There are two ways to interpret soil testing of the cations calcium (Ca) and magnesium (Mg), from the field: one by basic cation exchange ratios and the other by sufficiency. Base cation saturation ratios appear in most soil reports (see area circled in red below) and consist of percentages of calcium, magnesium and also potassium, sodium and hydrogen.  Tim Hartz from UC Davis addressed this topic head on at the recent UC Soil Fertility Short Course held on October 23.

Proponents for examining cation ratios, for example Albrecht and Stiener, have held that there is an ideal cation ratio, meaning that the optimum soil environment for crop plants occurs when the cation exchange complex is 65% Ca : 10% Mg : 5% K : 20% H. To some extent this makes sense, since it has been shown that the base saturation of an individual cation can affect its availability to plants and that the uptake of one cation may affect the uptake of others.

However, research testing a variety of ratios of cations in the soil, especially Ca and Mg, has not found much in the way of identifying a true ideal ratio. As an example given at the short course in Davis, a study in Wisconsin that doubled and even quadrupled the exchangeable Ca: Mg ratio resulted in no difference in yield of alfalfa hay per acre.

It was rather suggested that growers and managers should concentrate on making sure each nutrient is in sufficiency for plant uptake rather than on reaching some favorable cation saturation ratio.

Most California soils have an abundance of calcium and magnesium in them already. In the soil test shown below, which is from a representative raspberry soil of the Pajaro Valley, we have 4100 ppm (mg/Kg) of calcium and 670 ppm magnesium, which when we multiply times two equals 8200 lbs of calcium and 1340 lbs of magnesium in the top 6” of soil, compared to the approximately 100 lbs of calcium and 25 lbs or so of magnesium that we need for a decent berry crop. We have A LOT of calcium and magnesium in our soils.

Moreover, it’s pretty expensive to try and alter the calcium and magnesium ratios of a soil. For example, more than 1700 lbs of gypsum will be needed to be added per acre to raise the calcium by 20 ppm. So, outside of extreme misbalances of calcium and magnesium which result in structural problems like crusting, it’s probably not worth the effort and money to obtain a ratio which doesn’t seem to offer anything extra in the way of nutrient sufficiency.

In conclusion, the truth of the matter is really that even though cations do compete with another for plant uptake and have different effects on soil structure, it is nevertheless completely possible to have high plant productivity with a wide range of ratios of calcium to magnesium.

The link below is an article with a full discussion of the concept of soil cation ratios.

http://www.soils.wisc.edu/extension/wcmc/2004proceedings/Kelling1.pdf