lab 4

Sarah, try not to mix concepts

Buffer capacity is the ability of a soil to resist a change in pH. See lecture 15, slides 11 & 12.

Exchangeable acidity (measured in CaCl₂) is lower than active acidity (measured in water). In the lab when you add H₂O, you are measuring the H+ and Al3+ ions in soil solution. When you add CaCl₂, the Ca replaces H⁺ and Al³⁺ on the exchange complex (by mass action); i.e. H⁺ and Al³⁺ previously on the exchange complex are "driven" into soil solution, so the pH goes down (i.e. more acidic). See slides 9 & 10.

Thank you! For days I have been searching for that missing piece that wasn't allowing me to put everything together in my mind.... for some reason that little tidbit, "driven", did it for me. I'm so happy! Yah! So silly.