Cation Exchange Capacity: What exactly?

Here's a few things to ponder perhaps.

CeC - the degree that a soil can absorb and exchange cations.

Cation - a positively charged ion and in the case of soils, it refers to the basic cations, i.e. (NH4+, K+, Ca2+, Fe2+, etc...)

Anion - a negatively charged ion and again in the case of soils, it refers to (NO3-, PO42-, SO42-, etc...)

Soil particles and organic matter contain negative charges on their surfaces. Mineral cations can adsorb to the negative surface charges or the inorganic and organic soil particles. Once the minerals are adsorbed they're not easily lost when the soil is leached by water and these minerals provide a nutrient release available to the plant roots via the soil foodweb. Minerals can be replaced or 'exchanged' by other cations, hence 'cation exchange'

NOTE: Adsorption is the process of attraction of atoms or molecules (generically known as "monomers") from an adjacent gas or liquid to an exposed solid surface. Such attraction forces (adhesion or cohesion) align the monomers into layers ("films") onto the existent surface. [cite]

The amount of these positively charged cations a soil can hold is described as the CEC and is expressed in milliequivalents per 100 grams (meq/100g) of soil. The larger this number, the more cations the soil can hold. A clay soil will have a larger CEC than a sandy soil.

The soil texture and organic matter content level plays the major role in a soil's overall CeC. Generally, the more clay and organic matter in the soil, the higher the CeC. Clay content is important because these small particles have a high ration of surface area to volume. Different clays also vary in CeC.

Smectites have the highest CEC (80-100 millequivalents 100 g-1), followed by illites (15-40 meq 100 g-1) and kaolinites (3-15 meq 100 g-1).

Base Saturation

The proportion of CEC satisfied by basic cations (Ca, Mg, K, and Na) is termed percentage base saturation (BS%). This property is inversely related to soil acidity. As the Base Saturation% increases, the pH increases. The availability of nutrient cations such as Ca, Mg, and K to plants increases with increasing Base Saturation%.

Base saturation is usually close to 100% in arid region soils.

Base saturation below 100% indicates that part of the CEC is occupied by hydrogen and/or aluminum ions. Base saturation above 100% indicates that soluble salts or lime may be present.

Can you say 'Cal-Mag' lockout? I thought you could! LOL

CEC and Availability of Nutrients

Exchangeable cations may become available to plants. Plant roots also possess cation exchange capacity. Hydrogen ions from the root hairs and microorganisms may replace nutrient cations from the exchange complex on soil colloids. The nutrient cations are then released into the soil solution where they can be taken up by the adsorptive surfaces of roots and soil organisms. They may however, be lost from the system by drainage water.

Additionally, high levels of one nutrient may influence uptake of another (antagonistic relationship). K uptake by plants is limited by high levels of Ca in some soils. High levels of K can in turn, limit Mg uptake even if Mg levels in soil are high.

Anion Exchange

In contrast to CEC, AEC is the degree to which a soil can adsorb and exchange anions. AEC increases as soil pH decreases. The pH of most productive soils in North America is usually too high (exceptions are volcanic soils & most rock dusts) for full development of AEC and thus it generally plays a minor role in supplying plants with anions.

Because the AEC of most agricultural soils is small compared to their CEC, mineral anions such as nitrate (NO3- and Cl-) are repelled by the negative charge on soil colloids. These ions remain mobile in the soil solution and thus are susceptible to leaching.

Flush it away!

There's a smidgen on CeC as it relates to soils.

HTH

LD

 

I yield to LD as he is the one who brought the whole topic of CEC to our awareness here. "Light" was brought forth into "Darkness". LD is the resident subject matter expert on this topic.

Since LD initially brought the topic up it has grabbed me pretty thoroughly and convinced me that without this one not-so-simple basic understanding of soil fertility a lot of plant management is going to be required throughout the life-cycle; nutrient management, pH issues, overall plant health and vigor, etc. So, howmanymoreyrs if you, or anyone for that matter, is pursuing soil grows you have to really dial into the concept and the science of what is going on inside the soil. I believe that from this base point of understanding of soil science ALL other topics of soil horticulture spring forward.

That said, I've borrowed a paragraph from a document published by the Maryland Cooprerative Extension that provides an analogy of sorts for grasping the bigger picture of CEC. Perhaps this will be helpful for you. Science aside, it's a "simple" concept to grasp.

"The CEC of a soil might be compared to the size of a fuel tank on a gasoline engine. The larger the fuel tank, the longer the engine can operate and the more it can do before a refill is necessary. For soils, the larger the CEC, the more nutrients the soil can supply. Although CEC is only one component of soil fertility, all other factors being equal, the higher the CEC, the higher the potential yield of that soil before nutrients must be replenished with fertilizers or manures."


Soil CEC is the genesis of what follows in the plant life-cycle. It's the Alpha, if you will, of where to start.

 

howmanymoreyrs

Pretty simple actually in the whole scheme of things.................

1. Source the best/finest humus that you can. EWC, thermal compost, leaf mold, humus, et al. This will give you the tools to work with.

2. Application of AACT in conjunction with BOTH humic acids and chelated minerals, particularly these specific ones: sulfur, cobalt, boron, molybdenum, manganese, copper, iron and zinc.

3. Avoid any/all advice from a grow store employee

If you think that the CeC is interesting, check out the book "Teaming With Microbes" and see how a diverse aerobic microherd in the soil interacts with the minerals that you've added and how that whole process works.

One note - while the "Teaming" book is a great resource, you can pretty much avoid the entire chapter on 'Compost Teas' - 'nuff said.

HTH

LD

 

LOL

I remember going out to Tahquitz Canyon 40 years ago with friends and we would take several 5-gallon bottles of Arrowhead or Sparklets water and eat nothing but 'magic mushrooms' and peyote buttons for several days to relieve psychic boredom. Pretty intense by Day 3 forward, eh?

At first I thought that I 'saw God' but as things turned out it was simply the Moody Blues - 'Gypsy on a Dream' and 'Timothy Leary's Dead' era.

Thanks for the memories!

Wee!

LD

 

Thanks Lumpah,

This post on some soil chemistry is like a goodnight story for me....."and the hydronium ion and the hydroxide ion lived happily ever after"

But seriously, thanks for posting this, +rep.

 

Perfect and priceless! I kinda sorta have a vision how all of that must have gone down! LOL! Those purple cap mushrooms....Wowie indeed!

 

Sorry, To revive another long gone thread.. but I've been reading.

I Have read before that azomite contains a high amount of aluminum up to 20-30% I heard it quoted once. Would the aluminum effect the cec in a negative way?

I have also heard there can be to high of a CeC?

Wiki CeC

Azomite Faq