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RE: bone meal

> Bone meal is one option; it's a high-phosphorus soil conditioner. The
> phosphorus is there as calcium hydroxyl phosphate which is very poorly
> soluble.  In soils it release phosphorus very slowly and it 
> isn't favored as a phosphorus fertilizer.  

I believe it would depend on the pH of your substrate.  Bone meal is
practically insoluble under alkaline conditions, so if your substrate pH is
around 7 or higher, then little of the phosphorus would be available to your
plants.  Skeletons can remain in good condition for hundreds, even
thousands, of years while buried in alkaline soils.  

> A second idea that for now I'm just toying with is too mix 
> ferrous sulfate and sodium biphosphate in jar of water, 
> then bubble with air until most of
> the iron precipitates.  The combination could form some iron 
> phosphates
> (poorly soluble) and iron hydroxides that will pull some more of the
> phosphate down with it.  That precipitate, dried and crumbled, might
> provide stable iron and phosphorus to the substrate.  Any 
> thoughts on this
> little chemistry experiment?

Again, I think it depends on your substrate pH.  I know that phosphorus can
be precipitated out in acidic soils by Fe(III) and Al(III), so if your
substrate pH is acidic (and aerobic), then it may not become available to
your plants.

I think the key here is available, not necessarily total, phosphate.  In
nature, most phosphorus is recycled from organic matter, with decay and
conversion to phosphate necessary to make it available to plants.  This
sounds like a strong case to let detritus and mulm build up in your
substrate, though I have no idea how much of this "recycled" phosphorus
contributes to our total needs.  Very little, I suspect.