I've been pursuing information in this area lately and would recommend several sources that expand on this subject:
John Kempf was at the OEFFA conference this year. I've heard his presentation at a previous Acres U.S.A. conference, and the information is essential to raising healthy plants organically. The OEFFA lecture is available for purchase and highly recommended, however this link is for an interview that should cover the same presentation:
Shabari Bird has a fascinating lecture on plant ferments and infusions and the ability of specific plants to accumulate specific minerals into a usable formulation that can be gathered and processed by fermentation or infusion and applied to other plants. Her lecture at the 2014 Biodynamic Conference is a recommended purchase:
BD14-21 MP3 DOWNLOAD Weeds, What They Tell, and How to Ferment Them For Fertilizers
with Shabari Bird After ordering, you will receive instructions by email for downloading or listening online to your MP3.
Her website has a selection of podcasts that also provide information at no cost: