By legal definition, any substance sold as a fertilizer must contain a measured amount of nitrogen, phosphate and potash. The percentage by weight of these nutrients---abbreviated N, P and K---has to be shown on the bag or container. A typical analysis for alfalfa meal, for example, is shown as 2-1-2, while fish emulsion is usually around 5-2-2.
These, and other plant and animal byproducts, are called organic fertilizers. Also included are mined minerals, such as greensand, gypsum and other rock powders.
Under the rules of the U.S. Department of Agriculture's National Organic Program (NOP), materials sold as "organic" have to meet or exceed certain standards. These brand-name products are examined and approved by the Organic Materials Review Institute (OMRI). Products that have this approval are allowed to use the "OMRI Listed" seal on their packaging and advertising.
Because they do not contain a measured or predictable NPK nutrient analysis, some materials are called soil amendments rather than fertilizers. These include compost and animal manures. They usually contain plant nutrients, but the amounts will vary from one material or batch to the next.
Compost works as a soil conditioner, retaining moisture and plant nutrients. Manures are usually rich in nitrogen. Both boost the organic-matter content of soil, which is beneficial.
Several products from the meat and poultry processing industries are sold as fertilizers. Blood meal is dehydrated slaughterhouse waste, and is rich in nitrogen. Bone meal is steam-processed bones, and is a good source of phosphate (P) in soils that have a pH (acid-alkaline balance) on the acid side of neutral, or below pH 7.0. Feather meal is rich in slow-release nitrogen.
Fish and Seaweed
Fish emulsion typically has an analysis around 5-2-2, so is considered a good fertilizer for young plants, especially vegetables. Liquid fish-seaweed fertilizers have the added benefit of trace minerals and micronutrients that are essential for healthy plant growth.
Alfalfa meal, an animal feed, is used to boost the organic-matter content of soil while providing a small (usually around 2-1-2) nutrient addition along with a wide range of micronutrients. Corn gluten meal is so high in nitrogen (around 9-0-0) that it will inhibit seed germination. It is often used to fertilize lawns while deterring broadleaf weeds such as dandelions.
Cottonseed meal and soybean meal often contain pesticide residues and/or genetically modified (GM) organisms, so are best avoided.
Many valuable fertilizers are mined, such as greensand, feldspar, and potassium sulfate which are all sources of potassium. Gypsum provides calcium, an essential secondary plant nutrient. Nutrient analyses of rock powders varies widely.