Are you concerned about the tree? I'm a little confused, sorry. Trees don't like to have their roots smothered with top soil. Three or four inches of good organic mulch is the best thing. Fertilize in the late winter in your area. But if you are worried about the health of your tree, perhaps something else is the issue.

I am naturally concerned about the tree, I would not want to do anything to harm it. It is a large red oak. What I am wanting to grow healthier and larger is the variety of plants I have planted under it. I have a Lilly of the Nile, some Spider plants, Mother in Law's Tongues (I cannot remember plants actual names, the old fashioned ones my mother taught me are the ones that stuck) Dutchman's Pipes, some flowerless purple Cannas...I am experimenting to see what will grow best and so far nothing but the Spider plants do very well. I want to improve the soil without ripping out everything and digging it up. I try to keep pine straw and dead leaves on it. I want to put something on the surface that will help the plants and not harm the tree. Is this just wishfull thinking?

I don't think it's wishful thinking. As long as the tree roots at the base of the tree are not overloaded with mulch. The tree roots have to be able to breath. I have a shade bed around one of our huge oaks (not a redoak)and I put light mulch on that bed almost everyfall and ammend the soil in that bed. I think you'll be fine ammending your soil for your plants around your tree.

Dancey

I see, now! Wasn't sure if you were wanting to fertilize the tree or if you had other plants in that bed! Fall is not the right time to fertilize in your climate. You could encourage a new flush of succulent green growth that could become a frost target. The very best thing for tree AND for the plants under the tree is organic mulch.

3 to 4 inches would be tops for the mulch layer. You could top dress with compost, if you wish. Rainfall would help it infiltrate through the mulch to the soil, where it would do some real good. In the spring, a conservative dressing of a balanced fertilizer would be of benefit to your plantings.