CEC and Percent Base Saturation

Tom Miles

CEC and % base saturation
Richard Haard, Jim Joyner, December 13, 2007

Greetings As an extension of my conversation with Jim I am forwarding this farm in-house conversation on % Base saturation. Noteworthy is a interesting trend of higher Mg % saturation where I treated with charcoal powder 4 growing seasons ago. The following information from this Clemson University Soil scientist is a good way to get a working understanding of CEC and base exchange and how we might use it to manage soil nutrition here.


It is interesting also as an indicator of functional activity of charcoal in soil higher CEC did not show up in our block tests this year. The literature however indicates there is a weathering process and combination with soil organic matter < in the tropics> that brings on this activity. Also Cornell researcher Janet Thies showed continued rinsing and incubation improve CEC of charcoal in soils.
In our soils we are extremely low in the clay component. Across the farm, both the charcoal research plots our CEC is running about 10 to 13 MEQ cation/100g soil. This for a sandy soil is actually not bad as it is probably influenced by our organic matter content which is running 4 to 6 % In the west field where we currently have very high pH (7.1) and soil calcium (21-2700ppm) the CEC is about the same but the % of base saturation for calcium is the highest on the farm , (almost 90%).
The base saturation is the faction of the CEC that is occupied by basic cations. For us we need to look at magnesium availability in our soil. If we look at magnesium levels in our soil and % base saturation we can see that many of our readings are troubling. All fields tested are running 88 to 100 to 100ppm with several exception discussed later. These are classified by the lab as 'medium' . Converting this to pounds per acre/7 " depth we multiply ppm X 2 gives us levels of 176 to 200 lbs. of magnesium per acre. (not bad).
Looking at the % base saturation however we see that our levels of available Mg is at or below the minimum desired. Although background levels of Magnesium are adequate we need to determine if we are having magnesium deficiency in the fields
In the fields tested Mg % saturation at E 13/5,6 are quite low ( 6.0,5.7), W 6/6 is low at( 7.4,7.0) and our 2007 charcoal treatment sets is also low but showing an interesting trend of improvement over the season. Not all of our fields are low however. Interesting is that where the % saturation of magnesium is high the % saturation of calcium is low and visa versa. The trend does not seem to correlate with pH either.
In any case fields E1,2,and 3 have good readings for Mg % saturation (73%) and does E 11/7-11. (31-33%) The latter which has been suspect for nutritional problems (not magnesium). (Note that field 1 has charcoal from an old burn pile on the north end and field 2 had a 4' strip of charcoal powder applied 4 years ago whose exact location is unknown at the moment. )
Most interesting of all are analysis of sections of field 4 that I applied a tote of John's charcoal powder 4 years ago. This is the highest Mg % saturation measured (90.8%) and may be the result of this charcoal treatment. The controlled block study should answer this question over then next few seasons. Equally interesting is the lowest Ca % saturation occurs when the Mg % is highest. This must be competition for sites on the soil colloids.
Comments and questions are welcome
Rich H, 4CN, Bellingham, Wa
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