Answer: You didn't identify the trees so I'm just going to give general information. Some trees adapt quite nicely to clay soil so you may not have to amend the soil at all. Others need faster drainage. For these type trees you'll want to amend a large area (not just the planting hole) with some organic matter, or you'll want to plant them on berms of amended soil. In either case, mix compost or aged manure with the native soil to loosen it and help it drain faster.
Best wishes with your new trees!
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