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Cation exchange capacity

Cation exchange capacityCation exchange capacity is the first characteristic you should consider when you get your soil test results.  Cation exchange capacity (CEC for short) is closely tied to the amount of organic matter and clay you have in your soil since both provide spots for positively charged ions --- cations --- to cling to the soil.  In contrast, sandy soil without much organic matter will allow nutrients to leach away during heavy rains.  You're throwing away your money if you add soil supplements to raise your calcium, magnesium, or potassium levels without first increasing your CEC so that these essential nutrients will be held in place.

So what's a good CEC?  CEC can range from 0 to 100 meq/100 g, and your goal should be to reach or exceed 20 meq/100 g.  Although clay and any kind of organic matter will help you achieve this goal, humus is the most effective since it provides a lot more cation binding sites per unit area.  In case you're not familiar with the distinction, humus is organic matter that has broken down to a stable point at which it may endure for hundreds or thousands of years.  To make humus, add any kind of organic matter to your soil (compost, mulch, or cover crops) and make sure soil conditions are right for earthworms, bacteria, and fungi to turn that organic matter into high quality humus.

Let's take a look at my CEC and organic matter test results:


Mule (1) Mule (2) CP3 (3) Back (4) Front (5) Mom front Mom back
CEC 65.6 74.4 15.6 56 47.1 27.9 36.3
% OM 17 18.4 8.2 15 14.6 15.9 14.1

I've highlighted the one non-garden spot I sampled --- our chicken pasture, which was basically a lawn until we started letting chickens graze there this spring.  I also sampled some of my Mom's soil, which has been intensively gardened for decades.  Finally, the white columns are four different parts of my own vegetable garden.  By comparing these three areas, you can see:
  • Soil samplesWithin my garden, CEC values increase as percent organic matter increases.  Mark and I were both able to line up our soil samples from most to least organic matter by eye, so it would be possible to keep rough track of this information without sending off soil samples.
  • Mom's CEC values are lower than mine even though her soil has just about as much organic matter.  That's the difference between gardening in very clayey soil (my garden) and in a silty loam (her soil.)
  • My pasture --- where I've never added compost --- is the only area with a CEC below the 20 meq/100 g level.  As I use more intensive management techniques on this pasture, I hope to raise the CEC without adding any soil amendments.

Most organic gardeners believe you can't have too much organic matter in your soil, but soil scientists will roll their eyes at that statement and now I understand why.  If your CEC is low but your organic matter levels are high, that means you're doing something wrong and your organic matter isn't being broken down into a stable humus.  Maybe you've added too much high carbon material all at once (for example, tilling wood chips into the soil) or have sprayed pesticides that killed off your soil microorganisms.  That's why I recommend using CEC rather than organic matter as the most important "grade" on your holistic gardening "report card."  As far as I can tell, with CEC, more is always better.

Our chicken waterer never spills or fills with poop.


This post is part of our Holistic Soil Test Analysis lunchtime series.  Read all of the entries:





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