Forget existing soil. Use raised bed gardening instead. Add 1/3 compost (manure is in it )1/3 pearlite, 1/3 cocunut coir, garden soil, rock dust and fertilizer.
I think the best suggestion would include copious amounts of manure and compost worked into the soil to a depth of 12+ inches.
A soil test would help. Results would tell you which path to take for your preferred pH. If you have an agricultural extension office nearby, they will also have suggestions. As for the roots, they will deteriorate into compost with time.