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.

The best soil amendment for Northern Illinois soils will be composted organic matter. Organic matter will help structure the soil to improve drainage, feeds the soil micro flora which in turn will support plant growth, will retain soil moisture during dry weather. Rototilling to a depth of 18 inches will not be necessary or even desirable. Incorporation 6-8 inches deep will be adequate. You might amend any back-fill material on an individual planting hole to provide more organic matter. Sand can actually make the soil more compacted by filling in any remaining voids in the soil. Gypsum is a chemical reaction with clay that is only temporary. Perlite provides a physical property to keep the soil from compacting and can hold water as well.