Many home and commercial vegetable growers have their own secret recipe that helps produce the biggest and best-tasting vegetables. Often, forms of magnesium (Mg) are a part of that recipe. Magnesium is required by all plants. It is also the nutrient most likely to be deficient in vegetable soils.

Crops

Vegetable crops vary in their sensitivity to magnesium deficiencies in the soil. Beans, English peas, beets, radishes, lettuce, Swiss chard and sweet potatoes tend to be tolerant of soils that are low in magnesium. They seemingly use the available magnesium more efficiently that other crops. Deficiency symptoms in these crops tend not to develop until the levels of magnesium are critically low.

Solanaceous crops (tomatoes, peppers, eggplants, etc.) and Cucurbits (watermelons, cucumbers, squash, etc.) develop deficiency issues rather quickly when magnesium levels are limited. Many Brassica crops also have this problem, especially greens, broccoli and cabbage.

Diagnose

Growers should be proactive and prevent a magnesium deficiency before it causes a problem. The best way for a grower to determine if they need to apply magnesium is by performing a soil test. A soil test will determine which nutrients the soil is lacking. The Auburn University soil test lab will provide recommendations for which, if any, nutrients need to be added.

Symptoms

Magnesium is a mobile element, meaning it can travel though the plant from old growth toward new and developing growth. As a result, deficiency symptoms will appear on the older growth first. If symptoms are not corrected, these symptoms can progress to younger growth. Figure 1 represents typical magnesium deficiency symptom on collards. Note the yellowing between the veins of the lower leaves and the upward cupping of affected leaf margins.

Sources

There are several common sources of magnesium available for growing vegetables. Epsom salt (magnesium sulfate) is one of the most common sources and is found in many growers’ management plans. The Epsom salt typically used in vegetable production is the heptahydrate sulfate mineral epsomite. Some other common sources of magnesium include:

• Brucite (39% Mg)

• Sulfate of potash-magnesia (11.1% Mg)

• Kieserite (18.1% Mg)

• Magnesium chelates (2% to 4% Mg)

Application

Magnesium is often applied from dolomitic limestone (11% Mg) or high-magnesium limestones, especially if a pH adjustment is required. If a soil test shows no liming agent is required, magnesium should be applied as a fertilizer on low-magnesium soils.

If a vegetable crop develops a deficiency, apply 2 to 4 pounds of actual magnesium per acre using a drip irrigation system. Use a water-soluble form of magnesium, such as Epsom salt. To determine how much Epson salt to inject, growers should multiply the recommended level of magnesium per acre they want to apply by 10.136.

For example, if a grower wants to apply 4 pounds per acre of magnesium from Epsom salt, they would multiple 4 by 10.136 and get 40.54 pounds of Epsom salts. Do not apply all of this at once, but inject it over three or four irrigation events.

If a grower needs to side-dress — apply fertilizer between crop rows — he or she should use 25 to 30 pounds per acre of actual magnesium.

In an emergency, magnesium may be applied as a foliar spray. However, growers should not expect great results. Magnesium does not penetrate leaves well. A rate of 2 tablespoons of Epsom salt per gallon of water is generally recommended. Growers need to apply this multiple times to have an impact. Do not add any dish detergents or other soaps to the Epsom salt, as this will likely result in crop damage.

While magnesium is an important factor to vegetable growth, applying more than the soil test requires will not further improve vegetable growth. Applications only help when there are insufficient levels in the soil. Applying more that recommended levels is a waste of resources.

For more help or to ask questions, visit www.aces.edu or call 256-232-5510.

— Provided by Joe Kemble and Jeremy Pickens.