Chemicals and fertilizers aren't bad per se (everything is made out of chemicals, and there are plenty of organic fertilizers), but you're right that tobacco companies put a lot of unhealthy shit into their products. However, since that is one of the chief concerns I hear from pot smokers regarding legalization, I think there would certainly be a huge demand for organic weed, and millions of people (like me) would be willing to grow it themselves.. Most pot smokers, unlike most tobacco smokers, really care deeply about their health and care what's in the stuff they're smoking. For sure the tobacco companies would try to harness the market as their own and add all sorts of crap to their weed, but I think many of the existing local networks would continue to operate, just above-ground. How many local, organic weed growers are going to just give up when the tobacco companies start growing it?