Problems with precipitation of transition metal solution
Here's an idea.  Add the cations as their acetates or nitrates.  Control pH with acetate buffer. Fe(III) is a bad player.  "Ferric acetate," $\ce{[Fe3O(OAc)6(H2O)3]OAc}$, is water-soluble. You might sparge the solution with argon before sterilizing (sealed vessel) to prevent oxidation, and/or add ascorbate (or erythorbate, often biologically inactive) as anti-oxidant - if that is the problem.  

Lactate chelates, and may give greater solubilities.  Fe(II) lactate is E585 via the European Food Safety Authority.

(Note that my singular contribution to biochemistry was telling a lab its phosphatase assay bombed because...they used phosphate buffer.  I can't say they were grateful for acquiring the knowledge.)