March 24, 2020 Soils

Master Gardeners,
Again, great job connecting with class yesterday. You are amazing! I’m hoping to have all of your handouts ready and in the mail on Friday.
Consider following the Rogers Farm Facebook page. You’ll find plant ID challenges, miscellaneous gardening tips and ideas for keeping little ones learning while at home.
Please read:
Mentioned today
Keep in touch with any feedback about the class. I sincerely want to know if this is not working for you or if there are any accommodations I can make to make your experience more meaningful and fun.
PS Here’s the explanation of your question about what the (A) by the CEC refers to (quoted from Bruce Hoskins in the soil lab). Don’t be intimidated by this – John and I both agreed it’s WAY more than you really need to know. It’s likely more than what he and I need to know in most practical situations that we encounter. BUT, I know there are some of you who may really enjoy this level of chemistry. 🙂

The (A) by some but not all CEC numbers indicates that the calculated value was “Adjusted” to take into account any unreacted lime. This happens mostly on higher pH soils (usually around 6.5 pH or higher). CEC is calculated by adding up all the exchangeable cation nutrients (Ca, Mg, K, and neutralizable acidity). In higher pH soils there is usually some amount of unreacted lime (calcium and/or magnesium carbonate). Our acidified extract (pH 4.8) will dissolve some or all of this lime, leading to (in some cases) a grossly overestimated CEC – because some portion of the Ca and Mg is from dissolved lime and not from an exchange site.

There is a secondary estimate of CEC, based on soil pH and organic matter content. Not as exact, but much more accurate than the normal estimate. Ca and Mg are decreased proportionately until the (Ca+Mg+K) addition is close to the secondary estimate. The adjustment is labelled with (A) next to the CEC value. The original extractable levels are still listed in the lb/A listing, but CEC and percent levels have been adjusted. This has been in use for decades and works reasonably well in most high pH soils, though occasionally it does need tweaking. For illustration, take a high pH soil and divide lb/A Ca by 400 to get milliequivalents of Ca alone and compare this to the final CEC estimate. In some cases it is 2-3 times the secondary estimate.