Home gardeners have long used fish products, including scales, to fertilize plants. The products include liquid emulsions, as well as powders made from processed fishing industry waste. They key to the efficacy of any fertilizer, either homemade or commercially available, is to match the nutrient content of the fertilizer with the needs of specific plants. While fish scales and other fish derivatives are not complete fertilizers, they do supply nutrients needed by a large number of plants.
Fish Fertilizer Components
All plants need nutrients in varying amounts. The major plant nutrients are nitrogen, phosphorus and potassium. Commercial products sold in various forms that use fish scales along with other fish wastes contain all three major nutrients but have the highest concentration of nitrogen. Nitrogen is important because it aids plant rooting and growth, as well as fruit production. It contributes to the quality of foliage, especially in forage plants. Nitrogen is also a component of chlorophyll, essential to photosynthesis.
Plant Nitrogen Needs
Because nitrogen is a part of all living things, most edible and ornamental plants can use the nitrogen provided by fish scales and other fish derivatives. Some plants, however, need less added nitrogen because they can absorb atmospheric nitrogen by way of root nodules. These plants are members of the bean or legume family, including edible beans, peas, soybeans, alfalfa and clover. This nitrogen-fixing ability also helps other plants because some of the nitrogen captured by legume roots remains in the soil and can be used by other plants.
Fish scale/waste-derived products are available in several forms. Fish emulsion (5-2-2) is a liquid composed of fish parts processed with heat and acid. Another liquid, enzymatically digested hydrolyzed liquid fish (4-2-2), is also processed from fishing industry wastes, using enzymes instead of acid and heat. Applications of either product typically last one to four months. Two solid products, fish meal (10-6-2) and fish powder (12-0.25-1), are processed by drying and grinding fish waste. Meal is tilled into the soil, and powder is water-soluble. Meal lasts one to four months per application, and powder up to one month.
Given that the majority of edible and ornamental plants benefit from the nitrogen provided by fish scales and other fish products, the best way to assess fertilizer needs for an area is to do a soil test with a kit available at nurseries and garden centers. Some fish products, especially emulsion, have a distinct "fishy" odor, which may attract unwanted animals after application. Fish meal is a significant source of phosphorus in addition to nitrogen. Homemade fish emulsion can be made by combining 1 part fish scales, guts and other parts to 2 parts water, ripened in a closed container until fully emulsified.