Still we were better off than we'd been in some previous gardens where the good earth was about 8cm deep over an archaeological dig of old bricks, shards of delph, and broken bottles. It was with a frisson of empathy, therefore, that I read about straw bale gardening in Philadelphia via Metafilter. The recommendation is to buy as many straw bales as you can afford and water them well, applying some 'fertiliser' [unspecified] alternate days for a couple of weeks. After which you can pop your seedlings into a convenient hole and let photosynthesis do the work. Of course, you should lay out the bales so that the open ends aee top and bottom. The design of straw as a hollow tube works well for wicking and storing water. If you can't afford 'fertiliser' it's probably a good idea to pee on the bales because straw is quite nitrate-deficient; but go easy - urine is salty.
Some of the comments on the Metafilter page express worry about toxins in urban soil - Victorian paint factory round the corner etc. - but the contaminated soil is 450mm from the base of your lettuce if you plant on top of a straw bale. The added