Soil Sampling Agronomy Newsletter

by Dr. George Rehm, Extension Specialist-Soils
Minnesota Extension Service, University of Minnesota

The word "balance" means different things to different people. There are some who believe that a "balanced" fertilizer program is one where the amount of any nutrient removed by the crop is replaced in the fertilizer program for the following year. This approach to fertilizer or ag-lime applications does not make use of a soil testing program. This concept of a balanced fertilizer program can also be very expensive.

There are also some who believe that there should be an ideal balance among calcium (Ca), potassium (K), and magnesium (Mg) in soils. For example, it was once thought that crop yields would be maximized if 65% of the negatively charged sites in the soils would be occupied by calcium and 10-15% of these sites occupied by magnesium. Much recent research, however, has shown that yields are not dependent on the relationship of these three nutrients to one another.

For example, Wisconsin researchers varied the Ca:Mg ratio in soils from about 2.25:1 to 8.5:1 and measured the effect on alfalfa yield (see table). It is obvious that the Ca:Mg ratio had no impact on alfalfa production.

Effect of various Ca:Mg Ratios on alfalfa yields
Theresa Silt Loam Plainfield Loamy Sand
  Yield   Yield
Ca:Mg Ratio tons/acre Ca:Mg Ratio tons/acre
2.28 3.3 2.64 4.1
3.61 3.1 3.33 4.1
4.09 3.6 4.00 4.4
5.25 3.5 4.81 4.1
8.44 3.2 8.13 4.4

The Ca:Mg ratio in Minnesota soils varies from 1:1 to 8:1. Yet, this should not be a cause for concern. The fact that the Ca:Mg ratio can vary over a wide range and does not affect crop production should eliminate most of the controversy regarding calcitic and dolomitic limestone. Decisions on the source of ag-lime to use should be BASED PRIMARILY ON THE COST PER POUND OF EFFECTIVE NEUTRALIZING POWER (ENP) rather than what effect the material may have on the Ca:Mg ratio in the soil.

If we summarize all of the recent research with Ca, Mg, and K, we find that Ca:Mg, Mg:K, and Ca:K ratios are not important for crop production. There is no "ideal" balance among these three nutrients in soils. It is important, however, that these nutrients be present in soils in amounts that are adequate for crop needs in the same way that nitrogen and phosphorus are needed. If soil test K and Mg levels are low, these nutrients will be needed in a fertilizer program. The ratio of one to the other or the "balance" between the two is NOT important.

Simply put, a balanced fertilizer program is one which provides for adequate, but not excessive, supplies of all plant nutrients in the soil system. A large percentage of our soils in Minnesota are capable of supplying adequate amounts of many of the essential nutrients. When soil supplies of nutrients are low (as indicated by soil test) adequate amounts must be supplied in the fertilizer program. Soil testing, then, is the key to arriving at a balanced fertilizer program.