If you look around you can find all sorts of information about proper Ca:Mg ratios. You will also find that there are a few common ratios that are in literature as well, but where did this concept come from and how important is this for us?
The first conjecture at a "proper" Ca:Mg ratio came about in 1901 while looking at total Ca and Mg levels. The results here, suggested a ratio of 5:4 Ca:Mg.
Next, there was further research done in 1945. By this time, researchers had realized that total and exchangeable Ca and Mg levels were different and now considered exchangeable the more important factor.
From this, they determined the "proper" ratios of Ca:Mg based on CEC and saturation percentages.
They resulted in a saturation of; 65% Ca, 10% Mg, 5% K and 20% H. From this they determined that the "proper" ratios of Ca:Mg was 6.5:1 Ca:Mg.
As time passed and further research was conducted; suggestions of "proper" Ca:Mg ratios seemed to land at or between 5:1 and 8:1 Ca:Mg.
To add to this confusion, there are several other areas of research that have suggested a 3:1 Ca:Mg ratio.
Why would there be such a large difference in Ca:Mg ratio between various sources? Simple, they are both the SAME as they are stated from different perspectives.
I.E. 3 mmol of Ca = 60 ppm and 1 mmol of Mg = 12 ppm.
Here, you can see that a 3:1 ratio IN mmol of Ca:Mg is the SAME as a 5:1 ratio IN ppms.
So, now that we know where this came from and what "proper" ratios are; how does this effect productivity or growth then? For this we need to look into Ag and soils where Ca:Mg ratios are not kept specific; yet productivity or yield is tightly tracked.
Here are some links specific to this:
http://www.ipm.iastate.edu/ipm/icm/2003/4-21-2003/camg.html
From the first link:
""The results strongly suggest that for maximum crop yields, emphasis should be placed on providing sufficient, but nonexcesive levels of each basic cation rather than attempting to attain a favorable basic cation saturation ratio (BCSR), which evidently does not exist." Various greenhouse and field trials indicate that crop productivity is not influenced by ranges from less than 1:1 to more than 25:1--ratios outside of what is normally measured in soils.
....
"In summary, the Ca:Mg ratio concept is unproven and should not be used as a basis for fertilization or liming practices. Having sufficient levels of Ca and Mg is the proper method of evaluation, rather than trying to manipulate ratios."
From the second link:
"Thus, if adequate levels of calcium and magnesium are present in the soil, variations in the Ca:Mg ratio between 2 and 8 have no effect on yield, and varying the calcium saturation percentage from 32% to 68% and magnesium from 35% to 12% also do not influence yield.
.......
"Thus, the amounts of these nutrients taken up are determined by the selectivity of the roots. The remainder accumulates in the immediate vicinity of the roots. Therefore, it is doubtful that the supply of calcium and magnesium to the root surfaces would ever be limiting under Wisconsin conditions where soil pH is maintained in good growing range. The Ca:Mg ratio seldom will be the dominant factor determining calcium and magnesium uptake by plants. This was shown by the small variations in the tissue Ca:Mg ratio when the soil Ca:Mg ratio was varied
And from the third link:
"The optimum soil cation ratio concept, developed about 50 years ago, has been incorporated into some fertilizer recommendation philosophies in various ways. Recent field evaluations of this concept, however, show that the ratio of cations has no impact on the response of crops to Ca, Mg, and K in fertilizer programs. The optimum cation ratio concept has a major disadvantage in that even if the ratio of cations in the soil is considered to be optimum, a nutrient deficiency may still exist. A sufficient supply of available cations in the root zone is the most important consideration in making economic fertilizer recommendations."
The above papers also give results for yields in various ratios of Ca to Mg that go considerably outside any of the proposed ratios without crop production loss.
The last quote is probably the best takeaway in that sufficient levels in soils of each element is more important than their ratios to each other.
Happy Farmin'
