Write in detail about Passive Absorption of minerals salts.

Explanation

1. Ion-Exchange: Ions of external soil solution were exchanged with same charged ( 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 space 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.