Michigan State University Extension
Soils & Soil Management - Fertilizer - 05209703
07/10/97
Secondary and Micronutrients for Vegetables and Field Crops Extension Bulletin E-486, Revised August 1994
By M.L. Vitosh, D.D. Warncke and R.E. Lucas Department of Crop and Soil Sciences Michigan State University Extension
Magnesium is a part of the chlorophyll molecule in all green plants and is essential for photosynthesis. It also helps activate many plant enzymes needed for growth. Magnesium, a relatively mobile element in plants, is absorbed as the Mg++ion and can be readily translocated from older to younger plant parts in case of a deficiency.
Magnesium Deficiency Symptoms
In corn, magnesium deficiency symptoms first appear as interveinal chlorosis in the older leaves (Vis. 3). Symptoms often appear early in the season in cold, wet soils and may disappear as the soil warms up and dries. Severe deficiency may cause stunting.
In oats and wheat, the older leaves show a distinctive chainlike yellow streaking. In potatoes, the loss of green color begins at the tips and margins of the older leaves and progresses between the veins toward the centers of the leaves. The leaves become brown or reddish and very brittle during the advanced stages of the deficiency.
In celery, chlorosis begins on the tips of the older leaves and progresses around the leaf margins (edges) and inward between the veins. The oldest leaves are also the first to show symptoms of chlorosis in greenhouse tomatoes. The veins remain green while the interveinal tissues become yellow and then brown, and the leaves become very brittle.
Other responsive crops in Michigan are cauliflower, muskmelons, peas and rye. When deficiency occurs in these crops, the oldest leaves are mottled or lighter green than normal or new leaves.
Correcting Magnesium Deficiency
Present soil test criteria used for recommending magnesium in Michigan, Indiana, and Ohio are: (1) if the exchangeable magnesium level is less than 100 pounds per acre for mineral soils (150 pounds per acre for organic soils); (2) if the equivalents of potassium exceed magnesium (on a weight basis, this is about 3 parts of potassium to 1 part magnesium); (3) if the soil magnesium (as a percent of total bases) is less than 3 percent; or (4) if the equivalent ratio of calcium to magnesium is greater than 10. Similar criteria have been adopted by other North Central states.
On acid soils where magnesium need is indicated, at least 1,000 pounds of dolomitic limestone should be applied per acre. On non-acid soils, a magnesium deficiency may be corrected with 50 to 100 pounds of soluble Mg per acre broadcast, or 10 to 20 pounds Mg per acre row applied. Magnesium sulfate, potassium magnesium sulfate, or finely ground magnesium oxide are all satisfactory sources of magnesium.
Magnesium can also be applied as a foliar spray. Suggested rates per acre are 10 to 20 pounds of magnesium sulfate (Epsom salts) in 30 gallons of water.
Magnesium deficiency may be induced by high rates of potassium. Inadequate liming and excessive potassium applications will depress magnesium uptake. In some states, agronomists strive for at least 10 percent magnesium in the total exchangeable bases (equivalent basis). These rates are aimed at preventing grass tetany disorders in livestock that feed on lush grass. Anyone concerned about grass tetany should avoid excessive rates of potassium fertilizer and feed legume hay, which is generally higher in magnesium than grasses. Some magnesium carriers can be mixed with grain or salt rations. Contact your animal feed specialist for amounts and sources.
Magnesium uptake by celery may be related to its genetic makeup. Some celery varieties are unable to take up enough magnesium from the soil, resulting in magnesium deficiency, while other varieties grow normally. For those varieties that are inefficient users of soil- applied magnesium, foliar application of 5 to 10 pounds of magnesium sulfate per acre sprayed at 10-day intervals may be necessary.
Magnesium Toxicity
Excess magnesium has not been a problem in Michigan. Studies in Ohio and Wisconsin have shown that widely different levels of exchangeable calcium and magnesium can exist without causing a nutrient imbalance in the plant. Man farmers have used dolomitic limestone for years and have not had any nutrient imbalance.
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| Magnesium deficient corn. - 25K |
