Cation Exchange Capacity of Mediums and Affects on Fertigation

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Protaide

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In all the researching I've been doing recently, I wanted to post some information specifically on Cation Exchange Capacity (CEC) and how it affects elements in mediums. CEC has a large hand in most all forms of medium and interacts directly with elements. I'm sure that many people already know of CEC or even a great deal about it; however I wonder if many don't understand how it works. My hope here is to provide some info that I've found / learned, so that it may help others with medium choices; possibly even problem diagnosis. I will try not to be overly technical wherever possible.

If anyone has any additions or alterations that need to be done please feel free.

First some definitions (from wikipedia):

CEC - In soil science, cation-exchange capacity (CEC) is the maximum quantity of total cations, of any class, that a soil is capable of holding, at a given pH value, available for exchange with the soil solution.

So what are cations? They are a type of ion that, in our case, comes from electrolytes (salts) we use in fertilizers. I'll define from there.

Electrolyte - An electrolyte is a substance that ionizes when dissolved in suitable ionizing solvents such as water. This includes most soluble salts, acids, and bases.

The salts separate to form ions when dissolved into solution form. These Ions are either positive (Cation) or negative (Anion) in charge.

Cation - A cation (+) is an ion with fewer electrons than protons, giving it a positive charge.

Anion - An anion (−) is an ion with more electrons than protons, giving it a net negative charge

When looking at a salt compound; the first element is positive and second is negative. E.G. Calcium (+) Nitrate (-)


All of this should be pretty easy to understand. Mediums hold ions with charges. The greater the CEC, the more Cations that are held. However, now we should consider interaction in use. First we need to figure out what elements we are adding are Cationic or Anionic.

Cations encountered commonly:
Urea
NH4
Ca
Mg
K
Fe
Zn
Mo
Mn
Na
Sn
Cu
Sr
Al
H

Anions encountered commonly:
NO3
NO2
S
P
Cl
HCO3

Boron is Neither Anion or Cation without oxidation

As you can see there are many more Cations than Anions that would be encountered. This can be good or bad since CEC is all about holding Cations. It's a good time to point out that because the soil can only adsorb Cations; Anions are elements that are easily flushed / leeched out becuase the medium itself is negative charge. This is part of the cause as to why phosporous seemed to require higher additions / had higher utilization. As well, this is part of the reason that Urea is used in agriculture (beyond cost). Both Urea and ammonium are Cationic and will retain in mediums better than Nitrate; where leeching is a concern. Sulfur (sulfates) although Anionic and suffer the same issue; have not been previously required due to large environmental influx for replenishment. This has changed more recently as emissions have largely removed Sulfur compounds.

The next thing to consider, is that "not all Cations are created equal".
Cations all have positive charge, however the extent of their positive charge does also relate to CEC. The stronger the charge, the higher the medium's affinity to grab that element over another. I.E. - The medium will hold Cations in order from most strongly cationic to least cationic. The CEC in terms of capacity relates to the total amount of Cations that can be held; reguardless of strength of charge. This part of the reason why too much of certain Cations can cause deficiencies as the medium can get full of greater cation rather than a balance.
I've mostly been familiar with the electomotive series for elements in my past, but with Ions they use a different scale; The Lyotropic Series or The Hofmeister series. This is the order of Cations and anions relative to their charge.

Cations sorted by Charge. Most Cationic to least Cationic:
Al - H - Ca - Mg - Fe - Sr - Na - K - NH4
(unknown placement for Zn, Mn, Mo, Sn; but presumably in between Iron and sodium)

Anions Sorted by charge. Most Anionic to least Anionic:
S - P - Cl - NO3

Where the stronger Cations hold more easily; the stronger anions will leech more easily. I believe I've seen mention of HCL flush methods along with lower pH and you can see from the series above how flushing by displacement can occur. As well, this is probably part of the reason Coir requires more Calcium as it gives off Potassium.

It's important to note that mediums will attempt to hold cations at their CEC saturation point before easily "giving off" what has been adsorbed. Once the cation capacity has been reached. The medium will give off cations as new are brought in; thus "Exchanging" one for another in sort of a "pool ball effect" or "one in one out". Here, this is part of the reason why it's hard to get good ideas of medium conditions from run-off. Any anions in new water or already in medium will tend to flush easily; whereas the cations will be (mostly) displaced on their way through giving off stored cations in the run-off. Again to consider, the medium will always attempt to displace elements with the highest charge first. This means a Ca can displace not only itself, but more easily; any other element with less charge (Mg, Fe, K, etc).

There is one final piece to this puzzle of medium interaction and fertigation. That is, how plants best acquire these elements from a general stand point. Sure we all know "they get'em from the roots"; but there is a bit more that can be defined That is Nutrient movement through mediums.

There are generally 3 classifications for movement and corresponding elements that best suit each.

1) Interception: This is direct contact of elements with the root system and what everyone would be familiar with.
2) Mass Flow: This is classified as bulk movement of water through the medium and also includes evaporation.
3) Diffusion: This is where areas of higher salt concentrations will slowly move to areas of lower salt concentration. (similar to adding salt to water and watching it mix)

The primary nutrient movement in mediums:
1) Interception: Ca, Mg, Zn
2) Mass Flow: NO3, Ca, Mg, S, Cu, Mn
3) Diffusion: P, K, Zn, Fe


And Finally Some typical CEC ratings for various mediums in order (including pH)
Peat Humus = 160 - 200 (pH 5-7.5)
Hypnum Peat Moss = 100 - 200 (pH 5-5.5)
Vermiculite = 100 - 150 (pH 7-8)
Sphagnum Peat Moss = 110 - 130 (pH 4-5)
Reed Sedge Peat Moss = 80 - 100 (pH 4-7)
Coconut Coir = 39 - 60 (5.6-6.9)
Pine Bark = 40 - 60
Composted Bark = 8 - 60
Calcined Clay = 6 - 21
Perlite = 1 - 3
Rockwool = 0 (pH 7.5-7.8)
Sand = 0

Mixes CEC
Peat perlite 2:1 = 20-30
Bark Perlite 2:1 = 20-30

Other Random Tidbits of info.
Carbon to Nitrate ratio
Sphagnum Peat Moss = 48:1
Composted Bark = 106:1
Coconut Coir = 85:1

I think that covers CEC and interactions with fertilizers irrespective of biological or plant action. Hopefully this helps some with choosing or even diagnosing medium related issues. I did not cover water holding capacity and might add that later to help also.

Sorry to have been long. Thanks.
 
Coir

Coir

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I think it's also important to point out that while this information is extremely valuable and good to know, it can lead to people seriously overthinking and over complicating cultivation. I have seen so many growers that started out tremendously due to the fact they were unsure what they were doing and simply followed directions. It is once they started to try and reach magical numbers and balanced cation/anion ratios that they began having problems.
 
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Protaide

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Oh my yes. I did not provide this to cause complications. Merely to provide information and understanding with mediums. I don't / didn't expect people to alter mediums based on this; but that the information does play a role. It all depends on each situation and perspective. From a general point of view; In agriculture higher CEC is preferred, primarily as fertilizers are typically added once or twice. Lower CEC in agriculture would require more frequent additions of fertilizers, which is not typically practical.
 
caveman4.20

caveman4.20

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Oh my yes. I did not provide this to cause complications. Merely to provide information and understanding with mediums. I don't / didn't expect people to alter mediums based on this; but that the information does play a role. It all depends on each situation and perspective. From a general point of view; In agriculture higher CEC is preferred, primarily as fertilizers are typically added once or twice. Lower CEC in agriculture would require more frequent additions of fertilizers, which is not typically practical.
Please share what you believe involving CEC and reusing soil.....:cigar:
 
Coir

Coir

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Oh my yes. I did not provide this to cause complications. Merely to provide information and understanding with mediums. I don't / didn't expect people to alter mediums based on this; but that the information does play a role. It all depends on each situation and perspective. From a general point of view; In agriculture higher CEC is preferred, primarily as fertilizers are typically added once or twice. Lower CEC in agriculture would require more frequent additions of fertilizers, which is not typically practical.
A lot of that depends on what type of agriculture. Most field growers around here apply fertilizer at the beginning of the season and that's it. Those of us that grow vegetables or flowers in greenhouses have a completely different method. Whether growing in soil, soilless mixes, rockwool, coir, or even NFT or other mediumless growing systems, feed at a consistent level at all times. I use inline injectors to supply water/nutrient with every irrigation cycle in my coir and rockwool slabs. For my NFT systems, I monitor the EC and pH every morning and make any necessary adjustments throughout the day as necessary. I don't change the level of feed but rather irrigation cycles based on time of year, plant age, plant type, solar radiation, humidity, and temperature. Interestingly enough, I can run both coir and rockwool slabs on the same irrigation system with no adverse effects even though the CEC is quite different in both of them.
 
caveman4.20

caveman4.20

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A lot of that depends on what type of agriculture. Most field growers around here apply fertilizer at the beginning of the season and that's it. Those of us that grow vegetables or flowers in greenhouses have a completely different method. Whether growing in soil, soilless mixes, rockwool, coir, or even NFT or other mediumless growing systems, feed at a consistent level at all times. I use inline injectors to supply water/nutrient with every irrigation cycle in my coir and rockwool slabs. For my NFT systems, I monitor the EC and pH every morning and make any necessary adjustments throughout the day as necessary. I don't change the level of feed but rather irrigation cycles based on time of year, plant age, plant type, solar radiation, humidity, and temperature. Interestingly enough, I can run both coir and rockwool slabs on the same irrigation system with no adverse effects even though the CEC is quite different in both of them.
Are you running organic hydro?
 
ralphy

ralphy

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A little information in the wrong hands is a dangerous thing, ie people who read this but don't fully understand it risk messing things up more if they try to incorporate a limited understanding into their growing regime. It's good to understand the importance of CEC, but simplicity is often best.
 
caveman4.20

caveman4.20

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A little information in the wrong hands is a dangerous thing, ie people who read this but don't fully understand it risk messing things up more if they try to incorporate a limited understanding into their growing regime. It's good to understand the importance of CEC, but simplicity is often best.
The part that gives me faith is how huge of a factor the roots are in this process and how they behave in different media's for instance in soil it appears that the plant is adjusting cec and ph in response to everything....it's a deep rabbit whole ...the further I go the more I know that I don't knowshit!
 
Coir

Coir

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Are you running organic hydro?
I have run full organic in coir. As much as people don't want to hear it, after lab testing of the produce(tomatoes in this case) there was no difference in chemical or nutritional content between the full organic and the conventional nutrient except of the sodium content. The organic produce had much higher levels of sodium but that could have been a lab anomaly. The fruit showed the same brix levels as well. While yield was held similar after fine tuning with the organic nutrients, it was more expensive and more difficult to maintain than with the conventional nutrients.
 
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Protaide

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Please share what you believe involving CEC and reusing soil.....:cigar:

Presumably, since CEC itself is related to the mediums electrostatic charge and surface area; any mediums that can break down will lower CEC as well. However, these mediums also will lose porosity too.


@ coir in the case of your comparative NFT's. No you won't probably see adverse affects. Since there is a constant flow and primary nutrient delivery via Mass flow. here it's just that the higher CEC is not necessary. Of course if you were to take a sampling of the 2 mediums; the rockwool would have a cation ratio closer to the feed vs. coir (I.E. Coir would test higher for say Ca and K vs Rockwool). The only area you see much focus on CEC is where they Don't deliver nutrients often and is a case where low CEC would be detrimental. In your case the higher CEC just means more flush volume heh.

@ralphy. heh agreed again I hope no one messes their system up from this though I'm not sure how they would. I'm definitely rethinking posting ideas for manipulating plant enzymes and hormones internally via elemental ratios XD. :angelic:
 
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Protaide

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The part that gives me faith is how huge of a factor the roots are in this process and how they behave in different media's for instance in soil it appears that the plant is adjusting cec and ph in response to everything....it's a deep rabbit whole ...the further I go the more I know that I don't knowshit!

I like the rabbit hole at least to learn and understand, though I know not all do. For instance I was recently reading about there being 3 ion channels for potassium in the root system. One is the primary uptake channel the second is actually a channel for expelling potassium from the root mass and the third is a negative regulator channel that loses conductivity as potassium increases. E.G roots have their own potassium sensor built-in. ;) Movement within the channels is by electrostatic gradient.

Totally useless but so neat to learn!
 
johnnyrotten

johnnyrotten

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D99 over on ICMag used to talk about Cation Exchange in his PPK thread...I never really understand it but following what he said to do sure helped me grow some big plants!
 
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