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##Isotope tracers  Stable isotopes of especially Ca and Sr have been used extensively to source the origin of Ca in drainage water . water.  Applied to plant tissues, it could potentially trace plant nutrients back to their primary source.   It has been mostly used to study apatite weathering.  Apatite is a calcium-phosphate mineral, and as P has no stable isotopes, the uptake dynamics can only be studied via the Ca ion (or potentially the ¹⁸O/¹⁶O in the phosphate group).