The Myth of the Calcium to Magnesium Ratio

When you are involved in soil fertility, you sometimes hear people talk about a ‘calcium to magnesiumCaMgphoto ratio’.  A few agronomists follow this theory however many are skeptical of its value.  The theory is that for optimal crop production these two nutrients need to be in balance in the soil.  While this balance can be tracked in the soil, there are two things to remember.  First, plants tend to balance out the nutrients they require regardless of soil supply.  And second, given that an acre-foot of soil weighs 2 million pounds, significantly changing mineral ratios in soil is almost an impossible task and certainly not an economical one.

Soils generally contain ample amounts of the two cations calcium (Ca) and magnesium (Mg).  These two minerals are available as soluble in the soil water or in the exchangeable form on the exchange complex.  The amount of each mineral generally reflects the parent material from which the soil is derived as well as the type of lime or gypsum material that has been applied.  Most commercial laboratories measure and report exchangeable Ca and Mg and their proportion in the base saturation.  From a soil fertility perspective, the focus should be on the amount of available Ca and Mg while the ratio is less important.

Once exchangeable Ca and Mg are measured, ratio is calculated based on meq (milliequivalent) or electrical charge basis.  For example, if there is 4.88 meq Ca in 100 g soil and 1.72 meq Mg in 100 g soil, then the Ca to Mg ratio is 2.8:1 (Ca / Mg = Ratio) which might be considered too low.  For those that subscribe to the Ca to Mg ratio, the suggested ratio range is 5:1 to 8:1.  It is believed that the right balance will improve soil structure, reduce certain weed populations (foxtail and quackgrass), reduce leaching and increase yield.

An ideal soil is said to have a base saturation of 65% Ca, 15% Mg, 4% potassium (K) and 1% to 5% sodium (Na) with the remaining 10 to 15% being micronutrients and hydrogen.  This ratio provides supra-optimal levels of these nutrients to the crop and to the soil life.  Don’t get too hung up on these percentages though. They are general guidelines that can vary quite a bit depending on soil texture and other factors and they are hard to change quickly.

For the Ca-to-Mg theorists, it is thought that the Ca to Mg ratio determines how ‘tight’ or ‘loose’ a soil is. Generally speaking, theorists believe that higher Ca in the ratio equates to loose soil is and higher Mg in the ratio leads to tight soil.  The belief is that a soil high in Ca should have more oxygen, drains more freely, and supports more microbial life and aerobic breakdown of organic matter.  Inversely, a soil high in Mg should have less oxygen, drains slowly, and is less effective at breaking down organic matter. Those that subscribe to this concept believe that 60-80% of the exchange sites should be occupied by Ca and 10-20% by Mg and together, they should add to 80%.   And if they don’t add to 80%, just add some Ca or Mg to correct it.  Of course that is where the dilemma arises.  In 2 million lbs. of soil it takes quite a few pounds of material (and money out of your pocket) to make a measurable difference.

Because the chemistry of Ca and Mg in soils is similar and because they are normally well supplied by the soil (note that Mg can be deficient in sandy soils), there is no scientific evidence that the ratio of exchangeable Ca and Mg has an impact on yield or crop production.

The late Stanley Barber, distinguished soil scientist and professor at Purdue, also came to the same conclusion years ago.  He wrote, “There is no research justification for the added expense of obtaining a definite Ca to Mg ratio in the soil.  Research indicates that plant yield or quality is not appreciably affected over a wide range of Ca to Mg ratios in the soil.”  And I tend to agree with this view.

Dr. Davidson posts articles on soil health and management related subjects. If you have suggestions for topics or questions, feel free to contact him at [email protected] or call 402-649-5919