Each year, crop producers are blitzed with a new concept or product which, if used, will solve all production problems.  The latest promotion revolves around the  claim  that the ratio of calcium to magnesium in soils has a major effect on crop yield.  Looking at this concept in detail, there are claims that there is an IDEAL ratio of calcium to magnesium that can be easily adjusted.  Is this concept valid for Minnesota soils?  Is this ratio really something that I should be concerned about?  These are two of the many questions that crop producers are asking as we head into the 2003 planting season.

The ratio concept is not new.  It evolved from research conducted with acid sandy soils in New Jersey using alfalfa as the test crop.  In order to understand the concept, it's important  to describe some facts of fundamental soil chemistry.

The best place to start is to introduce the concept of exchange sites.  These sites are negative electrical charges associated with the clay size particles and soil organic matter.  Cations are plant nutrients with a positive electrical charge calcium (Ca⁺⁺), magnesium (Mg⁺⁺), and potassium (K⁺) are three cations found in a wide range of concentrations in all soils.

FIGURE 2.


These cations are found in a thin film of water that exists around plant root hairs and the clay particles.  These positively charged plant nutrients are attracted to the negative electrical charge of the clay minerals and soil organic matter.  This attraction is illustrated in Figure 2.

Those that promote the ratio concept believe that the ratio of calcium to magnesium associated with the exchange sites should be about 6.5 to 1.0.  They further believe that optimum yields cannot be produced unless this ratio is 6.5 to 1.0.

Since the introduction of this concept, various research projects have been designed to evaluate the effect of this ratio on crop production (yield).  Some of the results of those research  efforts are summarized in the tables that follow.

In research in Wisconsin, the Ca:Mg ratio was changed by adding either sources of calcium or magnesium to the soil.  A wide range in ratios was produced (Table 1).   Alfalfa was the test crop.  The ratio had no effect on alfalfa production.   Neither the low nor the high ratios had any effect on alfalfa yield.


Table 1.  Effect of various Ca:Mg ratios in two contrasting soils on alfalfa yield.

Source: University of Wisconsin


Working with corn and soybeans, McLean and coworkers at Ohio State changed the Ca:Mg ratio in soils at 13 sites.  The results of that study are listed in Table 2.

Looking at the five sites that produced the highest yield, the Ca:Mg ratio varied from 5.7 to 26.8 and 5.7 to 14.3 in 1975 and 1976 respectively.  For the five sites that produced the lowest yield, the ratio varied from 5.8 to 21.5 and 5.0 to 16.0 in 1975 and 1976 respectively.

Based on the results of this study, it was obvious that the ratio had no effect on corn production.  A similar consideration of ratios and soybean yields leads to a similar conclusion.

Table 2.  Calcium/magnesium ratios at five sites that produced the highest corn yields and five sites that produced the lowest corn yield.

More recently, a study was conducted near Rochester to evaluate the effect of calcium/magnesium ratios on alfalfa and soybean production.  The relevant properties of the soil at the experimental site are listed in Table 3.  Both the pH and soil test values for potassium were low.  Therefore, a response to potash fertilizer and lime would be expected.


Table 3.  Relevant soil properties at the experimental site at Rochester.

The treatments that were applied and the corresponding yields of soybean and alfalfa are listed in Table 4.  This initial calcium/magnesium ratio was not 6.5 to 1.  It was lower.

In looking at yields in Table 4, yield of both crops was increased by the application of lime.  With an initial soil pH of 5.6, this would be expected.  All treatments were fertilized with adequate rates of phosphate, potash, sulfur and boron.  Neither the type of lime (calcitic, dolomtic) nor  the use of additional magnesium affected the yield of both crops.


Table 4.  Soybean and alfalfa yield as affected by application of calcitic and dolomitic limestone as well as additional magnesium.

Crop yield did not decrease as the total amount of applied magnesium increased.  This is clear evidence that magnesium soils does not have a negative effect on crop growth and production.  In addition, the calcium/magnesium ratio was probably altered as the total amount of applied magnesium increased.  Since there was no change in yield, there was no indication that the calcium/magnesium ratio had any effect on crop production.

The effect of calcium/magnesium ratio on crop production was evaluated  by Dr. Black of Iowa State University.  His conclusion was: "The values suggested by Bear, Prince, and Malcolm (1945), Graham (1959), and Baker and Amacher  (1981) all represent a good educated guess regarding satisfactory exchangeable cation ratios in most soils for most crops. The fact is, though, that research has not demonstrated that there   is  an optimum exchangeable cation ratio for any crop."

This statement is an accurate summary of the research that has evaluated the effect of calcium/magnesium ratios on crop production.  There is no need to believe that the ratio should be used in making fertilizer recommendations.  It's an outdated, antique concept that has no value in high yield, modern agriculture.