Spectrum Agronomic Library

Competition between Ca and Mg for uptake by crops has become a perennial topic of discussion in agriculture. Generally, the discussion centers on the claim by some that there is (or is not) an “ideal” soil Ca/Mg ratio that should be achieved through fertilization.

It is reported that the first publication of an ideal Ca/Mg ratio came from New Jersey in 1901. This early work recommended a “total” Ca to “total” Mg ratio in the soil of about 5/4. As we know today, and was recognized soon after this publication, an analysis of the total soil content of a nutrient bears little relationship to its crop availability. Later, again in New Jersey, it was reported that the “ideal” alfalfa soil should have cation saturation's of 65% Ca, 10% Mg, 5% K, and 20% H. In the years since this claim was made, there have been many instances where record breaking alfalfa yields, not to mention other crops, have been produced on soils without this supposedly ideal cation balance. Fertile soils commonly have a Ca/Mg ratio between 5/1 and 8/1. However, this does not mean that the specific Ca/Mg ratio is required, best, or even related to yield. Research results show that this ratio can be as narrow as 2/1 or as wide as 11/1 without negative effects, assuming that there is an adequate amount of each nutrient in the soil.

In the mid-1980's the University of Wisconsin conducted research into the effect of Ca/Mg ratio on alfalfa growth. They found that while the Ca/Mg ratio in the plant tended to reflect the soil Ca/Mg ratio, the plant content of these nutrients was affected much less and in no case did the soil or plant ratio affect yield. In this work the plant Ca and Mg contents were never below the respective critical levels for each nutrient, even though the soil Ca/Mg ratios ranged from 2.28/1 to 8.44/1. They concluded that, assuming there are adequate levels of Ca and Mg present in the soil, variations in the Ca/Mg ratio over the range 2 to 8 have no effect on yield.

In 1999 the University of Missouri, Delta Research Center published the results of an investigation into the effects of soil Ca/Mg ratio on cotton. They amended plots with gypsum or epsom salts to create soil Ca/Mg ratios between 3.8/1 and 11.7/1. They found that cotton yields were not significantly different between treatments.

McLean, et al in Ohio, could find no specific cation ratios that predicted sufficiency or shortages of K, Mg, or Ca in several crops (Table 1). Notice that for all crops the Ca/Mg ratios of both the high and low yielding groups have essentially the same ranges. There is no trend or bias in the relationships between the Ca/Mg ratio and the relative yields of any crop. This indicates that the soil Ca/Mg ratio had little or no effect on yield and the researchers concluded the same.

The obvious conclusion is that crop yields are not significantly affected by the soil Ca/Mg ratio as long as both nutrients are present in adequate amounts.