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For blood as a food for animals, see Hematophagy.
Blood meal is dried, powdered blood used as a high-nitrogen fertilizer. It is one of the highest non-synthetic sources of nitrogen and if over-applied it can burn plants with excessive ammonia. Blood meal is completely soluble and can be mixed with water to be used as a liquid fertilizer. It usually comes from cattle as a slaughterhouse by-product. It can be spread on gardens to deter animals such as rabbits, or as a composting activator. It may also be used as an animal food supplement.
Blood meal, bone meal, and other animal by-products are permitted in certified organic production as soil amendments, though they cannot be fed to organic livestock. In the United States, fear of BSE (mad cow disease) and the related Creutzfeldt-Jakob disease prompted Demeter International, which certifies biodynamic farms, to fully prohibit use of bone meal and blood meal, since these could be routes of infection for BSE.
Blood meal is different from bone meal in that blood meal contains a much higher amount of nitrogen, while bone meal contains phosphorus.
Nitrogen is more typically missing from soils than the other elements provided by most fertilizers (phosphorus and potassium). Plants grown in soil lacking proper amounts of nitrogen will yellow from the leaves down due to nitrogen deficiency. Applying blood meal will help plants become green again