Write in detail about Passive Absorption of minerals salts.

Explanation

Ion-Exchange: lons of external soil solution were exchange with same changed (anion for anion or cation for cation) ions of the root cells. There are two theories explaining this process of ion exchange namely: i. Contact exchange and ii. Carbonic acid exchange.
i. Contact Exchange Theory: According: to this theory, the ions adsorbed on the surface of root cells and clay particles (or clay micelles) are not held tightly but oscillate within a small volume of speach called oscillation volume. Due to small space, both ions overlap each other's oscillation volume and exchange takes place.
ii. Carbonic Acid Exchange Theory: According to this theory, soil solution plays an important role by acting as a medium for ion exchange. The `CO_(2)` released during respiration of root cells combines with water to form carbonic acid `(H _(2) CO_(3)).` Carbonic acid dissociates into `H^(+) and HCO_(3) ^(-)` in the soil solution. These `H^(-)` ions exchange with cations adsorbed on clay particles and the cations from micelles get released into soil solution and gets adsorbed on root cells.