Bone meal is a mixture of finely and coarsely ground animal bones and slaughterhouse waste products. It is used as a dietary supplement to supply calcium and phosphorus. As a slow-release organic fertilizer, it supplies phosphorus, calcium, and a small amount of nitrogen to the plants.
The NPK rating of bone meal is typically 3-15-0 along with a calcium content of around 12% (18% CaO equiv.).
As the bone meal is water-insoluble, it needs to be broken down before the plant can absorb it. It can only be broken down in acidic soil (pH < 7.0) and releases its nutrients over a span of 1 to 4 months.