Organic fertilizers are soil amendments designed to aid plant growth by supplying one or more nutrients that may be lacking in the existing soil (see References 1). Unlike synthetic or inorganic fertilizers, organic fertilizers come from living things. In general, organic fertilizer releases nutrients into the soil more slowly and over a longer period than inorganic fertilizer (see References 2).
Plant-based organic fertilizers are made from dried, ground plant matter. The most common varieties in use are alfalfa meal, cottonseed meal and soybean meal. Soybean and cottonseed meal are good sources of nitrogen, with NPK (nitrogen, phosphorus and potassium) ratios varying between 6-2-1 and 7.5-0.7-2.4. Alfalfa meal is a more balanced fertilizer, with an NPK of about 2.5-1-1, and also contains a plant growth regulator called triacontanol (see References 3).
Fertilizers made from animal byproducts are an outgrowth of the livestock and fishing industries. These include feather meal, meat and bone meal, blood meal and fish meal. With the exception of bone meal, which has low nitrogen levels and very high phosphorus levels, animal-based fertilizers are high in nitrogen but low in phosphorus and potassium (see References 3). In addition to helping soil fertility, animal-based fertilizers keep waste from the meat industry out of landfills (see References 1).
In contrast to plant- and animal-based fertilizers, wood ash has no nitrogen, low phosphorus levels and moderate potassium levels, with NPK levels of 0-1-3 to 0-1.5-5. Wood ash is also a useful source of calcium and many other trace nutrients needed for healthy plant growth. Outside of the northeastern United States, the majority of the 3 million tons of wood ash produced annually ends up in landfills (see References 3).
Manure and Biosolids
Poultry litter, another byproduct of the meat industry, has a fairly balanced NPK ratio of 4-2-3 (see References 3). Manure from other animals, along with biosolids from sewage treatment, are more often associated with compost production than direct fertilizer application, yet both can be applied directly to soil without undergoing the chemical breakdown of the composting process (see References 1). With 860 million tons of manure and 6.9 million tons of biosolids produced annually, both are rich sources of soil nutrients (see References 3).
Based in central Missouri, Rachel Steffan has been writing since 2005. She has contributed to several online publications, specializing in sustainable agriculture, food, health and nutrition. Steffan holds a Bachelor of Science in agriculture from Truman State University.