Question

Write in detail about Passive Absorption of minerals salts?

Answer 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:

1. Contact exchange and 

2. Carbonic acid exchange.

1. 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.

2. Carbonic Acid Exchange Theory: According to this theory, soil solution plays an important role by acting as a medium for ion exchange. The CO₂ released during respiration of root cells combines with water to form carbonic acid (H₂ CO₃ ).

Carbonic acid dissociates into H⁺ and HCO_32in the soil solution. These H⁺ ions exchange with cations adsorbed on clay particles and the catipns from micelles get released into soil solution and gets adsorbed on root cells.