Understanding Base Saturation Ratios on your MSU Soil Test Report

May 15, 2019

Subscribe to our Newsletters
By James Isleib
The percent base saturation for potassium (K), magnesium (Mg) and calcium (Ca) are reported on every Michigan State University soil test report. According to the MSU Soil and Plant Nutrient Laboratory, “The percentages reported assume K, Ca and Mg comprise 100% of the exchangeable bases, and are used to determine potential magnesium deficient situations. Mg should be above 3% and greater than the percentage of K. For example, 6.8% K and 4.2% Mg indicates a Mg-deficient soil.”
Soil sampling on a Chippewa County farm. 
According to Lichtfield Analytical Services, a reputable and widely-used commercial soil testing lab in LIchtfield, Michigan, “The percentages of saturation for each of the cations will usually be within the following ranges: calcium – 40-80%, magnesium – 10-40%, potassium – 1-5%.” Lichtfield also suggests that the potassium:magnesium ratio can be important for certain crops. “On some crops, high magnesium levels may reduce potassium uptake by the plant. The ratio of potassium to magnesium should be between 0.2 and 0.3 for best uptake. Ratios below 0.2 could cause reduced potassium uptake.”
In the University of Wisconsin Extension publication “Soil calcium to magnesium ratios – should you be concerned?”, authors E.E. Shulte and K.A. Kelling state that the Ca:Mg ratio seldom limits plant growth if soil pH is maintained in the good growing range, based on Wisconsin research. They do not recommend applying calcium materials to soils simply to increase the Ca:Mg ratios.
Based on these sources, a set of take-home points from recent MSU Extension crop nutrient management across Michigan’s Upper Peninsula was distributed.
Take-home information on soil nutrient balances
  • Exchangeable Mg should be above 3%.
  • Exchangeable Mg should be greater than exchangeable K.
  • Normal ranges for exchangeable bases: Ca 40-80%, Mg – 10-40%, K – 1-5%.
  • The ratio of K to Mg should be between 0.2 to 0.3 for best uptake. Ratios below 0.2 could cause reduced K uptake.
  • Ca:Mg ratio rarely an issue. Ratio between 2 and 8 not shown to impact alfalfa yield in Wisconsin.