i would broil or roast over a fire until blackened. you dont need to clean the off prior. just move the rack up to the top so the bones get toasted up quickly (30 mins or so). then cool, smash with a hammer on cement
and mix with equal weight in vinegar mark this level with a pencil. cover loosely with a lid for one - two weeks. if the level has decreased from evaporation the top it off to the mark.
save these bones and mix with a small amount of bokashi. stuff in a small clay pot (or bamboo tube, or bull horn) and pad the face with peat. dig a hole in a nice garden bed add a little peat or leaf mold to the hole. bury you conainer of bones for two-three months. dig up and distribute where you want phos. you'll find you can crumble this up really easily.
excessive sounding right? you can also skip the initial calphos extraction and roasting and just beat with a hammer some then soak in vinegar until the vinegar naturally has evaporated. mix with bokashi and bury in that same manor. you can get creative with how you do it. these are just some ways I've tried with success.
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