News Feed Forums Holistic Management Questioning CEC Reply To: Questioning CEC

  • Corey McCain

    October 21, 2021 at 9:58 am

    Thanks Brian, I have read about higher ph showing a higher CEC but even that idea seems flawed to me. Is the ph itself actually truly effecting “capacity”? It seems like in reality hydrogen gets replaced by calcium/mag in higher ph soils and the CEC number goes up not because true capacity changed but simply because of how things are calculated.

    I have a small 1000 sqft garden plot that has very high ph around 7.9-8.2. 6 weeks ago I started an experiment with sulphur on it. I certainly applied it too heavy but this is an experiment and the plot isn’t critically important at the moment. In four weeks the ph came down to 7.3 from 8.2 (sulphur seems to work faster in my Florida environment) in that particular spot. I suspect at this rate it will be in the low 6 soon. Whether or not the soil can be stabilized at a ph under 7 I do not know. It has a good amount of limestone chunks on the surface. The supposed CEC is 13.28 with 2253 ppm of calcium, 160ppm of magnesium and 37ppm K, 21.5ppm Sodium. Along with sulphur I did add more K and Mag as well as other minor nutrients which are obviously not manifest in the numbers I just listed. I will do a soil test once the ph gets to around 6.4 to see what the supposed CEC is.

    My prediction(very limited experience so I could be very wrong) is that the CEC number is not going to move drastically if at all as I predict most of the calcium will still be there. In fact in this scenario it seems that the only + charge element that would increase is hydrogen so if nothing gets drastically lowered it seems to suggest to me that full capacity is not yet reached or time is needed for things to get kicked out and leached. This actually brings up what appears to me to be another flaw. Some minerals are in the soil solution but not attached to anything and could eventually leach or may later attach or be used. Does the soil test assume all the minerals extracted are attached to an exchange site or is this factored in the calculation? Can a mineral like calcium float in soil solution and not be attached to exchange site for years thus high calcium giving a false number on soil test for CEC? Maybe it does leach quickly if not attached but there is more Cal in the soil bound up and not seen on soil test and its releasing somewhat near the speed of leaching thus the next soil test still shows about the same level? I suppose a saturated paste test could help answer some of my questions. I’ve never done it. I’m sure there are many other factors as well.

    The point is I suspect even when this soil shows a ph of 6.4 or so the CEC on the report will not change much unless there is something in the calculation method that makes it lower. It will just be another example of what seems to me to be a flaw in CEC calculation method. Perhaps high ph soils don’t actually have higher holding capacity, they simply just have more calcium/mag.

    On a side note the same day that I amended this plot I planted a sunn hemp cover crop on it. I thought the sulphur application would kill it but it didn’t. 4 weeks later it was flowering as its really getting late in season now for sunn hemp. A week ago I planted Daikon radish in between the rows and they have since sprouted. I’ll update this thread when I eventually get a soil test done.