Arborists will tell you--emphatically-- that soil quality and quantity can be enhanced with compost and mulch, but did you know that simply applying your lawn clippings can make a high quality mulch/compost blend? You see, the nitrogen contained in the grass clippings, and the carbon in the (now shredded) leaves that were on your lawn are ready to start the process of breaking down into stable compost much quicker than either, alone; the "gestalt" principle. It is free, a good way to mulch for fall, and gives you a nice way to re-use that organic material to help your trees thrive into Spring. Give it a try! You'll be amazed at how quickly the clippings break down in the root-zone of your trees, and how the tree will respond so favorably in Spring. With natives, this ought to be enough to never worry about fertilizing again!