What is “usable soil volume”?
A. Usable soil volume describes the amount of soil available for tree root growth. For example, if a tree is planted on a severely compacted clay soil, the usable soil volume will be only the soil disturbed during installation or a few inches on the surface loosened by mulching—perhaps less than 2 or 3 cu. ft. Tree roots grow primarily in the top 2 feet of soil, and most grow very near the surface. For this reason soil below 2 feet would not be considered in soil volume calculations in most cases. For example, if a tree is planted in a sidewalk cutout that is 4 x 4 feet and the soil is uncompacted, then the usable soil volume could be considered to be 4 feet x 4 feet x 2 feet deep, or 32 cu. ft.
