When fertilization of Minnesota crops is considered, major attention, of course, is devoted to using adequate amounts of nitrogen, phosphorus, and potassium adjusted for the relative supply in soils as measured by an appropriate soil test. Magnesium (Mg) is also essential for optimum growth and development of all crops. Scince amounts required are not large, it is classified as a SECONDARY nutrient. By contrast, nitrogen, phosphorus, and potassium are classified as PRIMARY nutrients.
In the plant world, Mg is part of each chlorophyll molecule. Therefore, this nutrient is an essential factor in the process of photosynthesis—one of the most important physiological processes in the biological world. If plants are deficient in Mg, the leaf veins are usually dark green with the area between the veins showing a very light green color. The light green is a consequence of the lack of chlorophyll.
In soils, Mg exists in an available form as the ion with two positive electrical charges written as Mg++. Because of these positive charges, the Mg ion is attracted to (not attached to) the negative electrical charges of the clay sized particles and is, therefore, not leached from soils. Because of this attraction, Mg is also not very mobile in soils. The exchangeable Mg (the Mg associated with the clay size particles) varies over a wide range in Minnesota. The concentration of exchangeable Mg can be as high as 1,000 or 2,000 ppm for the calcareous soils in the western part of the state to 50 to 100 ppm in the acid sandy soils of the east.
Magnesium is not a villian in Minnesota soils. There are some who adhere to a belief, and it is only a belief, that high concentrations of Mg is soils have a negative effect on soil properties and, thus, can be harmful to crop growth and yield. This belief is not supported by research. In fact, research has shown that high amounts of added Mg have had no negative effect on yield (see Table 1).
Table 1. Increasing amounts of applied magnesium and the yield of first cutting alfalfa.
Treatment # Mg applied Yield
lb./acre ton D.M./acre
1. control 0 1.29
2. calcitic lime 0 1.42
3. calcitic lime + Mg 300 1.44
4. dolomitic lime 0 1.51
5. dolomitic lime + Mg 300 1.47
both liming materials were supplied at a rate to supply 3500 lb. ENP/acre
In this study, the calcitic and dolomitic limestone was supplied at a rate to supply 3500 lb. of Effective Neutralizing Power (ENP) per acre. The calcitic limestone contains no Mg while the Mg concentration in dolomitic limestone varies from 8% to 11%. When additional Mg was applied at a rate of 300 lb./acre, magnesium sulfate (epsum salts) was the source. It is very soluble and, therefore, readily available. All liming materials and the epsum salts were broadcast and incorporated in the early fall prior to seeding the alfalfa in the fall and the soybeans in the following spring.
The yield of both alfalfa (Table 1) and soybeans (Table 2) was not affected by the application of Mg. In this study, the amount of Mg added to the soil increased moving from treatment #1 to treatment #5 and yields did not decrease. Therefore, it’s easy to conclude that Mg WAS NOT harmful to the soil.
Table 2. Increasing amounts of applied magnesium and yield of soybeans.
Treatment # Mg applied Yield
1. control 0 42.4
2. calcitic lime 0 46.0
3. calcitic lime + Mg 300 46.3
4. dolomitic lime 0 49.3
5. dolomitic lime + Mg 300 46.3
both liming materials were supplied at a rate to supply 3500 lb. ENP/acre
Some believe that high levels of Mg in soils are responsible for hard soils and, therefore, increase runoff and erosion. The studies that produced this conclusion were conducted in the laboratory whith very high rates of Mg being used. Those conditions did not come close to simulating conditions that would occur in Minnesota soils. A brief consideration of properties of Minnesota soils leads to the common sense conclusion that Mg is not harmful to soils. In western MN, levels of exchangeable Mg are high. Yet, in eastern MN, exchangeable Mg levels are low. If other conditions such as soil moisture, amount of added N etc. are the same, crop yields will be the same.
A discussion of Mg in soils would not be complete without mention of calcium to magnesium ratios. The thinking that there is some “ideal” ratio for these two nutrients in soils is probably the most bogus concept in soil fertility and fertilizer use. There is no research evidence to support the concept of an “ideal”ratio. In fact, results of field trials in Wisconsin clearly show that the calcium to magnesium ratio can vary over a wide range without having any statistically significant effect on yield (Table 3) regardless of soil texture.
Table 3. Calcium to magnesium ratios and the yield of alfalfa.
Ratio Silt Loam Loamy Sand
———————-ton D.M./acre———————
2.28 3.31 —
2.64 — 4.14
4.76 3.40 —
4.81 — 4.12
8.44 3.22 —
The addition of Mg to a fertilizer program is sometimes needed for production of optimum yield. If there is a question of need, have soil samples analyzed for Mg. Suggestions for direct application of Mg to corn are provided in Table 4. If Mg is needed for legume production, it can easily be added as dolomitic limestone.
Table 4. Suggestions for magnesium use in corn production in Minnesota.
Magnesium Soil Test Relative Level Starter Broadcast
ppm —————lb.Mg/acre————–
0 to 50 low 10 to 20 50 to 100
51 to 100 medium trial, 10 lb./acre 0
Most Minnesota soils contain adequate amounts of Mg for optimum yield. When needed, Mg is important. There is no need to be concerned about the harmful effects of Mg.