What is the CEC (Cation Exchange Capacity)?

[andy10917]

One of the questions that I get asked the most when people get back their soil test results is "What is this CEC Number?". CEC stands for Cation Exchange Capacity, and it is a quick way for someone that is reviewing the soil test results to get a feel for how the soil will react to the addition (and depletion) of nutrients.

Let's get over the terminology first. "Cations" are the positively-charged things that occur when you add things to your soil. For example, when you add Lime (mostly Calcium Carbonate, or CaCO₃) to the soil, the water in the soil splits the Lime into Ca⁺⁺ (Calcium) and Carbonate (CO3^--) pieces, or "ions". Anything that has one or more "+" in it is a Cation. Soil particles are negatively charged, so there is an attraction between them. We say that they "bind" to the soil particles. They are held there until a plant (like your grass) picks it up through the plant's roots. The ability of the soil to hold nutrients is called it's "Exchange Capacity".

While you may not think of it this way, clay particles may be thousands of times smaller than the sand grains in your soil. Silt particles are in the middle - bigger than clay, but smaller than sand. If each particle can bind one ion (not exactly true, but close enough for this explanation), then there are thousands of more opportunities for clay to hold cations than there are for sand - within the same space of course.

So, you'd think we'd want a lot of clay, right? Not exactly. While clay can hold nutrients just fine, the tininess of the particles mean that they are so close together that it lacks space for water and air to move into the soil. The good microbes are "aerobic", meaning that they require Oxygen to do their jobs. Just like water puddles because it can't move through the clay, Oxygen can't move well either. Clay soils can hold tons of nutrients, but they can't get converted to usable products by the microbes. That combined with the physical work that grass must do to push roots through a clay soil makes for tough sledding for the grass plants.

So, what do we want? We want a mix of sand, silt and clay so that nutrients get held but Oxygen and water move freely. I personally like soils with CEC's between 8 and 20, and even the extremes of those numbers require adjustments in nutrient regimens. I also always look at the Organic Material percentage when I'm looking at a CEC number - it tells you if OM can be used to help the number or if a high CEC is coming completely from clay or not.

So now you're thinking that you can affect a not-so-good CEC with lots of OM coming from compost, right? Don't race off to the compost heap quite yet. Here's the real skinny on that one:

Each percent of OM in 1,000 square feet of soil equates to 450 lbs of organic material. Compost is normally about 40-50% OM after you remove the water it holds from the equation. Compost weighs about 1000 lbs per cubic yard. So, it will two cubic yards of compost per 1000 square feet to raise the CEC one point (2000 lbs - 1000 lbs of water * 45% average OM). That's a LOT of compost - way more than you'd ever dare put down at one time. Also, too much compost at one time can mat, and cause Oxygen deficiencies in the soil. This isn't knocking compost at all - it has many other uses beyond CEC management.

What does all of this mean when you consider the soil's CEC? It means that you should treat the soil's CEC as it's semi-permanent "temperament". You're only going to change it slowly. If your soil is very sandy (low CEC), you're going to need to feed it like a baby - small amounts and often. If it's clay, more like a stubborn old miser - it holds on to everything and gives up nothing. If you have a loamy soil, thanks the lawn gods - you have a healthy teenage boy that can eat large quantities of food and can do anything physically (although real teenagers don't actually demonstrate the "do anything" part much).

A longer Article is being written on this subject, including the interpretation of specific CEC values...