What takes place to soil that's been under tarmac and buildings for numerous years?

Some recent soil Tip on Aquaponic:
Question by Paul: What occurs to soil that's been beneath tarmac and buildings for numerous years?
In big cities the soil has been cut off from the light and has not been fertilised by rotting vegetation for possibly hundreds of years.
What chemical changes have occurred in that soil and why?


Very best answer:
Answer by AmphiboliteThis soil becomes compacted due to pressure forcing out liquids and gases. Any chemical changes would rely on the local chemistry

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