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.

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.