Role in Mineral Absorption

It seems logical to assume that minerals are absorbed into plant roots as water is absorbed. It is a logical assumption but an incorrect one. The uptake of water and the uptake of minerals are independent processes.

Minerals enter root cells through a permeable membrane when the concentration of the mineral salts in the soil solution is greater than in the root cell. Such a condition creates a concentration gradient. Since plants are continually using the mineral salts within the roots, the concentration gradient serves to explain how certain elements are absorbed. With others, absorption occurs even against a concentration gradient. The explanation is thought to reside with ion exchange or with contact exchange.

In ion exchange, a positively charged ion may be absorbed by a root cell if another positively charged ion is released from the cell. Another form of ion exchange can occur when both a positively charged ion (cat­ion) and a negatively charged ion (anion) are absorbed by the root cell together, thus maintaining the electrostatic equilibrium in the cell.

In contact exchange, the intimate association between the soil par­ticles and the root hairs is the key. A direct exchange occurs between the ions adsorbed to the particles of soil and those of the root cells.