Buffering capacity is the soils ability to resist changes in pH and relates to the CEC (cation exchange capacity). The higher the CEC the higher the buffering capacity and the more material is needed to change the pH. Clay and organic matter typically have a high CEC (because of their negative charge), silt not so much followed by sand. So as you say, it doesn't take as much lime or sulfur to move a sandy soil's pH up or down. The cations are the positively charged minerals which are held by the negatively charged clay so I'm not sure what you mean by buffering negatively charged nutrients?
