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Magnesium for crop production in Minnesota

George Rehm, Carl Rosen and Michael Schmitt

Although magnesium (Mg) is an essential element for plant growth, its use in a fertilizer program receives only minor emphasis in Minnesota. For most of the state, this lack of emphasis is justifiable. If Mg is limited in the diet, animals can develop grass tetany. Therefore, some special consideration is given to the Mg status of forage crops.

The role of magnesium in the soil

Magnesium is the central core of the chlorophyll molecule in plant tissue. Thus, if Mg is deficient, the shortage of chlorophyll results in poor and stunted plant growth.

Magnesium also helps to activate specific enzyme systems. Enzymes are complex substances that build, modify, or break down compounds as part of a plant's normal metabolism.

Magnesium in the soil

Magnesium is abundant in the earth's crust. It is found in a wide variety of minerals. Magnesium becomes available for plant use as these minerals weather or break down. The majority of the soils in western Minnesota have naturally high levels of Mg. For the acid soils of the eastern counties, the addition of dolomitic limestone in the crop rotation, when needed, should supply adequate Mg for crop growth.

Magnesium is held on the surface of clay and organic matter particles. Although this exchangeable form of Mg is available to plants, this nutrient will not readily leach from soils.

In Minnesota, Mg deficiency has only been observed on very acid soils. These soils usually have a sandy loam, loamy sand or sand texture. A Mg deficiency is not likely to occur until the soil pH drops below 5.5. In Minnesota, the acid sandy soils occur in the central and east-central part of the state.

The low levels of Mg in soils can occur where potatoes are grown on acid sandy soils or where corn follows a potato crop. Sometimes, grass tetany, a livestock disorder caused by low levels of Mg in the diet, is reported where high rates of potash have been applied to grass pastures. Research trials, however, have shown that the use of Mg in a fertilizer program for these pastures has not increased forage yields. For these situations, it is less expensive to supplement the animal diet with a salt that contains Mg.

Relationship of magnesium to calcium in soils

There are some who believe that there is an "ideal" ratio of calcium to magnesium in soils and one of these two nutrients should be added in a fertilizer program if this "ideal" ratio does not exist. The need for this "ideal" ratio has never been verified by various research efforts throughout the Corn Belt which have focused on the importance of ratios. In Wisconsin, for example, the ratio of calcium to magnesium in soils was adjusted in a range of two to eight by adding different amounts of calcium and magnesium in a fertilizer program. This variation had no significant effect on alfalfa and corn yields. Therefore, as fertilizer recommendations are developed, emphasis should be placed on providing adequate amounts of magnesium in soils rather than the maintenance of a certain ratio of one nutrient to another.

Deficiency symptoms


Figure 1. Magnesium deficiency in corn. The striping extends the full length of the leaf.

The loss of a healthy green color can be the first indication of a Mg deficiency. Color loss reflects the shortage of chlorophyll in the plant. As the deficiency becomes more severe, the area between the veins of the leaves becomes yellow while the veins stay green. In corn, there is a definite striping the full length of the leaf, appearing first on the lower leaves (see Figure 1).


Figure 2. Magnesium deficiency symptoms in potatoes. The loss of color begins at the tips of the lower leaves.

In potatoes, the loss of the green color begins on the tips of the lower leaves when there is a mild Mg deficiency. When the deficiency is more serious, the yellowing progresses between the veins toward the center of the leaf. In the advanced stages of Mg deficiency, leaf areas between the veins show small brown dead spots (see Figure 2). Diseases, herbicide damage, and environmental factors also cause leaves to die prematurely. So, care should be taken in identifying a Mg deficiency. Use plant analysis to be sure.

Predicting the need for magnesium

The critical plant tissue concentrations of Mg in selected crops are listed in Table 1. Since Mg is a mobile element in the plant, the concentration of Mg usually decreases from the top to the bottom of the plant. Also, the Mg concentration usually decreases as the plant approaches maturity. It is, therefore, important to indicate the age of the plant and the part of the plant that was sampled when samples are submitted for a measurement of Mg in plant tissue.

Table 1. Relative magnesium levels in selected tissue of several crops.

Crop Plant Part Time of Sampling Magnesium Status
Deficient Low Sufficient High
alfalfa upper 1/3 of plant 1/10 bloom <.20 .20-.30 .31-1.0 >1.0
corn ear leaf silking <.10 .11-.25 .26-1.0 >1.0
oats upper leaves boot stage - .13 .13-.40 >.4
potatoes petiole of most recently mature leaf bloom <.20 .20-.30 .30-.70 >.70
soybeans most recently developed trifolate pod set <.10 .11-.25 .26-1.0 >.10

The critical values in Table 1 have been established for specific stages of growth. When collecting plant samples, every effort should be made to sample the crop at the stage of growth that is listed.

A soil test to measure exchangeable Mg is offered by most soil testing laboratories. In Minnesota, the potential need for Mg in a fertilizer program is highest where sandy soils are very acid. If dolomitic lime has been used in the crop rotation, soils usually have a relatively high level of Mg and it is not necessary to test the soil for this nutrient.

Magnesium recommendations for corn production are summarized in Table 2. The Mg suggestions for fruits and vegetables are listed in Table 3.

Table 2. Magnesium recommendations for corn production.

Magnesium Soil Test Relative Level Magnesium to Apply
Starter or Broadcast
ppm ----lb./acre----
0-50 low 10-20 50-100
51-150 medium trial* 0
151+ high 0 0

*Apply 10-20 lb. Mg per acre in a starter only if a magnesium deficiency is suspected, or if a deficiency has been confirmed.

Table 3. Magnesium recommendations for fruit and vegetable crops.

Magnesium Soil Test Relative Level Magnesium to Apply
Starter or Broadcast
ppm ----lb./acre----
0-50 low 20 100
51-100 medium 10 50
101+ high 0 0

Sources of magnesium

The application of dolomitic limestone is the most cost effective method for applying the Mg that is needed. The Mg content of dolomitic limestone varies from 8-10%. To be effective, this Mg source should be broadcast and incorporated before planting.

There are fertilizers that are a combination of potassium sulfate and magnesium sulfate. The Mg content is 11%. The sulfur (S) concentration is 22% and the K2O percentage is 22%. This fertilizer is easily used in a starter fertilizer for corn or as a Mg source when there is no desire to increase soil pH.

Although the need for the addition of Mg to a fertilizer program is not widespread in Minnesota, this nutrient can increase crop production when needed. The potential for need should not be ignored. If there is doubt about the need, analyze the soil to be sure.

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