Bassanio - welcome to the forum.
I am facing the same situation. Last fall we had to have an ENOURMOUS southern live oak removed. The stump was ground and I have a large - 23' x 20' x 20" deep mound of sawdust tapering out to ground level on all sides. I want to use that as a base for a berm. I want to extend it to the left as viewed from the patio. After consulting with my County Agent and Master Gardener friends I have been adding grass clippings. The plan is to till that in then add compost, till in and add aged horse manure, tilled in. The process ill be repeated once or twice during the summer. My goal is to have the sawdust well composted by fall and the berm built to the depth, width and height that I want.
The problem has been the daily rain for the past 2 1/2 weeks.
I want to plant a Drake or Lace Bark Elm as a focal point of the berm. Fast growing, small (25' to 35' tall and wide) shade tree suitable for my subdivision lot and my growing region.
Back to your issue. You may want to consider having a soil test done to see what, if any, nutrients are missing from your soil. With random fertilization you may be doing more harm than good. A soil test gives you a base line to work from.
Adding organic matter is always a good thing. When you add organic matter be careful not to put it against your young tree. Leave at least a 12" radius from the trunk to your organic matter.
Elizabeth - or Your Majesty
Living and growing in Lafayette, La.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant. ~Author Unknown