Exchange Capacities and Base Saturations
This is the second in a series of articles related to understanding the metrics you get from laboratory soil test results. If you haven’t read the “Soil Management Part 1” article, it would be a good idea to do that first before tackling the material in this article.
In that article, I covered the major nutrients, secondary nutrients and micronutrients. Just to reinforce the material, there is no relationship between whether a nutrient is major or secondary and its importance – it is mostly the quantity of it that is needed that determines its typing.
Most of this article will utilize an analogy that I’ll spell out here.
You need to picture a swimming pool. The pool is filled to the top with a mixture of beach balls, tennis balls and marbles. Obviously, the total number of objects in the pool depends on the number of beach balls, tennis balls, and marbles in the mix. If it’s mostly beach balls, there will be fewer total objects in the pool, while if it is filled almost completely with marbles there will be many, many more objects in the count. The balls are called “colloids” in soil. Now add to that picture an interesting twist – each of the balls in the pool has an electrical charge that attracts particles of nutrients. They attract nutrients (called “cations”) that have the opposite charge in a manner similar to magnetism. But each ball can only attract and hold one nutrient particle. It doesn’t matter if it’s a beach ball or a marble – it can only attract and hold one nutrient particle. Which mixture would hold more nutrients – the pool of beach balls or the pool of marbles? Obviously, it’s the pool full of marbles.
In soil chemistry, we call the beach balls “sand”, we call the tennis balls “silt” and the marbles are “clay”. Believe it or not, the sizes of the particles are much more pronounced in soil. And the number of nutrient particles that can be held by all the balls in the pool is called the “Cation Exchange Capacity” or “CEC”.
CEC is critically important. If a soil can’t attract and hold nutrient particles, they go right through the soil and exit into the subsoil and deeper – eventually reaching the water table. We call this process “leaching”. Effectively, large-particled, sandy soils hold few nutrient particles and allow a higher level of leaching.
In general, for lawns, we classify the soil type by looking at the CEC. Sandy soils have a CEC generally under 10. Loam soils vary between 10 and roughly 18. At that point, we begin wandering into clay-based soils. Clay soils can go to from 18 (or so) up to a CEC of 100.
Let’s go back to the pool analogy. We’ve got beach balls, tennis balls and marbles. But potentially, we also could have a few little scraps of sponges scattered among the holes in between the larger particles. They can hold some nutrient particles, too. Want to guess what the “scraps of sponges” are in the soil chemistry world? Organic matter!
It may seem backward, but the CEC is determined by removing the cations from the colloids. We estimate the number of colloid particles (“balls” in the analogy). By knowing how many cation particles we get, we know the capacity of the soil.
Alright, so we know the soil colloids can hold important nutrients. But what are the nutrient cations? The primary ones are Calcium, Magnesium, Potassium and Sodium. There are others sometimes present. Soils that don’t have the primary cations filling all of the available receptor sites will have either Hydrogen or Aluminum filling the sites. Soils that aren’t full with the primary cations are acidic soils. In many postings, we call Hydrogen and Aluminum “riff-raff”.
Once we have the cations separated, we can identify each one with solvents. When we know what percentage of the cations are made up of each element, we have generated a second metric: the Base Saturations. For reasons too detailed for this article, the best mix of cations is when Calcium is between 60%-70% of the mix, Magnesium is between 10%-20%, Potassium is between 3%-5% and Sodium is present but lower than 3%.
CEC is different from TEC (“Total Exchange Capacity”) in that TEC accounts for what the capacity could be if the Hydrogen was replaced with primary cations. A little more sophisticated, it draws a better picture of the capacity in acidic soils. In alkaline soils, TEC and CEC are always the same.
Occasionally, the CEC can be overstated in tests. This happens when there is so much of a cation present that the soil has the cation present that is greater than what is bound to the soil. With a little practice, the soil test interpreter can spot likely cases of this situation.
So, with its higher nutrient-holding capacity, you might be tempted to think that a clay-based, high-CEC/TEC soil is better. Well, it is – but it has a cost. Clay soils tend to absorb water very slowly, have a lower level of air in the soil (low tilth) and a physicall-challenging to root growth.
But that’s a story for another article…