Magnesium is an essential secondary macronutrient that plays a role in plant metabolism and growth. It is the central atom within the chlorophyll molecule, making it indispensable for capturing light energy during photosynthesis. Magnesium also acts as a cofactor, activating hundreds of enzymes responsible for energy transfer, protein synthesis, and carbohydrate metabolism within the plant cells. Adequate magnesium supports the plant’s ability to use other nutrients efficiently, transporting phosphorus and ensuring proper sugar distribution from the leaves to the roots and developing fruits.
Recognizing Magnesium Deficiency in Plants
The most definitive visual indicator of insufficient magnesium is interveinal chlorosis, where the tissue between the leaf veins turns yellow while the veins themselves remain green. This pattern occurs because magnesium is mobile within the plant structure, allowing the plant to relocate it from older leaves to support new growth. Consequently, deficiency symptoms first appear on the older, lower leaves as magnesium is scavenged for use in younger tissues.
As the deficiency becomes more severe, the yellowing areas may develop rusty-brown or necrotic spots, and the leaf edges can curl upward. Distinguishing this from other deficiencies is important. Iron deficiency also causes interveinal chlorosis, but it targets the newest leaves because iron is immobile. Nitrogen deficiency causes a general, uniform yellowing of the entire older leaf, which does not follow the green-vein pattern characteristic of magnesium loss. Observing the location and specific pattern of the yellowing is the first step in accurate diagnosis.
Primary Sources of Magnesium for Soil and Foliage
Magnesium supplements offer different speeds of availability and secondary benefits. The fastest-acting source is magnesium sulfate, commonly known as Epsom salts, which is highly water-soluble. This quick solubility makes it immediately available for plant uptake, offering a rapid correction for visible deficiencies. Magnesium sulfate also provides sulfur, a secondary nutrient important for protein formation.
A slower, long-term soil amendment is dolomitic lime, which contains both magnesium carbonate and calcium carbonate. Dolomitic lime is beneficial when a soil test indicates a need for magnesium alongside a low soil pH, as its carbonate component raises the soil’s acidity. Because it is slow-release, it provides a steady source of magnesium but is unsuitable for immediate deficiency correction in alkaline soils, where it could raise the pH too high.
Many commercial complete fertilizers (NPK formulations) now include secondary nutrients like magnesium, often listed as magnesium oxide or another soluble salt. These products offer a balanced approach, providing magnesium as part of a regular feeding schedule. The choice of source depends on the urgency of the plant’s need and the existing soil conditions.
Step-by-Step Application Techniques
For the quickest absorption, magnesium sulfate can be applied as a foliar spray directly onto the leaves, bypassing the soil’s uptake mechanisms. The common mixing ratio is one tablespoon of Epsom salts per gallon of water. Spray the solution finely to both the upper and lower surfaces of the affected leaves, ideally during the early morning or late afternoon to prevent leaf scorch.
A soil drench is another rapid method for delivering magnesium, useful for potted plants or when wider application is needed. This involves dissolving a slightly higher concentration of magnesium sulfate, around two tablespoons per gallon of water. Pour the solution directly onto the soil around the plant’s root zone, where the magnesium is quickly absorbed by the roots.
For long-term correction using slow-release amendments like dolomitic lime, the product must be incorporated into the soil. Before planting, spread the recommended amount (often based on a soil test) evenly and till it into the top six inches of soil. For existing plants, apply the amendment as a top-dressing around the base, followed by light watering to help it dissolve and move into the root zone.
Avoiding Magnesium Overload and Nutrient Imbalances
Applying too much magnesium can be detrimental by creating a nutrient imbalance in the soil. An excess of magnesium can trigger cation antagonism, directly interfering with a plant’s ability to absorb other positively charged nutrients. Specifically, high magnesium levels can inhibit the uptake of both calcium and potassium, even if those nutrients are present in adequate amounts.
This antagonism can lead to secondary deficiencies, where the plant exhibits symptoms of calcium or potassium deficiency. For instance, excess magnesium may induce blossom end rot in tomatoes, a condition linked to poor calcium distribution. To prevent this, perform a soil test before applying large amounts of any magnesium amendment, ensuring cation ratios are balanced and magnesium is only added where a deficit is confirmed.