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.