In most cases, the soil is already there.

You can use various techniques to improve the soil without totally replacing it, sheet-mulching is a good one:

http://www.agroforestry.net/pubs/Sheet_Mulching.html

I live on the urban/suburban fringe and we've been planting different things based on the soil/light/drainage conditions. We have a dry gravel hilltop nearby that we've been planting things like black locust, raspberries, and jerusalem artichoke (with some added topsoil). In other places we have some fairly deep mucky soil that we've planted pawpaws, nut trees, and various maples. This is public land that is mostly used for recreation trails. No one cares because it's mostly full of invasive trees that we're trying to slowly use for firewood (seasoned buckthorn is a fruitwood!)

The local birds are doing a great job of planting things like mulberries in parks, and we help out with an occasional raspberry, shade tree, and nut tree. The only problem is that the wild rabbits like to eat most of what we plant, and any tree tubes we use to keep the rabbits out end up being bottle-rocket launchers around the 4th of July.

I live in a 9 story condo building in Jersey City. My wife is head of the building and grounds committee. I told her that we should be pooping in the yard. She says she'll have me arrested. :)

Seriously though, there should be a way to recycle urban wastes so as to enable urban gardening.

Edit:

Right now my disposal instructions specify cremation. I'm starting to wonder if there isn't a better way.

But then,

To what end
does one grow wiser
when one ends
as fertilizer?

In New Orleans, ALL soil is alluvial silt/sand, most of it with humus.

The neutral grounds (medians to y'all) have traditionally been, in some spots, used for herbs. No one needs very much, so a clump can serve the local community. Usually found in areas with no yards at all.

Alan

I live on an acre of land, and keep chickens [one dozen Light Brahmans]. The coop is next to the veggie-patch [nice solid deer-netting fence between the two]. In late fall/ winter, they run the entire acre, in summer during veg season, I coop them and feed them weeds and veg cuttings, and give them lawnmower clippings as coop bedding. In winter, I toss the compost onto the veggie patch and let it sit till I dig it in in spring. I also compost table scraps, etc.
The eggs are amazing, and the garden [which is small, but I am an old hand at intensive growing] puts out enough to keep us in veggies year-round.
And yes, in other places at other times I've been known to buy a truckload of manure, mix it in with my own soil, build raised weedless beds, and call it good. There are many ways to do this.