Magnesium is essential for healthy plant development, serving as the central atom in the chlorophyll molecule. Without sufficient magnesium, the plant cannot efficiently capture sunlight and convert it into the energy necessary for growth, a process known as photosynthesis. Magnesium acts as an activator for hundreds of enzymes within plant cells. These enzymes are responsible for complex processes like energy production, protein synthesis, and the transfer of phosphates, which power nearly every biological function in the plant. Ensuring adequate magnesium in the soil is therefore a direct way to support robust growth and maximize a plant’s ability to thrive.
Diagnosing Magnesium Deficiency in Soil
Before applying any supplements, it is prudent to confirm a magnesium deficiency, as adding the mineral unnecessarily can disrupt the balance of other soil nutrients. The most recognizable visual sign of a lack of magnesium is interveinal chlorosis, which appears primarily on the older, lower leaves. This discoloration presents as a distinct yellowing of the tissue between the leaf veins while the veins themselves remain green. Since magnesium is a mobile nutrient, the plant moves it from older leaves to support new growth, causing the deficiency symptoms to first show up on the mature foliage.
Although visual symptoms offer a strong indication, a professional soil test is the most accurate diagnostic tool to determine the nutrient status of your soil. A standard soil analysis will reveal the concentration of exchangeable magnesium, usually reported in parts per million (ppm) or as a percentage of the Cation Exchange Capacity (CEC). For most crops, an adequate level of exchangeable magnesium is considered to be 40 to 64 ppm, or 10 to 15% of the total CEC.
Testing is particularly important because magnesium competes for uptake with other positively charged ions, specifically calcium and potassium. High levels of either of these two nutrients, even with a seemingly sufficient amount of magnesium, can induce a functional deficiency in the plant. A soil test provides the Ca:Mg and K:Mg ratios, which are essential for making an informed decision about the appropriate type and amount of magnesium application. Over-applying magnesium without testing can create an imbalance that negatively affects soil structure and the absorption of other necessary nutrients.
Specific Sources for Supplementing Magnesium
Several materials are available to correct a magnesium deficiency, each with a different solubility and effect on soil acidity. The choice of source depends heavily on the existing soil pH and the urgency of the correction needed.
Epsom salts (Magnesium Sulfate Heptahydrate) are highly soluble and provide magnesium in a readily available form for immediate plant uptake. This source is often preferred when a quick fix is necessary or when the soil pH is already in the optimal range, typically between 6.0 and 7.0. Epsom salts contain approximately 10% magnesium and 13% to 14% sulfur, and their application does not significantly alter the soil’s pH.
If the soil test indicates a magnesium deficiency concurrently with low soil pH, dolomitic lime is the most cost-effective and beneficial long-term solution. Dolomitic lime (Calcium Magnesium Carbonate) is a crushed natural rock that provides both calcium and magnesium, typically 8% to 20% magnesium oxide. Since it is a carbonate, dolomitic lime is slow-releasing and works to raise the soil pH over time.
Kieserite (Magnesium Sulfate Monohydrate) is another effective magnesium source that is less soluble than Epsom salts but more concentrated, containing about 18% magnesium. This material offers a more sustained release of magnesium into the soil solution. Like Epsom salts, Kieserite is a sulfate and has a neutral impact on soil pH, making it a suitable choice for soils that are already at the correct acidity level but require a steady supply of magnesium.
Methods for Applying Magnesium
The method of application should be chosen based on the selected magnesium source and the desired speed of correction. For long-term soil health improvement and sustained nutrient supply, soil incorporation is the most effective approach, particularly for slower-release materials like dolomitic lime and Kieserite. This involves broadcasting the recommended amount of material uniformly over the soil surface and then tilling or raking it into the top few inches before planting. Broadcast rates for soil correction typically range from 20 to 40 pounds of magnesium per acre, or about 0.5 to 1 pound per 100 square feet, though this must be guided by soil test recommendations.
For an immediate, temporary correction of acute deficiency symptoms observed during the growing season, foliar feeding is the preferred technique. This method bypasses potential soil issues like high potassium levels or cool temperatures that inhibit root uptake, allowing the plant to absorb magnesium directly through its leaves. Highly soluble sources, such as Epsom salts, are ideal for foliar application because they dissolve completely in water without clogging sprayers.
A common guideline for a foliar spray is to dissolve 1 to 2 tablespoons of Epsom salts in one gallon of water and apply the solution to the foliage until the leaves are thoroughly wetted. The best time to apply foliar sprays is during the cooler parts of the day, either early morning or late evening, to prevent leaf burn and ensure maximum absorption. While foliar feeding provides a rapid visual improvement, it only delivers a small amount of the nutrient and should be viewed as a temporary measure to supplement a comprehensive soil-based program.