The rhizosphere, or root surface zone, is the radius of soil that surrounds the absorbing root surfaces. Influenced by root exudates, this is the site of interaction between soil microorganisms and the plant. Rhizosphere volume can vary from several millimeters in clay soils, up to one centimeter in sandy soils.

Reply to post by Peter Torres, on September 16, 1998 at 22:19:50:

Corrected version: I mixed up "volume" and "radius" the first time.
The rhizosphere, or root surface zone, is the volume of soil that surrounds the absorbing root surfaces. Influenced by root exudates, this is the site of interaction between soil microorganisms and the plant. Rhizosphere radius can vary from several millimeters in clay soils, up to one centimeter in sandy soils.