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Understanding CEC (cation exchange capacity)


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I wanted to know what is the best action if one by accident applies double dose of liquid fertilizer: If it is best to just re-water with correct dose, or flush it heavily, or what? Couldn't get any good response so I tried to calculate this myself, see below. I don't know if anyone here is into the "scientific approach" at all, if you are, you are welcome to comment, since I never tried to calculate this before. So here it goes:

I have a palm with 9L soil which drinks (up to) 3L water, and I usually use half dose of a Swedish liquid fert called Blomstra at every watering, which contains about 5g/100mL of nitrogen of which 3g is NO3- and 2g is NH4+. (Note: NH4+ are "cations", i e positively charged ions, and NO3- are "anions", and soil naturally has more or less negative charge, specified as "CEC", which holds on to the cations but lets anions get flushed out quickly.) Full dose of Blomstra is 4mL/L so for 3L of water I use 6mL Blomstra i e 2*(6/100) = 120mg NH4+ in Blomstra for one 3L watering.

These say 360 lb./acre (pounds per acre) NH4+ corresponds to 1 meq/100g, where "meq/100g" is the unit for CEC, i e if CEC=100 for my soil (which is close to the truth if it is mainly peat and/or vermiculite at least) then there is 36000 lb/acre, and one pound is 0,454g and one acre is 4047m2, and assuming soil depth of 2dm this means 4047*0,2 = about 800m3 soil, meaning 36000*0,454g NH4+ per 800m3 soil, which is 36000*0,454/800 = 20g/m3 or 20mg/L NH4+, meaning that my 9L of soil has the capacity to store 180 mg NH4+.

That should mean that the best action is to flush with plenty of clean water (which will flush out the NO3-, which is 60% of the nitrogen in this fertilizer) and not to water again with the correct dose. To water again with correct dose might make things worse. Actually, it is hard to say exactly what the situation with NH4+ will be, since the figures 120 and 180 mg are so close to eachother and my calculation rather coarse - perhaps rather much of it will be flushed or perhaps none of it, but much of it will remain in any case. And my calculations only cover nitrogen.

If you have a low CEC soil, the situation is quite different. Then you should probably first flush with plenty of clean water, and then you may water with fertilzer but maybe a slightly lower dose than normally, perhaps half of normal dose. However, I'm thinking with low-CEC soil, you probably shouldn't use liquid fert, rather use slow release fert.

I also think it is funny how instructions on my fert bottle takes no account of CEC of soil. Maybe they just assume everyone uses mostly peat moss like almost everyone does here in Sweden.

Does anyone else but me find this interesting, or am I just the biggest nerd here? :-)

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I absolutely find it fascinating, but have nothing to add...so I'll just wait for the replies to roll in :interesting:

Naples (inland), FL - technically 10a but more like 9b in the winter :hmm:

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I actually first sent my question to 2 sites saying "ask a horticulurist" but they had no clue.. In my country Horticulturist is a 5 year academic education, I assume it is similar around the world. How can someone like that not master this question I wonder.

There is a lot of useful stuff in the academic books & papers. My view is that there are many thousands of persons working full time with mainly supporting the professional growers, and then being tried out by the growers. So any question that helps them has been investigated in great detail. And my view is that what they do not cover is anything going on outside the area of professional growing. Which means they e g don't know much about influence of humidity since they know all growers have perfect humidity using machines you can't use in an apartment. So in that area the myths are vivid and unopposed... Fact is the academic world don't even cover how stomata work good enough. Sure there has been books and experiments, but they don't even know what parameter the plant reacts to (it is probably not "relative humidity" in itself that the plants actually care about, so watching your RH meter does not show humidity in the eyes of the plants exactly), and also stomata vary a lot between species (number, present on both sides of leaves or not, can be closed or not) but no one has tried to make a list of all plants. Just an example. Don't know if anyone else in this forum has been reading such books?

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Adding a missing part of the subject of this thread, namely impact of other salts than those of nitrogen:

I'm thinking only N-P-K have to be considered (since the other ones are in such small quantities), and of these, phosphor (P) should be no problem, since phosphates in common fertilizers usually have much lower salt index than nitrogen (N) and potassium (K), meaning 1 g of P is much less harmful than 1 g of N or K, and also my fertilizer has only 1 g of P and 4,3 g of K. Potassium is (probably) about as bad as nitrogen.

In any case, I think you don't have to consider the salts individually, meaning in my example I roughly have 11 g of salts of which 3 g can be leached. Also, leaching helps remove other salts already present in the soil, either if the soil is old (due to salt buildup) or fresh (due to fertilizer present in new soil).

So if you over fertilize with liquid fert a factor 2 too much as in my example, then leaching (maybe water volume 2-3 times soil volume) might well bring levels down to acceptable levels, preferably performed without delay. Next time it is time for watering, apply fert as usual if plant seems healthy (maybe half of normal dose otherwise).

Note: When I started calculation of CEC above, I was hoping storage capacity for fert due to CEC would either be much greater than amount in a fert dose or much less, so that I could have a clear opinion on the effect. The result above with figures 120 and 180 very close to eachother is the least useful I could hope for, making it hard to make conclusions. But looking at the leachable parts, one still gets some useful conclusions from this.

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How long was it before you realised that you had applied the double dose. If it was straight away I'd give it a good flush with water and leave it until it had dried out slightly before adding the correct dose. If it was some time before you realised your mistake I'd still flush with water and then not fertilize until your plant was due it's next feed. How often is this plant usually fed?

Regards Neil

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Thanks but it is not about a real case (although it has happened for me and probably will again), I am trying to understand the science of it, so that I don't have to guess or learn from mistakes to get it right. BTW I believe you are wrong - you should flush plenty, then NOT add any fert at all (not until it is time for next watering) since CEC causes more than enough fert to be stored even after flushing. And I always use liquid fert with every watering, that's the recommendation of two persons from the same university that developed Blomstra

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Adding some more discoveries and thoughts on CEC:

One practical impact (which was unexpexced to me before I thought of it) is that using ½ dose of liquid fert doesn't necessarily mean that the plant sees less fert. And a related thing is that if one waters excessively, it might have impact on level of fert in soil. As in my case, when I use ½ dose of liquid fert every watering and make sure water comes out the drain hole then empty the saucer (and I even read that the more water that comes out the better, because you get some flushing of salts). But if you water with ½ dose and use twice as much water as needed, it will actually give the same fert dose as using normal dose of fert and normal amount of water, due to CEC. For example 4L of water with 4mL fert gives same dose as 2L water with 4mL fert. This is true for the cationic nutritients but not the anionic, those get leached out. Another difference with using excessive amounts of water is that you do flush out salts that have built up (from tapwater). It also means flushing affects balance between nutritients, which is very important (I think excess of one nutritient may give symptoms of lack of the other nutritients) so I guess that's a good reason to fertilize consistently, e g fertilize at every watering.

I think that of the main nutritients N-P-K, it is only ammonium (NH4+) and potassium (K+) that are cations, and that both nitrate (NO3-) and phosphates (PO43-) are anions, which easily leach out (I didn't realize the phosphates are also anions until now). And it seems the soil stops the cations from flushing very effectively, since farmers even with CEC > 10 may apply N and K only once per year and even once for two crops.

I don't know how easy it is to flush the anions but people seem to recommend volume of water 2 or 3 times volume of soil for a good flushing, and I suppose you could just imagine putting a little table salt or sugar or whatever in soil and imagine how much water you need to flush most of it out, seems to me 1 times the volume of soil should flush plenty and that it also might depend a bit on how fast the soil drains.


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  • 5 months later...

I did some deep diving into this subject 3 months ago but I haven't had time to update this thread. What I wrote above is not correct. I don't think most people at PalmTalk find this subject very interesting (or at least that I am becoming too scientific) but in any case I want to end this thread with correct information. I read some books and publications and discussed it here, unfortunately that forum seems to be available only if you join. In summary:

  • CEC is not a factor in salinity buildup and leaching requirements. CEC holds the ions in place so that the roots can access them but not in a way so that salinity increases (meaning that from the point of the plant, the soil is not more "salty" and thus no risk of becoming more harmful because of it).
  • It also means I now believe I understand what the best action is if one accidentally over-fertilizes with liquid fert in a pot (with soilless media), which is one of the concrete reasons I want to understand this topic:
  • If I use up to twice the dose then I believe the best thing to do is just to do nothing right now and next time to use plain water (no fert) but not let much water come out the drainage holes, since at this watering the plant will use leftover fert from the last time, so try not to leach it out.
  • If I use a higher dose (or am not sure) then the safest thing is to as soon as possible leach the soil with clean water 2 (or 3) times the volume of the soil (which effectively removes close to all soluble salt) and directly after that water with fert - normal dose should be ok but maybe even better to use ½ dose this time to be on the safe side, especially if one did not leach very thoroughly.
  • This undestanding also has many other good implications, like understanding better how to act if ammonium toxicity occurs.

Below is a copy of the more detailed disussion in that forum, from my side, for anyone who is interested in the full details (I don't want to copy other people's posts there):

POST #1:
Actually understanding salinity and CEC?
I try to understand how salinity builds up in pots (with probably mostly peat moss but could also be some of the other common ingredients like bark, sand, vermikulite), both in general and specifically for ammonium.

For a 10L pot with such a soil mix, CEC might be 15meq/100cm3 which means it can hold roughly 15*39 mg per 0.1L which means 59g of K+ or 27g of NH4+ (simplified analysis) which sounds like more than I would expect but I can't find any error in my calculation.

A typical amount of N in watering can (when liquid fert and used every watering) is 50mg of N per L of water (which is 50ppm). Assume the 10L soil can hold 2L of water, that is 100mg of N. I look up in a table that 50ppm of N in water gives the water a salinity level of 0,2 mS/cm. Let's assume the plant consumes half of the applied fert. Assuming we start from zero (no fert and no water in soil), does that mean that at 2nd watering soil has 100+50mg of N, and after x waterings 100+50*x mg? And assuming this goes on without leaching, and assuming all N is NH4+, and no bacteria present to convert NH4+ to NO3- (because too cold or no oxygen), after 540 waterings I would reach the ceiling of the CEC at 27g and then have 27000 ppm and salinity of 108 mS/cm? Which is beyond insanely high but I still can't see any error in my calculations (and I realize situation is unrealistic).

Finally my most important question: I wonder to what extent CEC applies to NH4+ and K+? Because many say that ammonium is virtually unleachable if CEC is high (but those guys usually discuss clay), while at the same time Bunt in his invaluable book from 1988 says "The CEC of peat moss and bark are such that divalent cations (Ca2+ and Mg2+) are adsorbed but most monovalents cations (NH4+, K+, Na+) remain water soluble." meaning CEC does not apply to NH4+ and K+, that they are fully mobile and leach easily? Most sources seem to indicate CEC binds these fully, while Bunt seems to say they ar fully mobile and leach easily?

(Reasons for wanting to understand this is understanding salinity buildup, best action if have overfertilized, and scenario that leads to ammonium toxicity, just to mention a few top reasons.)

I have read a bunch of academic papers and books but get no definite answer, I can't even find a book that really delves into this.

POST #2:
[No useful answer so far]
I have managed to get external verification that my calculation is correct by comparing to this exercise by Dr. Thomas E Loynachan at Iowa State Univ: http://www.public.iastate.edu/~teloynac/354ppcecsol.html. Possibly 15meq is slightly high, so lets say 10 then. Then a 10 litre pot can hold not 59g but 39g of K+ (or 18g of NH4+). That is a big pile of salt in such a small pot! Who knew? And from experience, even after around 20 waterings salinity is often so high that flushing is required to avoid plant damage. So peat moss can hold around 10-20 times more salt (due to CEC) than what the plant thinks is too much. So CEC is dangerous if one does not keep an eye on it.

POST #3:
[I got a reply indicating that maybe CEC does not affect salinity, I thought the opposite was true before]
Are you saying that ions tied by CEC do not contribute to salinity level? If so, then CEC could never be harmful, and the higher CEC the better. That would be great news for me. But I thought the opposite was true: That ions bound by CEC do contribute to salinity level.

For some reason, texts on CEC seem to usually skip this very basic topic. I'm trying to find a credible source, and what I find right now is this (https://hortscans.ces.ncsu.edu/uploads/s/a/salinity_537b90737a53a.pdf) paper where they say on the topic of Salinity e g "If the CEC of a soil mix is very high, the salts will be held tightly in the mix and leaching will be much more difficult." and "It is known /..../ that organic matter, with its high CEC and large pore spaces, retards salt removal."

If this paper is correct then CEC could be a big risk, as seen in my calculations above. And if then cations are never leached, as you say, it could mean big trouble. Or maybe I misunderstood something?


POST #4:
I have read numerous books and academic (and other) texts on CEC, salinity, leaching etc so I already know what is typically in such books and texts. I have now also bought my favorite book [8], and I borrowed [5] and [6] to delve into the subjects of CEC and salinity in greater technical detail by reading selected chapters.

Above, I assumed CEC will cause ions to remain in the soil in a way that adds to salinity, but now I'm starting to think this is a big mistake. Unfortunately I have seen no one stating clearly if ions bound by CEC contribute to salinity or not, and many sources seem to indicate that CEC does affect salinity (and leaching). Here are a couple of good and interesting sources which may still cause some confusion (but worse exist):

    [1] says "Ammonium does not easily leach" in an article focused on indoors/greenhouse growing, but I suppose this is not true for soilless media (which is commonly used indoors and in greenhouses), assuming CEC is the reason he means that would affect NH4+ leachability.
    The "Field crops team" at Michigan State Univ Extension write in [2] that "NH4+ forms of N leach very little in clay. Ammonium forms of N can leach in coarse-textured sands and some muck soils.These are the only soils where ammonium leaching may be significant." which might be true when one focuses on outdoors soil like they do in this article; it is interesting they say for some soils NH4+ leaching is significant, and I guess this is also true for soilless mixes (peat and/or bark based at least).
    It is also odd how people have spent much time producing graphs on how salinity decreases with leaching for outdoors soils when they do not at all investigate and specify which ions are present, like in [3] and [4]

Bunt's book [8] doesn't explain this 100% clearly but it has several indications that CEC is not a factor in salinity buildup and leaching requirements:
    (page 88:) When listing main reasons for salinity, CEC is not mentioned.
    (page 88:) "A rise in salinity can be prevented, or corrected if it has already occurred, by controlled leaching" (unforunately the word "can" is not clear, he might mean "can always" or he might mean "can sometimes" I suppose, but I interpret it as rather being "can always").
    (page 65:) "The CECs of peat [moss] and bark largely indicate the potential amounts of exchangeable divalent ions, i e Ca2+ and Mg2+; most of the monovalent cations NH4+ and K+ will be water-soluble." (extremely intersting, and he doesn't say this but I'm guessing that in a peat moss based soil limed with dolomite, there will be an abundance of Ca2+ and Mg2+, which will occupy almost all of the CEC sites, making all monovalent cations easily leachable from any perspective)
    (page 65:) "Although plants can be grown satisfactorily in a wide range of materials irrespective of their CEC, management is usually easier when the medium has a reasonable CEC" (I assume he means "reasonably high")
    And then [5] (page 297): "In saline soils, soluble ions such as Cl-, SO42-, HCO3-, Na+, Ca2+, Mg2+ and sometimes NO3- and K+ can harm plants by reducing the osmotic potential" (it is interesting both how he specifically says that soluble ions can be harmful, as opposed to those bound for any reason including CEC I would think, and how he specifies harm coming from "reducing the osmotic potential")

Reading results in credible books (like above) is one way to get convinced. Another way is to try to understand root uptake of water and nutritients and how it might be possible for the soil to be full of ions but still not considered saline by the plants' roots. This is how I think it works. Please correct me if I am wrong:

    Matric potential expresses the force needed to remove water from the soil (caused by surface tension between water and soil). Osmotic potential expresses how different percentages of dissolved particles will cause water to want to move to even this difference out. And these particles in reality are ions (i e dissolved salts). This force can then counteract the suction the plant's roots is trying to create. (Mainly from [8, page 41])
    "Soil water availability can be expressed as the sum of the matric and osmotic potentials. As water content decreases, through evaporation and transpiration, both the matric and osmotic potentials decrease. /.../ The soluble ions cause an osmotic pressure effect." (From [5] page 297)
    The root cells in contact with the soil are within the "root hairs". The cell sap of the root hairs usually has a higher percentage of salts, causing a suction (through the cell membrane) due to the difference in osmotic potential. This is the normal way for plant roots to suck water from the soil. [7]
    "Plants are able to adjust to a [slow] increase in the soil osmotic potential by making a corresponding increase of the osmotioc potential of their cells" (i e the plants increases salt percentage in its cells in the roots, so that it keeps sucking) [8, page 87]
    So I'm guessing the ions bound by CEC (at least if I imagine CEC being saturated with Ca2+ and Mg2+) do not add to the osmotic potential since these ions don't really move (even if some should swap place it wouldn't affect osmotic potential, since they don't move in or out of the root cells and don't even, on average, move closer or further from the roots)

If this is right then it means CEC plays no part in salinity, and the higher CEC the better (since it provides a nutritient reserve), and high CEC can never be harmful. With one exception: Buildup of specific ions which for some reason the plant won't leave alone, which then cause toxicity. The main example in my mind is ammonium (NH4+). If available in too big amounts, the plant will suck it up and store it within the plant (seems like it can't resist taking it in "for a rainy day"), but unfortunately NH4+ is toxic. (E g some plants store the excess at the tips of the leaves, that's why they curl up and die then.)
Have I reached the end of this investigation now, or can you see eny errors?
[1] http://www.greenhouse.cornell.edu/crops/factsheets/AmmoniumToxicity.pdf
[2] http://fieldcrop.msu.edu/uploads/documents/Nitrogen%20losses%20from%20soil.pdf
[3] http://www.fao.org/DOCReP/003/T0234e/T0234E03.htm
[4] http://www.fao.org/docrep/x5871e/x5871e04.htm
[5] "Environental Soil Chemistry" 2nd Ed by Donald L Sparks, from 2003
[6] "Principles of Soil Chemistry" 3rd edition by Kim H Tan, from 1998 (not used)
[7] http://www.biologydiscussion.com/plants/absorption-of-water/absorption-of-water-in-plants-with-diagram/22718
[8] "Media and Mixes for container grown plants" by A C Bunt, from 1988

POST #5:
[I got confirmation I understood it correctly. And my post is essentially the same as my "summary" above. Mainly I discussed this with a guy from the Indian Institute of Soil Science.]


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