CEC formula?

Discussion in 'Fertilizer Application' started by Bunton Guy, Feb 27, 2013.

  1. Bunton Guy

    Bunton Guy LawnSite Bronze Member
    Messages: 1,919

    I've had some soil tests come back with low cation exchange capacity levels. Is their a formula to find just how much organic materials need to be added to get the levels up?

    What works the most effectively at getting the CEC level raised? We are dealing with red clay soils.
  2. Smallaxe

    Smallaxe LawnSite Fanatic
    Messages: 10,082

    Everything from microbes to earthworms and ants all the way up to the roots themselves, have something to do with Soil Organic Matter... no big mystery in that , it only involves what is rotting , where it is rotting and whether it is incorporated into the structure of the soil or not...
    Roots die and rot all the time but they also exude and shed which may or may not add significantly to SOM, but you'll want to look at this as a long term issue... to speed things up,,, a couple apps of compost after aerating will go a long way,, to providing the benefits of good SOM, quickly...

    Mulch mowing may or may not raise the SOM over time, even to the depth of 1 inch,,, for example... It all depends on whether the lawn debris is being carried into the root zone of the grass...
    It is a faulty idea to expect that there are formulas for living things and the habitats in which living things thrive... For example I have a large winter window garden that has different watering requirements and different fertilier requirements depending on the amount of direct sunlight as it goes lower in the winter and returns to be directly overhead,,, soon...

    There was no formula that I could give the caretakers, that would make clear the individual watering requirements , during my absence...
    Just focus on the cultural practices that increase SOM and steer clear of practices that destroy SOM and let nature take its course...
  3. Bunton Guy

    Bunton Guy LawnSite Bronze Member
    Messages: 1,919

    So in other words if my CEC level is 6 and it should be 20+ my profesional answer to the client should be lets get spiritual?
  4. Smallaxe

    Smallaxe LawnSite Fanatic
    Messages: 10,082

    You could getspiritual, I suppose... I don't know what it will do for SOM unless its one of these spiritual enterprises that are 'Full Of It'...
  5. t.land

    t.land LawnSite Member
    Messages: 16

    Amount of organic matter is going to increase your CEC the easiest/cheapest/fastest. CEC is calculated with this formula: K(Potassium) + Mg(Magnesium) + Ca cmol/kg (Calcium centimoles per kilogram) + H (Hydrogen). CEC can also be calculated using Organic Matter and clay, but this is the standard formula.

    The formula for determining CEC based on clay is this (this is an example):
    If you have a soil that is roughly 20% clay, you can calculate the CEC coming from the clay like this:
    0.2 (% clay in soil) x 40 (CEC of clay) = 8cmol/kg soil

    Now you must use the same formula to determine how many CECs are coming from the organic matter in your client's soil:

    We care going to assume that there is 5% organic matter (just for this example calculation):
    0.5 (% humus/organic matter in this soil) x 200 (CEC of humus) =10cmol/kg of soil

    8 cmol/kg (clay's CEC in this example) + 10 cmol/kg of soil (CEC from o.m.) = 18 cmol/kg soil

    So from this example you can see that the addition of organic matter will increase your client's CEC much faster than anything else, being that o.m. has a much higher CEC than anything else. Hope this helps
  6. Smallaxe

    Smallaxe LawnSite Fanatic
    Messages: 10,082

    Useable or accessable CEC is dependant on soil structure... all the math falls apart if the soil w/out aggregate structure...
    For example, if your 20% clay was a platy layer of water pounded dirt in which the OM floats on top of it,,, your cation exchange will not be able to find a root hair to exchange with, as a practical matter...

    Just how well would a clay based exchange site adsorb or grab hold of any nutrient from a fertilizer application when it is all packed tight in a platy structure??? Does water have to perculate through the clay plates in or for the NPK to find a cation exchange site???or will they move through as a result of "+", "-" attractions at the molecular level???
  7. t.land

    t.land LawnSite Member
    Messages: 16

    He asked for a formula, I provided one. The formula is a simple way to explain how those nutrients and the simplified composition of a soil affect CEC. There are many other factors that can/do come into play, but I highly doubt that we need to delve into every intricacy of this soil. That being said, I completely agree with both of your posts
  8. Smallaxe

    Smallaxe LawnSite Fanatic
    Messages: 10,082

    Determining whether the soil structure even allows the CEC number has any meaning whatsoever as "... devle[ing] into every intricacy..."???

    Aren't we then looking at numbers totally outside any context of meaning??? the purpose of those numbers on a "Soil Test Report",,, amounts to what???

    I am not interested in a pissing contest, so if you are offended at my questions then forget I brought it up... if there is a thoughtful reply available that shows me where I'm missing something, then that is what I'd like to hear... really... I mean no offense... :)
  9. Kiril

    Kiril LawnSite Fanatic
    Messages: 18,334

    @ Bunton Guy

    Ignore everything that has been posted in this thread, none of it is accurate.

    Bottom line, CEC is highly variable. Instead of trying to hit some target CEC, you should strive to maintain your SOM somewhere in the range of 5-10%.
  10. phasthound

    phasthound LawnSite Fanatic
    Messages: 5,158

    Sounds good to me.

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