Passive Absorption

Passive mineral absorption needs no metabolic energy from the plant and works chiefly through ion exchange between the soil solution and the surface of the root, explained by two competing theories. The contact exchange theory holds that ions adsorbed on the surfaces of root cells and of clay micelles are not held rigidly in place, but instead oscillate within a small local volume of space; because these oscillation volumes of a root-surface ion and a clay-bound ion of like charge can overlap, the two ions can simply swap places directly at the point of contact, without either one ever having to pass freely through the intervening soil solution. The carbonic acid exchange theory instead gives the soil solution itself an active role: CO2 released by the respiration of root cells combines with water to form carbonic acid (H2CO3), which dissociates in the soil solution into H+ and HCO3− ions; these H+ ions then exchange with cations that were …