Remove fill soil
Trees often decline because soil was spread over the root system. Examine the base of the trunk to determine if soil was placed over the roots. If the trunk becomes much wider as it approaches the soil line and the point where main roots are attached to the trunk is visible at the surface (left photo) then soil was not added close to the trunk. Soil may have been added to trees with trunks that do not flare out at the base (right photo). If this is the case, carefully remove soil at the trunk base until you find large roots growing from the trunk (left photo).
Removing all soil that is above the main roots could improve tree health. One of the best tools for this job is an air spade. An air excavation tool displaces soil with a supersonic stream of air but does not damage roots when properly used (See: air excavation tool). Highly trained arborists can provide this service. A stream of water can also be used but this is messy and takes muck longer, especially in clay soils. Careful digging with a shovel can remove soil, but this damages roots unless you are extremely careful. Injuring main roots is very damaging to a tree. Removing small roots during this process is usually not a concern.. In fact removing roots that cross over main roots or those that circle the trunk can help prevent future trunk girdling problems.
Trees in serious decline resulting from root burial may not respond to this treatment. It may be most efficient to remove dead branches from the tree at regular intervals to reduce risk from falling branches, or to remove the tree entirely.
If the soil level all around the tree appears higher out by the drip line than at the trunk base then soil may have been added. Check by carefully digging in the soil in the area where you suspect soil may have added. If there are few roots in the top several inches of soil then roots may have been buried. If the tree appears health then no treatment may be needed; if the tree is in decline health may be improved by carefully removing soil placed over the roots.