Final Exam Questions

Hi Maja, I was wondering about question 2 on the second practice final. The question provides a graph showing CEC of Humus and of Smectite changing with pH. I understand that the organic colloid will be entirely pH-dependent because of their carboxyl group, so CEC will change linearly with increasing pH. I also understand that smectite is similar to, or contains, montmorillonite and will have a high proportion of isomorphic substitutions which are pH-independent (CEC is constant up until pH 6.0)... however, I do not understand what is going on after the pH 6.0 mark for smectite. Why does CEC increase here? Thanks in advance!

Smectite (which is a group of phyllosilicates that includes mineral montmorillonite) predominantly has a constant (not pH-dependent) type of charge. However, OH groups on the broken edges & corners of smectite minerals will end up with the pH-dependent charges when reacting with either H ions (in acidic soil solution) or OH ions (in alkaline soil solution). Hence, the slight increase in CEC on smectite with the increasing pH.

By the way, that graph was taken out of your textbook and more details can be found there

Hi Maja, could you please explain what do C and N represent for in C/N ratio?

C = carbon, N= nitrogen Importance of the C/N ratio in soil organic matter is related to its susceptibility to decomposition. Organic matter with C/N < 25/1 is easily decomposed by soil organisms and as a result of that decomposition available N is released and it becomes available to plants