Magnesium (Mg) is an essential nutrient for crop production. When levels of this nutrient in soils are low, the addition of Mg to a fertilizer program will frequently improve crop yields. In Minnesota, the probability of measuring a yield increase from the use of Mg is highest when sandy soils are very acid.
Normally, there is a concern when there are low levels of Mg in soils. During the past winter, however, there have been several questions about the consequences of high levels of this nutrient in soils. Apparently, there are some who believe that high amounts of Mg in soils can "make the soil hard" or cause the soil to be "out of balance." Those who subscribe to this theory that high levels of Mg are bad also believe that there is an "ideal ratio" of calcium (Ca) to magnesium (Mg) in soils. According to this theory, the ratio of Ca to Mg should be in the range of 4.3 to 1 to 7.5 to 1.
Several researchers have thoroughly evaluated the impact of these ratios on crop production. In Ohio research, the ratios of Ca to Mg were varied over a wide range and corn and soybean yields were measured. In this study, the highest corn yields were measured when the Ca:Mg ratio varied from 5.7 to 26.8. The lowest corn yields were measured when the Ca:Mg ratio varied from 5.8 to 21.5. Soybean yields in this same study were not related to Ca:Mg ratio.
In Wisconsin, researchers adjusted the ratio of Ca:Mg in a sandy soil and then measured alfalfa yield. The results are summarized in the following table.
| Ca:Mg Ratio | Alfalfa Yield ton dry matter (acre) |
|---|---|
| 2.6 | 4.1 |
| 3.5 | 4.4 |
| 4.8 | 4.1 |
| 7.6 | 4.3 |
| 8.1 | 4.4 |
The amount of Mg in Minnesota soils available for crop production varies over a wide range. Levels of calcium carbonate and magnesium carbonate are high in the calcareous (high pH) soils. Yet, high levels of Mg in these soils have not caused problems with crop production.
George Rehm
Extension Soil Scientist
