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When your soils are tested does your agronomist or soil lab promote the virtues of tracking the Ca to Mg ratio?

Soil testing is important because it is the only way to track whether your soil is chemically fit to support soybean production. The test shows if you need to apply phosphorus, potassium or lime and the levels of other nutrients.

When a soil test is run, they analyze for many nutrients at one time including calcium (Ca) and magnesium (Mg). Both of these nutrients, along with sulfur (S) are considered secondary macronturients. Of course, extractable Ca levels are usually very high and Mg is normally sufficient. But in the Midwest the ratio of Ca to Mg is brought up from time to time as an important metric to monitor. Most often this  metric is downplayed in the end as not being scientifically relevant.

The University of Wisconsin published an article “Soil Calcium to Magnesium Ratios – Should You Be Concerned?”. The authors varied the Ca:Mg ratio in two soils by adding calcium or magnesium sulfate while P and K levels were maintained at optimum levels. They concluded that 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 didn’t impact yield. Click here for the full article. 

But this metric describing the balance between soil Ca and Mg levels gets revived frequently either by something we’ve read or an agronomist touting the value of this metric in solving soil problems. But generally speaking, don’t be too concerned about this ratio. However, in some circumstances this ratio should be checked.

So what happens if there is too much Mg relative to Ca in the soil? Of course too much Mg relative to Ca can impact soil structure. Cations like sodium (Na) and Mg tend to disperse soil particles while Ca flocculates soil particles. When soil particles flocculate they come together to form more stable peds of soil and better structure. Sodium and Mg destroy that structure.

If you lime your soils what is the composition of the lime? Is it calcium carbonate or is it a high-magnesium dolomitic lime. If your lime source is dolomitic you can have inadvertently been building up Mg levels relative to Ca. Some soils are sensitive to high soil magnesium levels and low calcium-to-magnesium (Ca:Mg) on fields where dolomitic lime has been applied repeatedly. A build up of Mg relative to Ca over time can degrade soil structure and porosity.

Usually soil test values can range from 500 to 4000 ppm for exchangeable calcium and 100 to 500 ppm for exchangeable magnesium. Generally, calcium levels less than 300 ppm and magnesium levels less than 35 ppm are low. The time though to be concerned is when magnesium base saturation is similar to calcium base saturation and the ratio is near 1 to 1.

A Michigan State University report states that overall quantity of calcium and magnesium is more important than ratios between these nutrients. They suggest the following recommendations:

• Adding calcium to alter the Ca:Mg ratio is not necessary unless magnesium equals or exceeds calcium on meq basis.
• Adding Ca may induce magnesium deficiency in sandy soil.
• The ratio is useful in selecting calcitic and dolomitic lime when lime is needed.
• Magnesium needs to be greater than 3% of soil base saturation.
• Percent Mg needs to be greater than the percent K.

In the great majority of cases, the Ca:Mg ratio is not a good indicator of the need for one or the other of these nutrients.

Agronomist Daniel Davidson, Ph.D., posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.