Soil Testing & Cation Displacement Tests

Neal Kinsey

Guest Blog by Neal Kinsey

Some years back I began encouraging those who had me come to conduct soil fertility courses to have clients that would attend, submit their good or bad areas to be tested. This way they could “test the soil tester,” knowing we had no way to tell other than by what the soil test was showing as to how the soil would perform.

This worked well until one time in England a soil that appeared to be an extreme problem turned out to be the highest producing soil on the farm. Of course, it was possible that the sample had been taken incorrectly, but the soil sent again from the same field looked just like the first one. We had several other soils sent for analysis and they all “looked like” serious problems, but some were and some were not.

In all cases, these soils, good and bad, had extremely excessive calcium levels and higher than normal Total Exchange Capacity levels. This brought to mind how the TEC is normally determined as “the sum of The Cations.” Dr. Albrecht used this method because it was more economical to measure the positively charged ions (calcium, magnesium, potassium, sodium, etc.) and establish the soil’s amount of negative charge from that. The thinking was that for every positively charged ion there was a negatively charged clay or humus particle to attract and keep it there.

This type of test works fine until there is an extreme excess of one cation or another, such as an extreme excess of calcium in the chalk soils of England, extreme magnesium levels in many heavy clay soils, and extreme sodium in desert soils, as examples to consider. Then the question becomes, is there so much of that cation there because it is being released faster than it can be utilized or leached away? Or is there really enough negatively charged particles for them to adsorb to and remain there?

Dr. Albrecht mentioned a test that could be done to accurately determine the true amount of negative charge beforehand, but it was not normally done due to the added expense and the fact that in most cases, it was not needed. We call this test the Cation Displacement test. As the program has shown, it only comes into play when there are severe excesses in the soil that will inflate the value of the true Total Exchange Capacity.

This type of test has shown the difference as to why some chalky soils, or other types with extremely excessive calcium saturations, that were excellent, appeared to be problems. With a high saturation of calcium, all the calcium ions were not actually able to attach to a negatively charged humus or clay particle, because there were too few to adsorb all the calcium ions. Thus, figuring that each calcium ion represented the correct amount of negative charge caused the TEC to be expressed as too high on the soil test. When we performed the Cation Displacement test on those extremely high calcium soils, which accurately determines the amount of a soil’s negative charge (the correct TEC), the true exchange capacity was much lower.

Then calculating that it is basically the excessive element that will not be attached to clay or humus, what we found was that even though the percentages looked about the same for the good and bad areas with normal testing, where the other cations (principally magnesium, and potassium when calcium is extremely excessive) showed to be adequate using the Cation Displacement test, there was no problem. But where they showed to be too deficient on the soils once the correct TEC was established by performing the Cation Displacement test, that (along with determined trace element deficiencies) turned out to be the problem soils.

As the testing procedures are developed, we foresee using this type of test for not only soils with severe excesses, but also to determine the correct TEC when soils must be sampled that are too dry, that have had N in the last 30 days, or large amounts of sulfur in the last 6- 12 months. Because of the added expense (around 50% increase per sample), we confirm the need first and only run the Cation Displacement test for those who tell us they want it done.

Because it is not a part of the normal test and the parameters are still being standardized, this testing requires individual attention and is still a very slow process. This cannot be helped, so if you are in a rush, skip the Cation Displacement test. We advise waiting, if possible, but until the testing protocols are completed to ensure speed and accuracy some clients may not feel they can afford the time.

This test has never been developed and offered as a part of soil testing to farmers and consultants before, because it can only be utilized and understood by correctly measuring TEC and base saturations in exactly the same way every time. If this type of testing were to be used for all soils with extreme excesses, it would soon become evident as to which testing programs can and cannot define the differences between good and bad fertility using their soil test numbers. And those labs or consultants that do not correctly understand, measure, and utilize TEC and base saturation will not understand, let alone capably utilize, this type of testing.

It requires time to get any new lab procedures correctly identified and defined. This is the stage we are in for Cation Displacement testing. The test has been so effective that we have received hundreds of requests per month instead of the few dozen we expected.  Progress is being made, but it takes time to correctly utilize this new system of approach. But for those who plan ahead in order to take advantage of such testing where needed it is paying big dividends by showing the true levels of needed fertility.

Learn more about Total Exchange Capacity Testing at Kinsey Ag Service.