Bones

Animal bones, used ground (bone meal) or calcined, rich in calcium phosphate.

Characteristics

Made mainly of hydroxyapatite, a calcium phosphate.
Bone meal has a typical NPK content around 3-15-0.
Insoluble in water: phosphorus is released slowly, mostly in acidic soils.
Cited in traditional recipes for fermented fertilisers and mineral salts.

Role in the preparation: Supplies slow-release phosphorus and calcium to fermented fertilisers or mineral mixes.

Caution: Animal by-products are regulated in the EU (Regulation (EC) 1069/2009): check local rules, avoid inhaling the dust and test the soil, since excess phosphorus is harmful.

Sources: Wikipedia - Bone meal · Washington State University - The myth of bone meal (L. Chalker-Scott)

Documentary information: this is neither a recommendation for use nor a plant-protection or veterinary claim. Check your country's regulations before any use.