Answer: The very best advice I can give you is to run some basic soil tests and find out for sure about the existing soil mix; there is just no way to tell just by looking at it, and top soil and mushroom soil can vary widely in their composition and analysis. As a rule of thumb, the complete fertilizer should be fine to use although it may be over or undersupplying one or more of the nutrients -- the soil tests will indicate where you need to be supplementing and where you don't. It is a good idea to keep on "feeding" your soil by regularly adding more organic matter such as compost or rotted leaves or aged stable manure and bedding. Using a natural mulch will also feed the soil as it breaks down over time. If nothing else, you really should check the pH. The only other possible addition would be a bit of sand to assist with drainage, but this would depend on how well draining the existing mix is. Your County Extension (622-4225) should be able to help you with the tests and interpreting the results.
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