Agriculture Reference
In-Depth Information
Cation Exchange Capacity (CEC)
A positively charged mineral in the soil is called a cation, pronounced “cat-eye-on” with
the emphasis on the cat. Years ago when I read about cations from a topic for the first time,
I spent a year walking around praising cations (as if it rhymed with passions), before I
figured out how it was supposed to be pronounced.
The cation exchange capacity of the soil basically refers to the ability of the soil to hold
onto these positively charged nutrients like calcium, magnesium and potassium so they
don't leach out. Since a majority of minerals in the soil are cations, this is important.
The use of the word “exchange” actually advances the definition a little bit because it
refers to the ability of the soil to attract, hold and release these nutrients. The people who
name things put that in there to show that it's an ever-changing, dynamic process, which is
important to note. Nutrients are always getting bumped off the soil molecule. Some are
taken up by plants, some are leached further down, and some attach somewhere else. I
guess if the soil held onto the minerals, the plants would get rather grumpy.
In fact, while hydrogen isn't considered a plant nutrient, microbes and plant roots use
hydrogen cations to trade for other cations sitting on a cation exchange site. Roots do this
by excreting carbon dioxide, which combines with water to form carbonic acid and hydro-
gen. Some of the hydrogen knocks other cations off the exchange sites, which the plant can
then take up. Brilliant. I prefer just to think of CEC as how good my soil is at retaining pos-
itively charged nutrients. Note that CEC doesn't tell me how many nutrients I have, just
how good it is at retaining and exchanging what is there. More on that soon.
As we discussed in the last section, the sand and silt don't help at all with CEC because
they don't have a charge. It's all about the clay, and specifically what kind of clay you
have. Some clays are better at holding onto cations than other clays. Gardeners have no
way of knowing what kind of clay you have other than researching the local soil reports or
asking local soil labs, but a soil test tells our CEC — an important number.
Pure clay can have a CEC anywhere between 0 and 150, with most non-tropical clays,
such as those in North America and Europe, falling in the 25-100 range. Interestingly, hu-
mus which is organic matter broken down into its most stable form, has a CEC of 100-300.
That's great news because we can therefore immensely improve the ability of our soil to
hold onto cations by building up the humus in our soil, which will be beneficial to almost
all soils — but most notably for low-clay soils.
By the way, if you're a math or science buff, you might like to know that the unit of
measurement for CEC is milliequivalents per 100 grams of soil (mEq/100g), but it's not
really important for our purposes.
