Sandybottom
Could you please take some pics and load them up. 800px wide max please.
Roots in sand tend to normally go deeper as the particles allow more air compared to say clay.
The reason why roots will tend to the surface is
# Species tendencies
# Lack of nutrient deeper
# Moisture
# Anchorage
Now in a sandy soil the moisture one is least likely, sand tends to dry out quick on the surface as anyone who walks the beach would know.
The nutrient issue, sandy soils leach easily (nutrients wash down in the profile). Thats said it could be that nutrients are more commonly refreshed/found at the surface.
Anchorage is vital, a wide spread buttress system would be of benefit.
Anyway, here's a page about soils.
We are all about trees - soil
Now whilst it's based here in Brisbane it does give you some ideas for working with it.
I would say topping over with some sandy soil a little at a time is OK, by a little I mean maybe 1" at a time.. but do keep away from the trunk by say 6".
Pics will allow us a little bit more of an idea what you have there, and perhaps to remodel your garden.