Fertilizer...collect some stinging nettle seeds and spread them around your neighborhood. They grow fast and everywhere, so you don't have to use your own garden to mass produce them. Anyway; harvest a bin full; submerge them in water; put a lid on it and 4-6 months later you got yourself a beast of a fertilizer. Get a bunch of neighbors to sacrifice some space for barrels or rain bins, and you all get endless fertilizer out of it. Seal them tho; because the odor is quite nasty.