Need advice, please: Starting a project to establish a mostly native planting area in a sandy, usually sunny, traffic island in my neighborhood. Plan is to "frame" out 4 feet on 3 sides, the existing concrete slab that bears a single bench (former bus stop) with landscape timber. Plan to remove some soil (50%?), mix in soil amenders (top soil and aged horse manure), and top with mulch. (1) Do you see any flaws in this? (2) Should we insert some type of shield between the new and old soil, perhaps 4 to 6 inches deep, and if so, what material, cheapest way to go, and is there a scroungable freebie material that would work? All advice appreciated!