Magnesium is the eighth most abundant mineral in the earth's crust. In cultivated soils, the key is in the minerals, a small part is called 'exchangeable' and an even smaller proportion is in the dissolved state in the soil solution.
Magnesium is essential for the development and behaviour of plants. It is a particular component of chlorophyll, essential for photosynthesis.
The choice of fertiliser is extremely important for our soil. Absorption through the leaves is marginal compared to that taken in through the root systems. The sulphate form (kieserite) is the most easily assimilated. Hydroxides, oxides and magnesian lime present variable solubilities depending on the modes of calcination.
As for carbonate forms (dolomite), they are almost inert in our soils.
Organic products also carry significant quantities of magnesia and in soluble mineral form: spreading often distributes the whole mineral supply.