I'm very much enjoying the conversation on urban permaculture.
I like to think I'm at least 1 on the permaculture scale, but I have a lot of the same issues that Rick mentions. I try to incorporate some of the ideas (minimizing inputs, using plants that help each other, tons of organic matter rather than fertilizer), but I'll never be able to have a true permaculture system here. I compost all of my food waste and most of my garden waste, but I still can't produce enough compost on 1/8 acre (that's the whole lot, not just the garden) to keep my garden going without any outside sources of nutrients. I prefer to get around the problem by collecting and composting my neighbors' garden waste rather than buying high-NPK fertilizer - a step toward sustainability for myself, but obviously not sustainable if everyone tried to do it.
I think there's an interesting difference in definitions of sustainability, not just how the permaculture system is defined, going on here. Many urban farmers, even those who are proponents of permaculture, encourage composting of materials from the surrounding housing, not just the one lot where the food is grown. There is nothing sustainable at all about putting tons upon tons of food waste in trash trucks heading to landfills, so keeping it out of that cycle is more sustainable on a global scale, even if it means that an outside input is going into my garden.
