Questions about som

I see the confusion. Slide #15 from that lecture says that "between pH 4 and 6, 85-90% of carboxylic groups are dissociated" which would mean an INCREASE in CEC, but slide #17 says "at low pH carboxylic groups on humic substances are undissociated", which would be a decrease in CEC…as far as I understand.

1. the "acidic character" of FA is different from soil pH. FA is more acidic because it does have more COOH groups, and can therefore contribute more H+ ions to the soil solution under the right conditions. pH will then determine how the functional groups behave.

3. Fulvic acid has more COOH groups (per unit mass) than Humic acid, which means it will have a higher CEC. And behavior of humic substances is determined by: 1) # of carboxyl groups AND 2) particle size. FA is much smaller with less molecular weight, so it has a higher SSA and overall it is more reactive and mobile in the soil.

Hope that helps for now. I just wanted to put in some thoughts, since it's Saturday and I know what it's like to have burning questions.

thank you for your help!

But i am still confused by the "acidity" of FA. if this is not equal with pH value. What does it imply? High acidity means more H+ in the soil?

One more other question about Ammonium fixation: is Ammonium fixation the same with isomorphic substitution ?

you're right. fulvic acid does have a low pH. an acid is just a compound or ion that contributes to H+ activity in the soil. isomorphic substitution occurs during the weathering of rocks and minerals, when primary minerals are altered or transformed into secondary minerals (ie. 2:1 phyllosilicate clays). remember the octahedral and tetrahedral sheets of montmorillonite for example. Mg can substitute for Al INSIDE the crystal structure, leading to a net negative PERMANENT charge on the colloid. ammonium fixation is when NH4 gets stuck BETWEEN two layers of vermiculite or illite (fine-grained mica) because it is just the right size. this also happens to K because of similar size, remember lyotropic series: Al3 > Ca2 > Mg2 > K=NH4 > Na. in this state the nutrient are non-exchangeable and only slowly available to plants, so it represents a decrease in available N. hope that helps.

Hi there, For the FA and HA, I am thinking that the FA is more acidic than HA since FA has more H+, and relatively it should has higher CEC. Since there will be more coo- that attract other cations such as fe3+ or al3+ and so on, FA "SOLUTION" (pure FA) will have higher CEC Refer back to lecture 17 slide 16, it says "CEC IS HIGH AT HIGH PH AND LOW AT LOW PH". Is it because coo- is organic anion that can lead to flocculation? (Since FA is so soluble that large amount of H+ being released with COO- leftover). Correct me if I am not thinking it in a right way. Thanks