pH / BS

Edited by 2 users.
Last edit: 00:12, 17 April 2017

Sarah, you have to be careful not to mix concepts. Your statement - "If you add OM to a soil, reactivity increases in the soil, allowing the soil to retain more ions/nutrients, which increases CEC, which means there's higher buffering capacity..." is true only if OM that we add has low C:N ratio and is comprised or relatively simple organic compounds. If we add woody type of OM with lots of lignin this will not be the case.

"does this then change pH because more of the active acidity can how be held as exchangeable acidity, increasing the pH?"
Here you mixed several concepts,
-addition of simple OM compounds (with higher number of functional groups per unit mass) will indeed increase CEC and allow more of the exchangeable cations to be adsorbed, but this will have no impact on active acidity (which represents number of H+ ions in soil solution). Furthermore, addition of OM does not directly have any effect on increase of soil pH. Addition of Al and Fe (acid-forming) ions will lead to an increase of soil acidity
Addition of OM, has no impact of base saturation, which is proportion of the CEC occupied by the base-forming cations (ie Ca, Mg, K and Na).

MajaKrzic (talk)22:50, 16 April 2017

Ok, I think what is confusing me is buffering capacity...

Sandra mentioned in the review session that the difference between the active acidity and exchangeable acidity relates to the buffering capacity. The smaller the difference is the higher the buffering capacity, is what I thought she said. I'm trying to relate that to other concepts to understand it better.

SarahKruk (talk)01:02, 17 April 2017

re: pH in water versus pH in CaCl2

pH in water measures the pH of soil solution

pH in CaCl2 measures exchangeable acidity (thus pH in CaCl2 will be lower than pH in water). The lower the pH in CaCl2 relative to the pH in H2O, the greater the exchangeable acidity.

SandraBrown (talk)02:01, 17 April 2017
 

I guess my question here is related to this topic:
To my understanding, a coniferous forest floor (specifically mor) tends to be more acidic than deciduous forest floor because the very slow and incomplete decomposition of conifer litter due to high C/N ratio and lignin favors low HA/FA ratio (FA is more acidic than HA).
If this is correct and definition of organic matter includes both non-humic and humic substances, couldn't adding organic matter that is rich with FA acidify a soil? Or, is that because any change in proton conc. by adding FA rich OM to a soil is insignificant and readily buffered by clay mineral and other soil colloids?
I'm bit confused. Thank you for the clarification!

TakuhiroSomeya (talk)01:31, 17 April 2017
Edited by another user.
Last edit: 17:16, 17 April 2017

Taku, try not to over complicate. Coniferous litter (e.g. pine) returns less base-forming cations to the soil than deciduous litter (e.g. maple)

SandraBrown (talk)02:03, 17 April 2017

Oh, OK. Thank you Sandra!

TakuhiroSomeya (talk)02:25, 17 April 2017