Nothing grows here

There is a rectangle of dirt east of our back door. Over two years we tried various annuals. Last fall we planted 3 Dwarf European Canberry bushes. Lo and behold they leafed out this spring and appeared to be maintaining although not thriving. After about a month the leaves began to develop black spots. Now the shrubs appear to be dying (as has everything planted there). Before planting the shrubs we dug out the old dirt down about 10 inches and replaced it with good soil. We have come to the conclusion that the ancient silver maple nearby is sucking all the life out of the soil. Our next move will be to cover the dirt with plastic and rocks and use planters... unless someone has a better idea. Help!