Cation ratios can help in identifying soil structure problems, and are a great tool for identifying problems. There is much research that has been done on the subject, most of which does not definitively show benefits to having the “right ratio” of Calcium to Mag. However many of these research papers also dicuss right N:S, MG:K, P1:P2 ratios.

Why are some ratios important and others not? Why are the proper ratios of nutrients important in animal and human nutrition and not in plant nutrition?

Research on Ca:Mg ratio was often based on total calcium and magnesium levels in a soil. The Mehlic 3 method of testing does not relate to the 'functional' fraction of calcium and magnesium in the soil, i.e. that proportion actively being exchanged between soil colloids, soil solution, plant roots, microbes etc.

A better way to truly determine Ca:Mg ratio would be to measure the soluble cations in the soil and take plant tissue samples. This would help to better determine a true plant available ratio. It is also important to point out that using Ca/Mg Ratios in isolation (without taking into account ppms) can lead to erroneous interpretations, calcium and magnesium levels can both be low, yet have an ideal ratio; or both can be high, yet have an idea ratio.

What is the correct Calcium to Magnesium ration?

As stated above, there is no definitive ratio, but our experience has shown that as long as the calcium and magnesium ppms are sufficient a Ca:Mg ration of 4:1 to 7:1 should offer a soil with better structure, better aeration, and better productivity.

Problems with high Mag Soils

Calcium is the element that causes the soil particles to move apart for aeration and drainage. Magnesium makes the particles stick together. One soil consultant has determined that is some soils the excess magnesium is held as trimagnesium ortho phosphate, Mg3(PO4)2-22H2O. Notice that the last part of the formula is twenty-two molecules of water. Is it any wonder that soils high in magnesium tend to dry and crack when water is tied up in a compound instead of available to plants? Note the two phosphates that are tied to the magnesium.

High mag soils cause potassium and calcium deficiency in plants. Soils with high magnesium tend to have poor structure. Typically these soils will have more sodium cations attached to the clay as well. Having high magnesium and sodium causes the clay particles to disperse when wet and set like concrete when dry.

The magnesium ions sitting on the clay surfaces have a 50% greater hydrated radius than calcium which causes these soils to absorb more water. This excess water tends to weaken the forces that hold soil particles to together resulting in less aggregate stability and greater dispersion of soil particles reducing infiltration rates and hydraulic conductivity (drainage). These soils tend to swell when wet and become very hard when dry, often forming a hard surface crust and becoming very difficult to till.
 
Excess magnesium causes a collapse of soil structure

Soils containing greater than 300 ppm of ammonium acetate extractable magnesium are considered high as well soils with base saturations greater than 15%. Soils that are saturated with magnesium can also show low potassium levels as of less than 100 ppm.

The addition of high rates of potash fertilizers can actually make matters worse and cause even more dispersion of soil particles. Potassium and sodium have similar properties, in sodic soils sodium causes lack of structure by causing dispersion of soil particles, soils tend to remain saturated and poorly drained.

For corn, 50-100 Mg ppm in soil and .25% to 1% in tissue is sufficient.

Correcting/Improving High mag soils

One solution for high mag soil is managing for soil quality with manures, crop rotation, and applying gypsum in an effort to displace magnesium. Gypsum is calcium sulfate, the sulfate ion mobilizes the magnesium out of the root zone, with calcium taking its place. Adding calcium carbonate can improve the structure of soil and build calcium levels faster.

Lime High pH Soils?
In the case of high mag soils the high pH is due to magnesium hydroxide, sodium hydroxides, and potassium hydroxides. pH has nothing to do with calcium, it only measures the hydrogen or hydroxide ion. Adding lime adds calcium, the stronger binding calcium cations displace the weaker held sodium (or magnesium) cations from the clay particles. Clay particles can then bond together with the calcium to form a well structured soil.

Application of potash in the starter at rates in the 30 pound range have shown much greater response than large application rates.  High rates of potash in the presence of high levels of magnesium (>600 ppm) can aggravate the dispersion of soil particles if crop rotation and manure additions are not practiced. Moderate rates of K applications are needed when potash is low to medium.

Guideline for Soil amendment use.

Photo credit DONTSOVA-PURDUE