How Important is Soil Ca:Mg Ratio?

The concept of soil cation balance, or specifically, the ratio of calcium to magnesium in the soil seems to be discussed each year. Some commercial fertilizer programs promote the use of products such as gypsum or calcitic limestone to bring soils into a better cationic "balance." The theory is to raise the calcium level in the soil relative to the amount of magnesium. By doing so, those that favor this type of fertilizer program argue a plethora of beneficial crop production and soil benefits will occur.

The controversy concerning Ca:Mg ratio has raged for nearly 100 years. To promote this concept of a high ratio of calcium to magnesium, proponents usually cite the research of a man by the name of William Albrecht who performed several studies at the University of Missouri back in the 1930's and 1940's. Many of these studies were done under greenhouse conditions and lacked statistical analysis. In fact, the graduate student who worked with Albrecht during this time, E.O. McLean, later did his own research on the topic and concluded that "for maximum crop yield, emphasis should be placed on providing sufficient but not excessive levels of each basic cation (for example, calcium and magnesium) rather than attempting to attain a favorable basic cation saturation ratio which evidently does not exist."

Recently, the University of Wisconsin has completed two additional studies where the concept of cation ratio was examined. The first was a 5-year study comparing varying phases of the program being promoted by Midwest BioAg, Inc. (Blue Mounds, WI) to that of University of Wisconsin soil test recommendations. In this study, a number of variables and responses were evaluated including crop yield (alfalfa and corn), crop quality, and economic returns. At the conclusion of the five years, the researchers concluded, in part, that "adding calcium or micronutrients when sufficient levels are already present in the soil does not appear to be useful."

In a second study, calcium and magnesium soil ratios were adjusted by adding various amounts of these nutrients to both a sandy and silt loam soil. Test crops in the three-year study were corn followed by alfalfa and parameters measured included crop yield, crop and weed populations, earthworm numbers, crop quality, and soil compaction. Once again, the study showed that there was no justification for using calcitic lime over the magnesium-containing dolomitic lime or to be concerned about Ca:Mg ratio as a guide to optimize any of the above mentioned measured criteria.

To summarize, crop producers should not be concerned with trying to manipulate Ca:Mg ratio as a means of improving crop yield, quality, or soil "health". There have been many research trials investigating cation balance over the past 100 years to verify this recommendation. It seems that if Ca:Mg ratio were as important as some proponents of high calcium programs make it out to be, surely this fact would have been shown in a significant number of these trials. This simply has not been the case. As it now stands, unverified farmer testimonials are often the only means of selling the idea of high soil calcium programs to potential clients.