I recently moved to a new home and want to create a large garden bed in the backyard that will include native Illinois ornamental trees, shrubs, and perennials. The current soil is mostly clay and the drainage is poor. I'm researching soil amendments and am coming across some conflicting advice. I was planning on adding organic compost and pine bark mulch and roto-tilling that in to about 18 inches deep. Some websites suggest adding other amendments such as sand, gypsum, perlite, or peat. What do you think is the best way to amend clay soil and in what quantities? The space is about 500 square feet.