Availability of minerals in vitro is a dynamic system and involves many processes such as: the input of minerals, interactions between minerals (precipitation), interactions between minerals and other medium constituents (EDTA, O-C, sugar and gelling agents), binding of minerals to the gel matrix, solubility, water availability, and movement of minerals by diffusion and mass flow. There are three principle mechanisms by which minerals in the medium may reach the surface of the explants and are ready to be taken up by the explants. (1) The minerals directly exchange with the explant surface (explant-interception within the medium). (2) The minerals may be carried towards the explants by mass flow in the water flow. (3) The minerals will diffuse from high to low concentration . (4) The combination of two s or more processes. The maximum amount of minerals that can come in direct contact with the explant is dependent on the area of explant contact with the gel, which is very small.
