# Is this how CEC works?



## awohld (Dec 11, 2005)

As far as I understand, CEC is a measure of the ability of a substrate to hold nutrients and make them available to the roots.

I'm looking at getting Profile aka Schultz Aquatic Plant soil.

If profile has a high CEC, does this mean it'll absorb Nitrogen and store it for plants?

If the above statement is true, can I load it by soaking it in fertilizer?

Will it absorb nitrogen out of the water column?


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## troy_h (Jul 12, 2005)

You could load it to some extent by soaking it, but there would also be an initial release of nutrients. Soaking it as opposed to adding few srategically placed root tabs until the bed has matured would offer no real benefit that I can discern. At least with the product I used to sell at the store, which is pretty much the same material as Profile, Turface etc, it's ability to store nutrients fed to it in a burst is minimal and pretty much limited to what it can store in it's surface pores, wax on wax off.


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## awohld (Dec 11, 2005)

Will profile grap Nitrogen out of the water column and save it for the plants?


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## plantbrain (Jan 23, 2004)

Yes, and it'll also release it back, you can add some, but it will not last forever without adding N back.

Regards
Tom Barr

www.BarrReport.com


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## troy_h (Jul 12, 2005)

I'm under the understanding that the nitrogen available in substrate comes predominantly from the breakdown of detritus within the substrate as opposed to extracting it from the water column, is this incorrect?


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## plantbrain (Jan 23, 2004)

In the long term, you are quite correct.
But other sources such as NO3/NH4 from waste etc can be directly added through simple diffusion down there also. So the Organic breakdown and the inorganic sources both are used in the water column as well a the root zone.

Regards, 
Tom Barr


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## kennyman (Oct 19, 2005)

Cation Exchange Capacity. 

This relies on the negative charge of the media to attract the positively charged cations. N is a macro that is negatively charged and therefore does not bond.


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## awohld (Dec 11, 2005)

kennyman said:


> Cation Exchange Capacity.
> 
> This relies on the negative charge of the media to attract the positively charged cations. N is a macro that is negatively charged and therefore does not bond.


So does that mean Nitrogen doesn't bond? If not how do you tell what bonds?


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## DelawareJim (Nov 15, 2005)

Cation exchange capacity (CEC) is the total of exchangeable cations that a soil can adsorb (the attraction of ions or compounds to the surface of a solid.). The CEC is expressed in milliequivalents per 100 grams of soil or other adsorbing material like clay/laterite. This is part of why laterite was used in Dupla systems. The laterite additive increased the CEC of the gravel which is generally pretty poor.

A high CEC is good because more cations ar available for root uptake. CEC increases with increasing pH. As pH is raised, the hydrogen held becomes ionized and becomes replaceable by a plant nutrient cation. Also, any adsorbed aluminium hydroxy ions (which are very strong cations and toxic to plants) are removed increasing the number of cation exchange sites making more nutrient available. So, in short, increasing the pH increases the number of nutirent exchange sites, and makes more nutrient avaialble at each site.

So, for those who calculate their CO2 concentrations using the pH/KH table, people who have the target CO2 concentrations with low pH and KH should probably increase their pH and KH levels to increase nutrient avalability to the roots.

Cheers.
Jim


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## plantbrain (Jan 23, 2004)

kennyman said:


> Cation Exchange Capacity.
> 
> This relies on the negative charge of the media to attract the positively charged cations. N is a macro that is negatively charged and therefore does not bond.


Ummm, what about NH4+???
Is that a cation or not?

Regards, 
Tom Barr


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## plantbrain (Jan 23, 2004)

DelawareJim said:


> A high CEC is good because more cations ar available for root uptake. CEC increases with increasing pH. As pH is raised, the hydrogen held becomes ionized and becomes replaceable by a plant nutrient cation. Also, any adsorbed aluminium hydroxy ions (which are very strong cations and toxic to plants) are removed increasing the number of cation exchange sites making more nutrient available. So, in short, increasing the pH increases the number of nutirent exchange sites, and makes more nutrient avaialble at each site.
> 
> So, for those who calculate their CO2 concentrations using the pH/KH table, people who have the target CO2 concentrations with low pH and KH should probably increase their pH and KH levels to increase nutrient avalability to the roots.
> 
> ...


I don't know about these things you mention.
As the pH increases, the oxidized forms become more prevalent, and less bioavailable in form.

There is a trade off there.

Fe3+ vs Fe2+ for example. 
I'm not sure high CEC is really that great either.
We have the various products and their known CEC's and we see little variation in terms of long term growth. 
Jamie Johnson's CEC anaylsis showed this.

Al toxicty is non issue in our tanks as well.

If you want to play with the pH issue som, consider turning the CO2 off at night and seeing if using the pH controller is really worth while.

I do not think you'll find much significance in terms of CEC , you will find significance in terms of actually NPK or the soils/sediments etc, CEC does play a role certainly, but it's not quite what many seem to believe.

You need to gab the nutrient, then bind it and have the roots remove it, ten have new nutrients attach and bind and repeat.

If you think about CEC/plant roots like a magnet, sometimes that helps conceptually. It's a form of short to moderate term nutrient storage, but unless something removes the nutrient and then is brought in to replace the missing nutrient, and if the water column is limited, then it can play a role.

We generally add all the nutrients into the water column in most plant tanks, and all plant tanks after a few month/a year unless you replace the substrate/add jobes etc.

Regards, 
Tom Barr

www.BarrReport.com


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## DelawareJim (Nov 15, 2005)

Also since most plants, terrestrial and aquatic, are able to uptake N03- and NH4+, N is always available in a useable form.

Actually, since N is available for uptake as either a weak cation (+), or weak anion (-), it is available in the widest pH range of any of the nutrients (pH 4.5 - 8.5) in a water soluable form and is rarely considered in a substrates CEC.

Cheers.
Jim


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## DelawareJim (Nov 15, 2005)

Tom;

What you mentioned above is the quandry that I wrestle with constantly.

Although I remember quite a bit of my college plant nutrition and soil science, and did a bit of research in that area, the principles I learned were to get the nutrients into the most readily available water soluable form at the least cost for terrestrial plants.

Since were growing aquatic plants, CEC becomes less (non) important.

Also, since aquatic plants do not have the economic impact as say tomatoes in the internation market, there have been significantly fewer research dollars directed our way. And I'm sure, that any nutrition research being conducted by the likes of Tropica et. al., are strictly propriatary in nature as a leg up on the competition.

So, since none of us have tissue sampling labs in our basements for determining nutrient uptake by the plants each of us have in our tanks, I'm at as much of a loss as to what's really happening in my tank as the next person.

Cheers.
Jim


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## Phil Edwards (Jan 22, 2004)

As far as I know CEC is pretty much useless in our tanks. CEC is really only important in environments where cations can be leached out of the root zone.  In a confined soil/substrate environment where water isn't dissolving and leaching nutrients out of the system it's a non-issue. 

What's most important is providing a substrate with good porosity and surface area. If the plants have enough surfaces to attach root hairs to and the substrate is porous enough so that it doesn't compact and go anaerobic you've got a perfect hydroponic medium. 

Regards,
Phil


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## SCMurphy (Jan 28, 2004)

This is where you guys forget about the rhyzosphere that plants create by pushing O2 into the roots, where some O2 leaks into the surrounding interstitial water of the substrate. The daily change from anoxic to oxic conditions combined with the bacteria in the substrate work to change the bioavailability of nutrients in the substrate. What was locked becomes oxidized, or is released by bacteria, then can be reduced, then taken up by the plants, or used by bacteria, or captured by the CEC component of the substrate until needed, or oxidized. 

The rhyzosphere is something you can see around the roots of plants in a soil tank by looking at the bottom of the aquarium during the day. There is a zone around roots where the substrate will change colors. In my tanks its a lighter color where the substrate is oxic and darker where it is anoxic.


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## kennyman (Oct 19, 2005)

plantbrain said:


> Ummm, what about NH4+???
> Is that a cation or not?
> 
> Regards,
> Tom Barr


Good point! They never covered that in Agriculture college 

I also wonder about the whole root zone issue in tanks. As I remember it is a two way street with many things going on like fungal symbiosis. Does that play any role in the aquatic world?


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## plantbrain (Jan 23, 2004)

There are qute a few things progressing with fungal associations in aquatic macrophytes, it was previously assumed incorrectly that they are rare and play little role.

I went to a confernce on wetlands about a year ago on fungal associations.
There are many species that use it.



DelawareJim said:


> Tom;
> 
> Also, since aquatic plants do not have the economic impact as say tomatoes in the internation market, there have been significantly fewer research dollars directed our way. And I'm sure, that any nutrition research being conducted by the likes of Tropica et. al., are strictly propriatary in nature as a leg up on the competition.


True, but rice is the world's largest ag crop
Each plant species acts different. But you can get some insight and see if it works in practical application. Often it does.



> So, since none of us have tissue sampling labs in our basements for determining nutrient uptake by the plants each of us have in our tanks, I'm at as much of a loss as to what's really happening in my tank as the next person.
> 
> Cheers.
> Jim


I'd not fret about the CEC issue. Vac you gravel once a year or so in stages, about 1/4 to 1/3 at a time.
Sean also mentions that plants actively alter their environment, they are NOT passive.

All that O2 drives fungi, drives much higher rates of mineralization, prevents issues when replanting of pulmes of reduced compounds that drop O2 levels way down very fast and bring up NH4.

Regards, 
Tom Barr

www.BarrReport.com


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