Answer: Both sand and clay based soils are improved by the addition of organic matter such as compost and/or mushroom soil and/or old rotted leaves and/or aged stable manure and bedding. Top soil is not a regulated material so I would not recommend adding that. You will need to loosen the existing soil down about a foot and work in the organic matter. This process will create a slightly raised area but it will settle over time.
Before you begin, it would be a good idea to run some soil tests to help guide your preparation process. Your local county extension should be able to help you with the testing and interpreting the results based on the plants you want to grow.
I should mention that blueberries have very specific needs in terms of soil, requiring a low pH and a texture that stays evenly moist yet drains well. I am also a little concerned that your space may not be large enough to accommodate even one or two of each fruit along with a small patch of strawberries. Please be sure to check the mature size of the specific varieties you want to plant. Your extension staff should also be able to recommend specific varieties that seem to grow best in your local area and soil type.
Enjoy your new garden!
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