I help run an urban farming collective in the Netherlands, we're setting ourselves up as a connecting bridge and knowledge-sharing platform between several smaller gardens. One of the things that I'm trying to spread the gospel about is composting and feeding the soil.
Thing is, there is this sweeping assumption that 'oh, all the ground is polluted'. Right now, most of the gardening is done in garden beds, with 'anti-rooting' fabric at the base of the beds. The city council has provided woodchip (sprayed? I don't know) to create lots of paths. In several places, the kids are told they shouldn't eat the wild blackberries growing because 'the soil is polluted'.
Nobody seems to know what is precisely in the ground, and how that is differentiated across the town. I have applied to the city council for the 'soil analyses', and the only thing they can tell me is whether there are old oil tanks in the ground or not, or whether there is industrial pollution or not. The vast majority of the city still needs to be researched. The council is doing various types of cleaning the ground, though clearly not where we have the gardens. Just to be clear - there are no obvious signs of pollution, i.e. no factories nearby, no spills, etc., though we don't know the history of the land we're on.
"Polluted land" is such a broad sweeping term, it's difficult to know what it is and how to tackle it. Also, how to tackle the fear and detachment that people feel from the land.
Things we are trying:
Building healthy soil in the beds themselves with mulching and compost.
We are figuring out where we can send soil tests to find out the ph, mineral composition, etc., so that should give some insight.
I've thoughts that if you do a proper lasagna mulching, you won't need go deep into the ground anyway.
I am also slowly suggesting small-scale hugelkultur to also be 'above ground', though in the community garden context several places have insecure land tenure in that in principle they could be evacuated at a month's notice so the time frame is an issue - though that doesn't mean we shouldn't try anyway.
Although there is plenty of toxic soil (and air) in cities, as you are suggesting it's wrong to assume that all the soil is polluted. Assumptions like that don't make sense. The soil at each specific garden site needs to be tested for organic and metal pollutants. Many cities in the US offer free lead tests, lead being the most common soil pollutant in cities, but we need to be doing more comprehensive tests, looking at other metals and petroleum-derived contaminants.
I see a certain irony in worrying about polluted soil while we are inhaling pollution with every breath in many urban environments (not to mention those people who are eating GMO and chemically treated food).
I think you are doing the right thing in testing, creating mineral-rich soil (which can reduce uptake of heavy metals) and building raised beds. Most feeder roots in soil are in the top 6-12 inches so growing in raised beds and hugelkultur makes a lot of sense.