Final Exam Questions

Hi Maja, I am feeling a bit unclear about determining soil pH from CEC and BS (question 7c on practice final #2). Could you explain the relationship? I thought that there would be a high amount of base cations on the colloid surfaces at low pH, because the base cations could not be in their ionic form in acidic conditions... but then I read somewhere that for a soil like the one in this example (loamy texture, base saturation 87.5, CEC 32) that the pH would be near neutral. Thank you!

Soil that has high base saturation (BS) has to have alkaline pH, since its exchange complex (or diffuse double layer)is dominated by the base-forming cations.

Cation exchange capacity (CEC) on its own does not tell us anything about the type of cations, it only gives us the total number of cations that that soil can adsorb. Hence, from the CEC itself (and without any other info) we cannot make any assumption about BS.

I am not sure what exactly was the question 7c (don't have time to go and look for it now) but I believe that it asked something along the line to choose one of 4 pH ranges based on the various types of info given in that question. Based on the high BS in that soil the most likely pH range would be around 7.5 to 8.5