Question
I have a 30 year old Schwedler Maple tree that I want to build a 13 ft. diameter 16 inch tall circular bed around. I will construct an inner wall 8 inches out from the trunk of the tree to keep the soil away from the bark. It is a shallow rooted tree (the roots within the circle are below ground now , but only about 4 inches deep,and that fact brings me to the question that I have for you.Will the additional 16 inches of soil that will be covering the roots when I'm done building the raised bed injure the tree?And what what ingredients and what percentage of each should I use to make well draining soil for this raised bed?

Answer
Hi Bonnie,  
You should not see any ill effects from a bed that size on a tree that old.  Most people think tree roots extend to the dripline when in fact they go well beyond the dripline.  The average is about 2.5 times the distance from the trunk to the dripline and some trees extend up to 7 times the distance.  Covering the entire rootzone area with even 2 inches of soil can kill it, but your flower bed over such a small area should be fine.
Your local bulk soil company should have a soil mix that is a clay/pine bark/sand/compost mix.  For perennials and annuals, I'd like to have clay make up about 25% of the mix.  For shrubs about half should be clay.  
In an elevated bed, we need the clay for water and nutrient retention.  Otherwise, in extreme periods of heat, watering becomes a real problem and winter damage is possible is soil that dries too quickly.
You will have to monitor you watering closely, because that tree will get most of it.
Jim