I don't know if anyone has seen "The High Road With Mario Batali", but episode 4 was about the new trend of putting massive greenhouses on top of the buildings in NYC. The greenhouse featured was 15,000 sf, hydroponic and fully automated. They made the point that they only grow non-GMO crops.

Although it is different than the organic way of soil that I prefer, it was a very interesting segment - and perhaps a great solution for the mega-cities.

there was an article in dwell about a roof-top greenhouse in brooklyn, in school. looked really good too.
http://www.dwell.com/outdoor/a...

That could possibly even be a for profit venture, and wouldn't clean hydroponics be organic?

hydro is usually not organic - synthetic fertilizers are used, since otherwise it is very difficult to keep it free of contaminants (algea, etc).

I see your point, it would be difficult to use fish emulsion or manure tea on that scale.