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Little River, TX | Keith has a very good explanation. A number of Labs pay lip service to CEC but make no adjustments in their thinking for a high soil.
For 10 or more years I used the computed CEC values to compute the desired levels of cations reported in the soil test. For simplicity sake any CEC that is below 20 meq/100g you can use the standard levels of K, Mg, Ca, reported by the labs as gospel. Above 20 CEC things become a little more complicated. Most labs refuse to consider a CEC above 30 meq/100g. In general I have two clay soil types. One averages in the 40 meq/100g range and the other in the 50 meq/100g range.
A computed CEC is determined by looking at 4 or more cations, in a simple equation. As Keith says the true value is determined by the amount of clay in the soil. Something that does not change. After applying or depleting NH4, K, Mg, Ca &/or sodium the computed CEC value will change. On a regular bascis my computed CEC would change 20% to at times 40%.
Therefore I sprang for the measured CEC values at $40/sample. The good thing is the CEC value really does not change & if a field is reported to be some magic number, that magic number will remaine valid for the rest of your life and probably for your great grandchildren.
Now for a reasonable soil with less than a 20 CEC value the text book values for cations work. For my best soil with it's 50 CEC value I want the potassium level to be at least 400 ppm K. If it is not then my hay fields will not make it through our summer drought. Not until the soil test is reported above 520 ppm K will I not see a positive response to potash fertilizer. Depending on who's text book you are looking at that high value runs from 140 to 180 ppm K.
If that is not confusing enough not all clays are the same. Some only have cations attach to the surface of the clay using the difference in charge. Some clays are like a book with many pages. When the clay has a little moisture some elements, both cations and anions, can become trapped between the layers. When dry these layers crack open and expose the hidden elements. With time and weathering these hidden elements become available to your crops.
This is more than you were asking for. | |
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Cec of soil
