I’ll still take the ones growing in dirt. The most commonly found pesticides are fungicides (I would think you would need these in hydroponocs, fungus loves water) and insecticides (again, inside and around water insects and mites are going to do really well). It’s all about what sites they tested and who is running it. Some land probably has heavy metals more. Some greenhouses will have more pesticides and plastic. I’m not sure I trust this to be completely objective and conclusive. Still interesting.
Second question: Are vegetables grown in inhospitable environments like Yuma, AZ lower in some toxins?
Heavy metal contamination in soil is not something I’ve seen very often and I’ve looked at 1,000s of soil analyses at this point. Excess heavy metals that make their way into crops usually come directly from synthetic fertilizers, fungicides and other pesticides. Phosphate fertilizers in particular are a well documented source, since phosphate fertilizer is considered an important source of heavy metals, especially Cd, which originates from phosphate rocks. Copper based fungicides are another source and in organic agriculture poor quality compost, which is very prevalent, is often the source. All of this can be mitigated with mineral balancing, proper crop nutrition, and biological fortification etc.
The primary differentiator between hydroponics and soil based ag is population density and diversity of beneficial microbiology. Some microbes do colonize hydroponic solutions and root zones, but soil simply hosts a far larger and more diverse population than any hydroponic system. Soil’s microbial density and diversity is what allows nutrients to be delivered to the plant through microbial metabolites rather than as raw ionic inputs. In soil system plants receive ions as the end product of a massive microbial supply chain that also feeds the plant biologically active metabolites along the way. This is what drives the phytochemical, vitamin, and flavor quality. The difference in nutritional quality, phytochemical richness and flavor between proper soil grown crops and hydroponics is dramatic.