Magnesium is considered somewhat a secondary nutrient and somewhat a micronutrient. It is right on the border of secondary/micronutrient due to the level at which it needs to be present. It is a central atom in chlorophyll and helps assist with photosynthesis. Magnesium has a very particular relationship with two other nutrients, calcium and potassium. Magnesium and Calcium have an extremely antagonistic relationship. If they are not in the proper ratio, severe nutrient uptake issues can occur (i.e. if you have too much magnesium and not enough calcium, severe lockout will occur). Magnesium toxicity is extremely rare, so more often than not, there is not enough calcium in the system to balance magnesium levels. Magnesium also helps contribute to protein synthesis with the assistance of enzymes.