it may seem contradictory at first, two chemicals that are functionally identical may be approved in some cases under the USDA's NOP recommendations and banned in other cases. In some cases, some banned substances, like ammonium carbonate and ammonium chloride, can be used in small quantities as a leavening agent. Substances like pure ammonium are approved if the ammonium is from a natural source, but not approved if the identical chemical is derived from an artificial source. Other substances, like ascorbic acid, may be used, regardless of the source of the component.
Natural Versus Organic
Through looking at the approved ingredient list of the USDA's NOP, it becomes clear that natural and organic are not synonymous. In some cases, natural fertilizers cannot be legally called organic fertilizers. In other cases, synthetic substances like ascorbic acid are approved for use in organic fertilizers without regard to natural or synthetic origins.
When considering whether to use a USDA NOP-certified organic fertilizer versus a commonly held organic fertilizer, look at a number of factors. If you want to grow natural vegetables that are also organic, using the traditional definition will be more suitable. However, if you are working with unique requirements or larger commercial agriculture, using the consistency of a USDA-approved organic fertilizer may be more suitable.