Soil testing 101 and too much of a good thing gone bad

I've been working on a project that involves a backyard that is a death zone for plants. It's a brownstone that has had the same owners for over ten years and everything they've planted has died, over and over again. They chose the appropriate plant material for the shaded space, they adjust the irrigation when needed (since backyards tend to get over watered in the shade) and they also added fresh manure based compost at least once a year. What gives?
I've finally solved the mystery by doing a soil test from the Cornell Soil Lab, since the diy soil kits weren't telling us squat. Apparently, the soil has an abnormal amount of organic material (wayyy too much compost) which caused the ph to be abnormally low and turned the soil into poison for plants. I won't get into the technicalities but I advise anyone with a backyard to take a couple of soil samples from their back yard and sent it in to the Cornell labs for a very detailed report of your soil. If you have questions or want to discuss your soil report with someone they have a very helpful representative based here in NYC for all of your questions. I don't recommend vegetable growing directly out of any backyard in NYC but if you are tempted, check out the metals in the soil before you plant (the report will give you that).
As for the garden I mention above, we are adding 5-7lbs of garden lime per square foot to increase the ph and hopefully be able to plant something by the end of spring.