You are correct Mr. David CEC management is important.

No one? I already asked 3 horticulturists but they all seem to have skipped CEC class..

I have now 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.

I still hope to find a clear answer to my 2nd question: To what extent does CEC apply to monovalent cations such as NH4+ and K+ ? Most sources seem to indicate CEC binds these fully, while Bunt seems to say they ar fully mobile and leach easily?

You are correct Mr. David CEC management is important.

Cation exchange capacity(CEC) of materials used in pot culture/potted studies is, as such, not a problem.Its major benefit is its nutrient ( cationic) holding power, higher the CEC of the medium used in pot , greater is the nutrient cation holding power. CEC is due to humified organic matter (humus) and clay minerals present in the materials in the medium.The cations Ca,Mg,K ,NH4 and Na etc are common cations contained in soil or irrigation water.Divalent cations are held more strongly than monovalent cations on exchange sites . pH of the medium ,concentration of nutreints (ions) in solution and hydrated size of the ion decides its placing power. As long as the cation exchange sites are unsaturated,the cations are attracted and adsorbed on exchange sites.This facilitates low ionic concentration in solution phase and so no much build up of salinity .As the the exchange sites get saturated,the solution concentration of cations builds up.The cations like Na in association with anions like chloride and sulphates builds up more salinity.Apart fron the ions in nutrient solution,irrigation water also adds the ions which contribute to salinity (as reflected in high EC).Cations present on exchange sites are not leached unless replaced by ions of higher charge and sufficient concentration are not build up in solution phase. Divalent ion Ca can easily replace K,NH4 ,Na etc .But at higher concentration in solution the monovalent cations can replace divalent cation .So the CEC of the medium , its ionic composition, cations supplied through nutrient solution and water composition (quality) and number of irrigations given decide the salt build up and the measured EC.

Thanks for the replies.

I am looking for clues on the specific topic of how easily monovalent cations like K+ and NH4+ are leached.

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?

Mr.David Strand,I appreciate your questions.There are two aspects in the discussion- first ,nutrient adsorption and retention on cation exchange site/ complex (nutrients are conserved against loss by leaching) and second,the release and contribution to nutrient uptake ,leaching and salt build up.High CEC contributed by peat,fine mineral particles/vermiculite etc help to retain the nutrient ions in adsorbed/ exchangeable form.The nutrient ions are exchanged with other cations in solution or desorbed due to dilution with water and will be available to plant uptake.This is beneficial aspect of CEC.Irrigation with water containg both cations and anions will accumulate in soil leading to salt build up , increasing EC.The dominant cation accumulated in the container medium can replace the cation held on exchange complex. Drainage water can leach this replaced cation from exchange sites/complex.One of the main reasons for accumulation of salt in container medium is improper drainge. As mentioned in the attached article,peat-sand mixtures are more leachable than peat substrates made with finer loam.More fine fraction or vermiculite may contribute to more CEC but incomplete leaching leading to salt accumulation. So providing proper drainage (through both from macro and micropores) is more important to reulate the salt build up.The composition of water used for leaching is also imporant . Na among cations and Cl and SO4 ions in water used for leaching lead to more saline environment in the pot medium (Osmotic effect and interfere in plant water uptake). Carbonates and bicarbonates in excess of Ca and Mg in leaching water may lead to excess build up of sodium carbonate/bicarbonate and sodicity in the medium.Please ask futher questions if I am not clear.

Thanks but as you can see even in my initial post, I am trying to understand how salinity is connected to CEC. 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'tmove 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

Mr.David Strand, I appreciate your interest in reading of many books and university publications concerning your field of specialization/interest.I appreciate your analytical points (3) which are creating confusion and four points which clarify that CEC is not a factor in salt build up and leaching requirement. As you may understand now, the CEC has positive role in cation holding and release , as it can partly remove the cations added through irrigation or fetilizers from solution phase (depending on exchange sites available/permitted for occupation) and avoids salt build up to that extent .In salt build up one may be more interested in cations like Na, K and NH4 and anions Cl and SO4 in irrigation water and added those through fertilizers and manures.As mentioned by you in your third point from Bunt's Book (I did not read this book),if the CEC is predominatly saturated with Ca and Mg, there will be less scope for monovalent cations to go and occupy the exchange sites.But if you apply NH4 salts as fertilizers and if sufficient NH4 ion concentration builds up in solution phase ,the NH4 ions can replace some quantity of Ca and Mg and occpuy exchange sites.Same is the case with K ions from fertilizers.Predominantly the NH4 and K ions (also Na) will be in solution phase and can be leached easily.Relatively porous medium containig peat moss,bark and sand etc. may not interfere with leaching.Only fine particles of clay/vermiculite etc if present in the medium in more amount may create problem in leaching ions especially from micropore .So as I understand it is the concentration of cations in irrigation water, added fertilizers and drainage/permeability of the medium used and the quality of water used for leaching may decide the extent of salt build up in the pot and consequent high EC measured.

Thanks, then I feel that I understand this.

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.

It is nice to actually understand things, so that one can make good decisions based on reasoning, and not just mimic whatever advice one hears :-)