I'm a bit worried about soil quality in an area of a trash-filled landfill after ~50 years...
You should! Having quite a bit of experience I can safely state that older landfills are ticking timebombs... They pollute the groundwater and soil, leaking all sorts of "pleasant" chemical compounts to the surrounding area. Especially as rainwater falls on the landfill and trickles trough the waste. The problem is that the pollution doesn't necessarily stick around the landfill itself. Underwater groundflows tend to disperse the nasty stuff over a far greater area.
Modern landfills are build to handle this problem and collect the percolatewater and treat it, so no pollution can enter the soil.
On the other hand, I think we humans should start at the source and reduce the amount of waste beeing produced, so we don't have to find new ways to get rid of it in the first place! As a prime example, I'm finally going to install me a compostheap, to help reduce my wasteproduction!