[robbedpeter]

Probably the only reasonable way of guaranteeing clean plant food sources is to use hydroponics, and to manufacture fertilizer yourself. Distilling water is easy, and buying the chemicals for fertilizer seems doable. But you'll want to make your own hdpe containers, and silicone tubing, so you don't get unlisted phthalates or plasticizer. GRAS and "food safe" classifications are fine, for things that are actually looked for, but mistakes are made, and most failures seem to favor producers, not consumers.

The question boils down to the actual harms you're mitigating and what constitutes the appropriate response.