What Is Organic Fertilizer?

What You Need to Know

Organic fertilizers are fertilizers that are naturally produced, as opposed to chemically manufactured synthetic fertilizers. The term organic in this context refers to the Principles of Organic Agriculture, which determines whether a fertilizer can be used for certified organic farming, not simply whether the material contains organic compounds.

The main categories of organic fertilizers are animal wastes, plant wastes, minerals, and treated sewage sludge (biosolids).

Animal-based sources are among the most widely used. Manure from cattle, poultry, horses, and other livestock adds nitrogen, potassium, phosphorus, sulfur, magnesium, and calcium to soil while also increasing organic matter and water infiltration. Cattle manure alone reaches an estimated two billion tons annually in the US. Chicken litter (manure plus bedding) is considered particularly effective for conditioning soil, containing trace amounts of copper, zinc, magnesium, boron, and chloride. Bat guano, prized since the Incan civilization, is high in nitrogen (about 10%), carbon, sulfur, and phosphorus. Fresh manure must be composted before use because uncomposted manure contains high ammonia levels that burn plant roots, and may harbor pathogens like E. coli and Salmonella.

Animal by-products from meat processing also serve as fertilizers. Blood meal (12-0-0) is one of the fastest-acting organic nitrogen sources. Bone meal (3-15-0) provides slow-release phosphorus and calcium. Fish emulsion (5-2-2) delivers balanced nutrients quickly. Feather meal (13-0-0) provides slow-release nitrogen over 4 to 6 months.

Plant-based organic fertilizers include compost, humic acid, grain meals, seaweed extracts, and green manure from decomposing crop residues. Compost provides modest direct nutrients but significantly improves soil stability through increased organic matter and supports microorganism populations that make nutrients bio-available. Grain meals (corn gluten, alfalfa, cottonseed, soybean) supply nitrogen and potassium, though fresh applications can temporarily increase soil ammonia. Algae captured from agricultural runoff has been shown to produce growth comparable to synthetic fertilizers when dried and applied to crops.

Mineral sources include greensand (anaerobic marine deposits), limestone (fossil shell deposits that raise soil pH to improve microbial activity and nutrient flow), and rock phosphate (fossil guano). Ash from plant combustion is an important potassium source. Sulfate of potash (0-0-50) is mined from natural mineral deposits.

Organic fertilizers offer several advantages over synthetic alternatives. Research shows they improve soil biodiversity and long-term productivity, with studies finding that potential mineralizable nitrogen was 182 to 285 percent higher in organic mulched systems than in synthetic controls. They increase organic matter which improves water retention and aeration, support beneficial organisms like mycorrhizal fungi that extend root nutrient absorption by 10 to 100 times, release nutrients slowly to reduce burn risk, and reduce groundwater pollution from nutrient runoff.

The main disadvantages are lower and less precise nutrient content (requiring more product by weight), variability between batches (without testing, exact nutrient amounts cannot be known), dependence on soil temperature and microbial activity for nutrient release (slow in cold soils), and higher cost per pound of nutrient. Some products like blood meal and fish emulsion also attract animals.

In practice, many farmers and gardeners use a combination approach: organic fertilizers build long-term soil health and biological activity, while targeted synthetic fertilizers fine-tune nutrient supply for specific crop demands. Cover crops and leguminous green manures are also grown to enrich soil through nitrogen fixation from the atmosphere.