Two broad philosophies exist as the basis for making fertilizer cation (potassium, magnesium and calcium) recommendations. The first philosophy, the sufficiency level philosophy, states that there are defined levels of cation nutrients that are sufficient and independent of the soil test levels of other cations. Potassium (K) recommendations, for example, are based on a soil critical level (ppm of nutrient), the soil cation exchange capacity, the crop to be grown and yield potential of that crop.

For example, according to Michigan State University Extension bulletin E2904, “Nutrient Recommendations for Field Crops in Michigan,” a soil with a cation exchange capacity of 11 with a soil test level of 91 ppm exchangeable potassium, growing corn with a 175 bushel per acre yield goal, would generate a recommendation of 65 pounds per acre K2O. The recommended K2O would be the same regardless of the soil test magnesium (Mg) and calcium (Ca) levels. This is the philosophy supported by Michigan State University and other land-grant universities.

A second philosophy, the basic cation saturation ratio philosophy, states that there are ideal ratios of calcium, magnesium and potassium in the soil. That is, levels of calcium, magnesium and potassium relative to each other and not their absolute values generate soil test recommendations. The basic cation saturation ratio philosophy was first developed in the early 1900s by a researcher named F.E. Bear and his research partners. Today, the values generally used are 65-85% calcium, 6-12%  magnesium and 2-5% potassium exchangeable bases, although values can vary from these, according to “A Review of the Use of the Basic Cation Saturation Ratio and the ‘Ideal’ Soil” by Peter Kopittke and Neal Menzes.

Crops have been shown to grow to maximum yield over a wide range of cation ratios through a series of field trials, such as in the Kopittke and Menzes publication and “Soil Cation Ratios for Crop Production” by the University of Minnesota Extension. It has been shown that the basic cation saturation ratio method results in costlier fertilizer applications. Also, on very sandy soils with low organic matter (low cation exchange capacity), recommendations based on the basic cation saturation ratio philosophy could result in less than necessary recommended potassium and magnesium. MSU does recommend soil amendment based on calcium:magnesium ratio if the amount of magnesium exceeds calcium on an equivalence basis—a rare occurrence.

There are advantages and disadvantages to both approaches. However, MSU adapts the first method. Growers should test fields regularly and fertilize based on available nutrients and crops grown in the rotation and expected yield.