21.17: Chapter Appendix- The Basic Cation Saturation Ratio System

The basic cation ratio system was discussed earlier in this chapter. This appendix is intended to clarify the issues for those interested in soil chemistry and in a more in-depth look at the BCSR (or base ratio) system.

Background

The basic cation saturation ratio system attempts to balance the amount of Ca, Mg and K in soils according to certain ratios. The early concern of researchers was with the luxury consumption of K by alfalfa—that is, if K is present in very high levels, alfalfa will continue to take up much more K than it needs and, to a certain extent, it does so at the expense of Ca and Mg. When looking with the hindsight of today’s standards, the original experiments were neither well designed nor well interpreted and the system is therefore actually of little value. But its continued use perpetuates a basic misunderstanding of what CEC and base saturation are all about.

With very little data, Firman Bear and his coworkers decided that the “ideal” soil—that is, an “ideal” New Jersey soil—was one in which the CEC was 10 me/100g; the pH was 6.5; and the CEC was occupied by 20% H, 65% Ca, 10% Mg and 5% K. And the truth is, for most crops, that’s not a bad soil test. It would mean that it contains 2,600 pounds of Ca, 240 pounds of Mg and 390 pounds of K per acre to a 6-inch depth in forms that are available to plants. While there is nothing wrong with that particular ratio (although to call it “ideal” was a mistake), the main reason the soil test is a good one is that the CEC is 10 me/100g (the effective CEC—the CEC the soil actually has—is 8 me/100g) and the amounts of Ca, Mg and K are all sufficient.

Problems with the System

When the cations are in the ratios usually found in soils, there is nothing to be gained by trying to make them conform to an “ideal” and fairly narrow range. In addition to the practical problems and the increased fertilizer it frequently calls for above the amount that will increase yields or crop quality, there is another issue: The system is based on a faulty understanding of CEC and soil acids, as well as on a misunderstanding and misuse of the term percent base saturation (%BS). When it is defined, you usually see something like the following:

%BS = 100 x sum of exchangeable cations / CEC

= 100 x (Ca⁺⁺ + Mg⁺⁺ + K⁺ + Na⁺) / CEC

First off, what does CEC mean? It is the capacity of the soil to hold onto cations because of the presence of negative charges on the organic matter and clays, but also to exchange these cations for other cations. For example, a cation such as Mg, when added to soils in large quantities, can take the place of (that is, exchange for) a Ca or two K ions that were on the CEC. Thus, a cation held on the CEC can be removed relatively easily as another cation takes its place. But how is CEC estimated or determined? The only CEC that is of significance to a farmer is the one that the soil currently has. Once soils are much above pH 5.5 (and almost all agricultural soils are above this pH, making them moderately acid to neutral to alkaline), the entire CEC is occupied by Ca, Mg and K (as well as some Na and ammonium). There are essentially no truly exchangeable acids (hydrogen or aluminum) in these soils. This means that the actual CEC of the soils in this normal pH range is just the sum of the exchangeable bases. The CEC is therefore 100% saturated with bases when the pH is over 5.5 because there are no exchangeable acids. Are you still with us?

As we discussed earlier in the chapter, liming a soil creates new cation exchange sites as the pH increases (see the section “Cation Exchange Capacity Management”). Laboratories using the BCSR system either determine the CEC at a higher pH or use other methods to estimate the so-called exchangeable hydrogen, which, of course, is not really exchangeable. Originally, the amount of hydrogen that could be neutralized at pH 8.2 was used to estimate exchangeable hydrogen. But when your soil has a pH of 6.5, what does a CEC determined at pH 8.2 (or pH 7 or some other relatively high pH) mean to you? In other words, the percent base saturation determined in this way has no relevance whatsoever to the practical issues facing farmers as they manage the fertility of their soils. Why then even determine and report a percent base saturation and the percentages of the fictitious CEC (one higher than the soil actually has) occupied by Ca, Mg and K?

If you would like to delve into this issue in more detail, see the articles listed below. We specifically note the 2007 review article that concluded: “Our examination of data from numerous studies […] would suggest that, within the ranges commonly found in soils, the chemical, physical and biological fertility of a soil is generally not influenced by the ratios of Ca, Mg and K. The data do not support the claims of the BCSR, and continued promotion of the BCSR will result in the inefficient use of resources in agriculture.”

Appendix Sources

Kopittke, P.M. and N.W. Menzies. 2007. A review of the use of the basic cation saturation ratio and the “ideal” soil. Soil Science Society of America Journal 71: 259–265.

McLean, E.O., R.C. Hartwig, D.J. Eckert and G.B. Triplett. 1983. Basic cation saturation ratios as a basis for fertilizing and liming agronomic crops. II. Field studies. Agronomy Journal 75: 635–639. Rehm, G. 1994. Soil Cation Ratios for Crop Production. North Central Regional Extension Publication 533. University of Minnesota Extension: St. Paul, MN.