The exchange capacity, also called Cation Exchange Capacity (CEC) or Total Exchange Capacity (TEC), is essentially the soil’s ability to hold and exchange nutrients. While this certainly includes all nutrients, there are five minerals that represent about 95% of the minerals that the soil holds and exchanges. These five minerals (calcium, magnesium, potassium, sodium, and hydrogen) represent what is called the “Base Saturation” on soil tests, and are what we will focus on for today’s discussion.

The exchange capacity of a soil is largely determined by the amount of organic matter and clay that the soil contains. By extension, this means it is largely determined by the soil type, with sandy soils having a low exchange capacity, and clay soils having a higher exchange capacity, although it can certainly by influenced by increasing humus and soil organic matter, adding carbon, and improving the “structure” of clay soils.

Why does all this matter?

This matters because soil is not all meant to be treated the same. A soil with a lower exchange capacity does not need as much minerals as a soil with a higher exchange capacity. Percentage wise, yes. PPM or pounds per acre, not so.

Think of it this way: imagine your soil’s exchange capacity as being the size of a bucket that you then want to fill with 68-72% calcium, 12% magnesium, 4-7% potassium, 1.5% sodium, and 5-10% exchangeable hydrogen. A 3-gallon bucket will take three times more minerals than a 1-gallon bucket to maintain these percentages.

What ultimately happens when exchange capacity is not understood is that all soils are treated the same. What then happens in a lighter soil is that calcium, magnesium, potassium, and sodium, having a much higher atomic weight, force exchangeable hydrogen off the soil colloid because it is over-saturated. The bucket is full and overflowing. The pH is high and things just don’t work like they did when the soil was properly balanced and still had exchangeable hydrogen.

Now, let’s say that we did NOT understand exchange capacity, and that we have a desired value of 3000 ppm calcium, which is certainly a reasonable calcium number for some soils: In the following example, imagine how deficient the calcium on the last test (the column on the right) would be and how much calcium we could put on and it would still test deficient!

If that amount of calcium was applied, how would that impact the balance of the other minerals?

Did you notice that, in the above example, each field has a different desired ppm of calcium? That’s because each desired value is based on the exchange capacity of that particular soil. That’s why we need to know the exchange capacity of soil – so we know what the desired value is for each of these nutrients.

Sometimes, soils that we would expect to have a high exchange capacity come back testing as lower than we expected. This can happen when soils don’t contain enough calcium to properly structure the clay, when magnesium levels are too high and create a sticky soil, or when clay becomes “aged” because of wrong product applications, and especially using anhydrous ammonia or potassium chloride (muriate of potash).

I would also like to point out that lighter soils, having a lower exchange capacity, are easier to change than heavy soils with a high exchange capacity. The question is whether or not the soil has enough capacity to feed the crop for the entire year. On the flip side, clay soils are harder to change, but are then easier to maintain once the balance has been achieved.