Building Soils for Better Crops, Third Edition

The Basic Cation Saturation Ratio System

SARE Outreach
Fred Magdoff and Harold van Es | 2010 | 294 pages
PDF (6.8 MB)

This title is temporarily out of print. We expect to publish an updated edition in the spring/summer of 2021.

This section deals with a somewhat complicated topic and is intended to clarify the issues for those interested in soil chemistry and a more in-depth look at the BCSR (or base ratio) system.

Background

The basic cation saturation ratio system, which attempts to balance the amount of Ca, Mg, and K in soils according to certain ratios, grew out of work in the 1940s and 1950s by Firman Bear and his coworkers in New Jersey and later by William Albrecht in Missouri. 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 provided by more than a half century of soil research after the work of Bear and Albrecht, the experiments carried out in New Jersey and Missouri were neither well designed nor well interpreted, by today’s standards. The methods for determining cation ratios, as well as the suggested values that the cations should have, have been modified over the years. Recent work indicates that the system is actually of little value. 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. On the other hand—as mentioned in the previous discussion—there are some, relatively infrequent, situations in which the problem of a high level of a particular cation needs to be addressed and can be addressed with either the BCSR or sufficiency system.

In addition to the lack of modern research indicating that it actually helps to use the BCSR system to make recommendations, and the problems that can arise when it (in contrast to the sufficiency system) is used, its 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 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

In addition to the practical problems with using the base ratio system, and the increased fertilizer it frequently calls for above the amount that will increase yields of crop quality, there is another issue: The system is based on a faulty understanding of CEC and soil acids, as well as a misuse of the greatly misunderstood term percent base saturation.

When percent base saturation (%BS) 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 on to 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 in chapter 20, liming a soil creates new exchange sites as the pH increases (see the section “Cation Exchange Capacity Management”). The hydrogen affected by the lime is strongly held on organic matter, and, although it is not “exchangeable,” it does react with lime and is neutralized—creating new exchange sites in the process. So what does the percent base saturation reported on some soil test results actually mean? The labs 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. In other words, the hydrogen that could be neutralized at pH 8.2 was added to the exchangeable bases, and the total was called the cation exchange capacity. 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? Actually, it has no usefulness at all. As the percent base saturation is usually determined and reported, it is nothing more than the current soil’s CEC as the percent of CEC it would have if its pH were higher. In other words, the percent base saturation 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 percents of the fictitious CEC (one higher than the soil actually has) occupied by Ca, Mg, and K? Good question! Although we understand that many farmers believe that this system helps them to manage their soils better, it is our belief— based on research—that it would be best to stop using the system.