@EddieB that is a very large and complicated question, I would start with the advancing eco agriculture webinars on you tube that address sap analysis. As succinctly as I can make it, all nutrients in the sap come from the soil or the leaves, the sap carries all the active nutrients so measuring it is a great way of telling what nutrients the plant has access to.

All the nutrients have relationships to at least some other nutrients and also to pest and disease susceptibility.