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Discussion Starter · #1 ·
Hi everyone! I figured out how to convert CEC into something I understand...namely, how many grams of a cation (positively charged ion like ammonium, calcium, sodium, etc) the substrate can hold. Here is the equation;

Milligrams[mg] of cation substrate can hold = (Mass of substrate/100 grams) * Meqs/100 grams * molar mass of cation

Let us say, for example, that we have a 1 inch (2.53 centimeters) thick substrate of turface MVP (with a density of .593 grams per ml or cubic centimeter and a CEC of 30 meq/100 grams) in a 10 gallon (which has a floorplan of about 50.6 by 25.3 centimeters). To find the mass, multiply the length, width, height and density together like so;

2.53 * 25.3 * 50.6 * .593 = 1,920.64 grams

Divide the mass in grams by 100;

1,920.64/100 = 19.20 (we'll leave the decimals out for simplicity)

Then plug this into the equation above with the molar mass of your desired cation (we'll use ammonium...ammonium has a molar mass of 18.039, but we'll just use 18 here for simplicity) and turface's CEC of about 30 meq/100 grams;

19.2 * 30 * 18 = 10,368 milligrams of ammonium

So, this one inch thick layer of turface mvp in an aquarium could hold about 10,368 milligrams, or 10.368 grams, of ammonium (a milligram is 1/1,000th of a gram), assuming ammonium was the only cation being retained. Ammonium is a relatively small cation; the larger the cation, the more of it the substrate can hold (calcium, for example, has a molar mass of just over 40; this same turface substrate could hold over 23 grams of calcium).

Hope you guys find this useful :)
 

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It's important to keep in mind that CEC changes based on pH, soil structure, and different cations adsorb differently. Calcium, for example, is held more tightly than ammonium, so if you have equivalent amounts in solution there's going to be more calcium adsorbed than ammonium.

I think for our purposes it's easiest to think about CEC categorically than trying to precisely quantify it. Gravel and sand has almost no CEC, clay-based products have a good bit, and stable organic matter (i.e. humus, maybe mulm? I need to look into that.) has the most. There are differences in CEC within those groups of course, but the difference between them is one or two orders of magnitude.

The thing I don't really understand about how this works in a tank (I learned this in relation to agriculture) is how well nutrients diffuse in the tank between the soil water and the water column. Undoubtedly there's a gradient and it's going to be very substrate dependent, but I don't have a good idea about how a nutrient heavy substrate (I'm thinking root tabs here) interacts with a lean water column and vice versa and how CEC affects that relationship.
 

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Discussion Starter · #3 ·
It's important to keep in mind that CEC changes based on pH, soil structure, and different cations adsorb differently. Calcium, for example, is held more tightly than ammonium, so if you have equivalent amounts in solution there's going to be more calcium adsorbed than ammonium.

I think for our purposes it's easiest to think about CEC categorically than trying to precisely quantify it. Gravel and sand has almost no CEC, clay-based products have a good bit, and stable organic matter (i.e. humus, maybe mulm? I need to look into that.) has the most. There are differences in CEC within those groups of course, but the difference between them is one or two orders of magnitude.

The thing I don't really understand about how this works in a tank (I learned this in relation to agriculture) is how well nutrients diffuse in the tank between the soil water and the water column. Undoubtedly there's a gradient and it's going to be very substrate dependent, but I don't have a good idea about how a nutrient heavy substrate (I'm thinking root tabs here) interacts with a lean water column and vice versa and how CEC affects that relationship.
It does indeed change based on PH...not sure how much turface is affected by this (though some quick research suggests that clay's CEC increases with increasing PH), but peat moss's CEC can double if its PH goes up from the 3's to 8. Most of our aquariums are going to be in the range of 6-8, so assuming peat moss in the substrate does not reduce the PH of the water below that its normal CEC of 100-120 could go up considerably.

Funny enough, while organic matter has a higher CEC than clay by mass, it does not necessarily have a higher CEC by volume (the important thing in an aquarium), as organic matter (again, peat moss is a good example) often has a much lower bulk density than, say, turface (itself not outrageously heavy).
 

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It does indeed change based on PH...not sure how much turface is affected by this (though some quick research suggests that clay's CEC increases with increasing PH), but peat moss's CEC can double if its PH goes up from the 3's to 8. Most of our aquariums are going to be in the range of 6-8, so assuming peat moss in the substrate does not reduce the PH of the water below that its normal CEC of 100-120 could go up considerably.

Funny enough, while organic matter has a higher CEC than clay by mass, it does not necessarily have a higher CEC by volume (the important thing in an aquarium), as organic matter (again, peat moss is a good example) often has a much lower bulk density than, say, turface (itself not outrageously heavy).
Yeah, the degree that pH affects CEC is itself substrate dependent, even between different clays. (I seem to remember smectite is not affected much by pH, but don't quote me on that.) I don't know anything about turface specifically. My overall point is that it can be difficult to know the actual effective CEC, but luckily we don't need that much precision.

Good catch about mass vs. volume.
 

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The thing I don't really understand about how this works in a tank (I learned this in relation to agriculture) is how well nutrients diffuse in the tank between the soil water and the water column. Undoubtedly there's a gradient and it's going to be very substrate dependent, but I don't have a good idea about how a nutrient heavy substrate (I'm thinking root tabs here) interacts with a lean water column and vice versa and how CEC affects that relationship.
There is leaching in both directions and substrate type not only affects the leaching, but also affects nutrient absorption by plants mainly, I believe, as a function of pH variations. The hydroponics industry has studied this extensively, both in terms of soil type (dirt, CEC, etc) and soil-less. If you search "hydroponics pH nutrient uptake" you may find some interesting articles that will give you a better feel for the dynamics.
 
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