The question of balance between soil magnesium and calcium levels seems to revive among farmers every few years. On soils where lime is recommended, and the lowest cost source is high-magnesium dolomitic lime, some farmers are especially sensitive to the relatively high soil magnesium level and low calcium-to-magnesium (Ca:Mg) ratio they observe on soil test lab reports for fields where lime has been applied repeatedly. However, the actual levels of plant-available magnesium and calcium are much more important to crop performance than the Ca:Mg ratio. Generally, calcium levels of less than 300 parts per million (ppm) and magnesium levels less than 35 ppm are considered low. Concern should begin only when the Ca:Mg ratio becomes very small, with magnesium base saturation nearly equal to, or greater than, calcium base saturation.
Questions about the ratio of bases in soils have been addressed with research in Wisconsin and other Great Lakes states. In E.E. Schulte and K.A. Kelling’s University of Wisconsin Extension publication, “Soil calcium to magnesium ratios – Should you be concerned?,” they varied the Ca:Mg ration in two Wisconsin soils by adding gypsum (CaSO4) and Epson salts (MgSO4). Phosphorus, potassium and sulfur were maintained at optimum levels. They found that if adequate levels of calcium and magnesium are present in the soil, variations in the Ca:Mg ratio between 2 and 8 had no effect on alfalfa yield, and varying the calcium saturation percentage from 32 percent to 68 percent and magnesium from 35 percent to 12 percent also did not influence yield.
Michigan State University studies also indicate that the overall quantity of Mg and potassium (K) is more important than ratios between these nutrients, with the following recommendations:
- Adding Ca to alter the Ca:Mg ratio is not necessary unless Mg equals or exceeds Ca on meq basis.
- Adding Ca may induce Mg deficiency in sandy soil.
- The ratio may be useful in selecting between calcitic and dolomitic lime when lime is needed.
- Mg needs to be greater than 3 percent of soil base saturation.
- %Mg needs to be greater than %K.
MSU Extension has been called upon occasionally by Upper Peninsula farmers to interpret magnesium, calcium and potassium ratios on high magnesium soils. The most concern comes from the eastern and western Upper Peninsula farming areas, where soil pH is low and dolomitic lime was locally available and affordable for many years. The MSU Soil and Plant Nutrient Laboratory provided a listing of 874 Chippewa County soil test reports from July 2003 to August 2016. Of these soil tests, 1.1 percent had a Ca:Mg ratio smaller than 1.5, and none were lower than 1.16. The average Ca:Mg ratio for these Chippewa County soil tests was 3.14. Baraga County submitted 221 soil samples in the same period with 1.4 percent having less than 1.5 Ca:Mg ratio and none lower than 1.32. The average Ca:Mg ratio for these Baraga County samples was 4.78. None of these samples was deficient in calcium to meet crop needs. Several were deficient in magnesium and potassium. In the great majority of cases, the Ca:Mg ratio was not a good indicator of the need for one or the other of these nutrients.
Source: Jim Isleib, Michigan State University Extension