The area of the solid particles in a given quantity of soil or porous medium. (i) BET surface area is that area on which gas molecules, such as N2 or O2, can adsorb which normally does not include the planar surface of expanding clays such as smectites. (ii) EGME surface area is that area on which ethylene glycol monoethyl ether can adsorb which normally includes the planar surface of expanding clays such as smectites.