Most feeder roots for a tree are in the top 6 -12 inches of the soil. Many
soil nutrients remain in the top few inches of soil due to the inability of
the molecules to move through soil. In fact, if potassium and phosphorus
containing fertilizers are not dug into the soil about 6 inches, they are
unavailable to the plant. The roots also require air to function properly.
It is the moist air between particles of soil that allow moisture to be
taken up by the root system. Most people think of roots as something that
sucks up water like a straw but that is not how water molecules are
transfered into plant tissue.

By piling two feet of dirt on top of a trees established root system, you
basically suffocate and starve the tree before it has time to put new roots
up to the surface where they can function properly. It is common
horticultural practice that the soil level around established trees NEVER be
altered. This is why you see some trees with a deep well around the base.
In that instance someone has needed to raise the soil level and has
protected the tree by keeping the soil level adjacent to the tree at the
lower level while the rest of the land can be regraded to the new higher
level.

Maybe this would be a solution for you? I didn't catch why you wanted to
raise the soil level but putting a well around the trunk of your tree will
allow you to raise the soil levels in the rest of your garden higher. Just
remember to allow enough space between the well and the trunk so that the
surface feeder roots have enough area to prevent suffocation.

From: "Nehmo Sergheyev"
Newsgroups: alt.forestry,rec.gardens,sci.bio.botany
Date: Sun, 06 Feb 2005 21:49:04 GMT
Subject: Stem Griddling Roots After Raising Ground Level?

- Travis -
Do you mean you are going to put a couple of feet of soil on top of
the
ground where the tree is planted? That will kill the tree for sure.

- Nehmo -
Yes, that's what's going to happen. I didn't know it would kill the
tree. Are you sure? Why would it?