Hi JustinL,

Your post:
Cation Exchange Capacity in Soils, Simplified
(so that even I can understand it.)

©2007 by Michael Astera
All rights reserved
July 2007

Revised Oct.31, 2008
Adsorb vs Absorb

adsorb (ad sôrb, -zôrb), v.t. Physical Chem. to gather (a gas, liquid, or dissolved substance) on a surface in a condensed layer: Charcoal will adsorb gases .

Please note the definition above, taken from my handy dictionary, flower press, and child booster seat, the real hardbound Random House second edition unabridged. It's not absorb, it's adsorb , with a "d". We all know that a sponge absorbs water, a cast iron pot absorbs heat, a flat-black wall absorbs light. None of those gathers anything on the surface in a condensed layer, they soak it right in, they absorb it.

Adsorb is different, because it means to gather on a surface in a condensed layer . This is pretty much the same thing as static cling, like when you take a synthetic fabric shirt out of the clothes dryer and it wants to stick to you. You don't absorb the nylon blouse, you adsorb it. Everyone got that? Good. On to Cation Exchange Capacity.

The Exchange Capacity of your soil is a measure of its ability to hold and release various elements and compounds. We are mostly concerned with the soil's ability to hold and release plant nutrients, obviously. Specifically here today, we are concerned with the soil's ability to hold and release positively charged nutrients. Something that has a positive (+) charge is called a cation, pronounced cat-eye-on. If it has a negative charge (-) it is called an anion, pronounced ann-eye-on. (Both words are accented on the first syllable.) The word "ion" simply means a charged particle; a positive charge is attracted to a negative charge and vice-versa.

Positively charged particles are known as cations. There are two types of cations, acidic or acid-forming cations, and basic, or alkaline-forming cations. The Hydrogen cation H+ and the Aluminum cation Al+++ are acid-forming. Niether are plant nutrients. A soil with high levels of H+ or Al+++ is an acid soil, with a low pH.

The positively charged nutrients that we are mainly concerned with here are Calcium, Magnesium, Potassium and Sodium. These are all alkaline cations, also called basic cations or bases. Both types of cations may be adsorbed onto either a clay particle or soil organic matter (SOM). All of the nutrients in the soil need to be held there somehow, or they will just wash away when you water the garden or get a good rainstorm. Clay particles almost always have a negative (-) charge, so they attract and hold positively (+) charged nutrients and non-nutrients. Soil organic matter (SOM) has both positive and negative charges, so it can hold on to both cations and anions.

Both the clay particles and the organic matter have negatively charged sites that attract and hold positively charged particles. Cation Exchange Capacity is the measure of how many negatively-charged sites are available in your soil.

The Cation Exchange Capacity of your soil could be likened to a bucket: some soils are like a big bucket (high CEC), some are like a small bucket (low CEC). Generally speaking, a sandy soil with little organic matter will have a very low CEC while a clay soil with a lot of organic matter (as humus) will have a high CEC. Organic matter (as humus) always has a high CEC; with clay soils, it depends on the type of clay.

Base Saturation %

From the 1920s to the late 1940s, a great and largely un-sung hero of agriculture, Dr. William Albrecht, did a lot of experimenting with different ratios of nutrient cations, the Calcium, Magnesium, Potassium and Sodium mentioned above. He and his associates, working at the University of Missouri Agricultural Experiment Station, came to the conclusion that the strongest, healthiest, and most nutritious crops were grown in a soil where the soil's CEC was saturated to about 65% Calcium, 15% Magnesium, 4% Potassium, and 1% to 5% Sodium. (No, they don't add to 100%; we'll get to that.) This ratio not only provided luxury levels of these nutrients to the crop and to the soil life, but it strongly affected the soil texture and pH.

The percentage of the CEC that a particular cation occupies is also known as the base saturation percentage, or percent of base saturation, so another way of describing Albrecht's ideal ratio is that you want 65% base saturation of Calcium, 15% base saturation of Magnesium etc. Don't get too hung up on these percentages; they are general guidelines and can vary quite a bit depending on soil texture and other factors.

It's still a little-known fact that the Calcium to Magnesium ratio determines how tight or loose a soil is. The more Calcium a soil has, the looser it is; the more Magnesium, the tighter it is, up to a point. Other things being equal, a high Calcium soil will have more oxygen, drain more freely, and support more aerobic breakdown of organic matter, while a high Magnesium soil will have less oxygen, tend to drain slowly, and organic matter will break down poorly if at all. In a soil with Magnesium higher than Calcium, organic matter may ferment and produce alcohol and even formaldehyde, both of which are preservatives. If you till up last years cornstalks and they are still shiny and green, you likely have a soil with an inverted Calcium/Magnesium ratio. On the other hand, if you get the Calcium level too high, the soil will lose all its beneficial granulation and structure and the too-high Calcium will interfere with the availability of other nutrients. If you get them just right for your particular soil, you can drive over the garden and not have a problem with soil compaction.

Because Calcium tends to loosen soil and Magnesium tightens it, in a heavy clay soil you may want 70% Calcium and 10% Magnesium; in a loose sandy soil 60% Ca and 20% Mg might be better because it will tighten up the soil and improve water retention. If together they add to 80%, with about 4% Potassium and 1-3% Sodium, that leaves 12-15% of the exchange capacity free for other elements, and an interesting thing happens. 4 or 5% of that CEC will be filled with other bases such as Copper and Zinc, Iron and Manganese, and the remainder will be occupied by exchangeable Hydrogen , H+. The pH of the soil will automatically stabilize at around 6.4 , which is the "perfect soil pH" not only for organic/biological agriculture, but is also the ideal pH of sap in a healthy plant, and the pH of saliva and urine in a healthy human.

So we are looking at two new things so far:

1) The Cation Exchange Capacity, and

2) The proportion of those cations in relation to each other: the percent of base saturation (% base saturation) and their effect on pH.

We are also looking at two old familiar things, clay and soil organic matter, and these last two need a bit more clarification.

http://www.soilminerals.com/index.htm


Answer: Again not everything may be as it may 1st seem to be?

One could measure out each Mineral by weight and mix it together in the supposed ratios discussed above and Yet not get Top Quality production?

Minerals are still Minerals and I do not know of any plant that eats Minerals?

Once again Plants may be like people in that they Live and grow from the Electro-magnetic Energy Released from the needed Minerals !

So we then need to ask ourself How can the plants have a Steady food supply of this needed Electro-magnetic Energy?

Once again: Soil Life !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Without Soil Life our soils will not have Organic Matter, which means they will not have Moisture control or Temperature contol or a Steady Flow of needed Electro-magnetic Energy to help feed the plants?

Soil Life that is alive and working properly may supply a steady supply of Complexed Acids to react against these Raw Minerals to cause the needed Release of Electro-magnetic Energy needed to help the plants grow?

This brings in another question?

Where does the plant take most of it's growth from: The Air or the Soil?

Hmmmmmmmmmmmmmmmmmmmmmmm?

Actually it is the Air that may provide most of the Plant material in the form of Carbon from Carbon Doixide, Hydrogen and Oxygen !

Carbon, Hydrogen, Oxygen make Carbohydrates, of which make up the major part of Plants and this comes from the Air !

And the supposed Environmentalist are Claiming we don't want Carbon Dioxide?

Hmmmmmmmmmmmmmmmm?

The Plants cannot Live without Carbon Dioxide in the Air!

I wonder what other Crazy ideas the Worldly Educated who may have been Educated beyond their own Intelligence will come up with?

Carbon Dioxide which may form Carbonic Acid is not the problem ! How else do we supposed Carbonate Rock deposits could be formed without Carbonic Acid?

The problem may be the Excess Nitrogen fertilizers and Pesticides and Herbicides and Fungicides and the Rest of the Industrial Poisons dumped into the water ways that finds it's way to the Oceans that may be the problem !

As usual not everything may be as it may 1st seem to be !

Cause and Effect !

Smile Tis your choice.