Magnesium serves two main roles that affect plants. One is improving plant health indirectly via the soil, and the other is improving plant health directly.
In the soil, magnesium stimulates bacteria to activity and speeds up the process of decomposing organic matter.
In the plant, magnesium takes part in enzyme reactions and in the absorption and metabolism of other minerals. It is also the central part of the chlorophyll molecule.
Because magnesium is necessary for the decomposition of organic matter, healthy compost usually contains magnesium.
Other sources of magnesium that plants can use include Epsom salts and dolomitic lime.
Soils that are low in magnesium tend to be those that suffer a good deal of nutrient leaching, perhaps due to sandy texture, excessive moisture, or insufficient organic matter. Acidic soils are also prone to magnesium leaching.
In some cases, a deficiency may be induced in plants that have absorbed too much calcium or potassium. Likewise, if plants do not receive enough water, they cannot absorb magnesium from the soil.
Symptoms of deficiency include:
- Yellowing of older leaves between veins and around margins, perhaps with brown or purple spots.
- Upward cupping of leaf margins.
- Leaf death.
- Plant death.
When faced with a magnesium deficiency, it is typically best not to supply large applications of high-magnesium fertilizers, as this can create a new mineral imbalance. Instead, use generous amounts of compost to supplement your plants’ magnesium take, and then work on improving the overall health of the soil. In particular, correct the pH and amend the soil with organic matter.
Magnesium toxicity is fairly rare because, by the time a plant has absorbed enough magnesium to show any symptoms, other nutrient imbalances have already emerged.
Magnesium toxicity can occur naturally when the rock underlying the soil is high in magnesium. It can also be induced by the overuse of dolomitic limestone and similar amendments.
Symptoms typically include:
- Yellowing or striping of leaves.
- Reduced yield.
- Calcium deficiency.
- Potassium deficiency.
Should this occur in your garden, your best bet is to bring calcium and potassium back into balance. Of course, you will also want to discontinue the use of high-magnesium soil amendments.