Agriculture Reference
In-Depth Information
Cation Exchange Capacity (CEC)
A soil test will include a value for CEC. The more clay and organic matter, the higher
this number will be. Here are some recommended numbers, although they will vary de-
pending on who you ask. Sandy soils low in organic matter will have a CEC between 1-10.
They have very little ability to hold onto cations. The 20-25 range is getting to be decent,
but 25-40 is ideal and would probably mean you have good organic matter and a decent
amount of clay. Above 40 is high clay and/ or organic soil that may include a lot of material
such as peat. Pure organic soils, such as a peat bog, go as high as 100 CEC.
The CEC value on a soil test isn't always particularly accurate. A better indicator of
CEC may even be the simple ribbon test. Take half a cup or so of your soil, make sure it's
moist, and squeeze it into a ball. Form it into a ribbon. If you can't do that, your CEC is
low (sandy soil). If you can form it into a long ribbon, your CEC is high (clay). A simple
test, but potentially more effective.
A lower CEC soil that has a lot of sand needs to be fertilized and probably watered more
often, because it can't hold onto either for very long. On the up side, it's easier to balance
the nutrient ratios since a smaller amount of fertilizers added can make a big difference to
the small amount of nutrients already there. A higher CEC soil that has a lot of clay and/or
organic matter doesn't need much fertilizer or water, but if the nutrient ratios are out of bal-
ance, it can take a lot of fertilizer to correct them.
