^ Humus is what 'buffers' pH, yes. Humus and clay have a high cation exchange capacity; sphagnum peat moss has CEC also which varies depending on the source of the peat, but is less than that of soil/humus (considering humus/clay has greater bulk density). Again what this means is that there are all these negatively charged 'exchange sites' which attract and hold onto positively charged ions (cations), including plant nutrients (most of which are cations). Ultimately what this means is that a higher CEC media (such as one with lots of clay and/or humus) tends to better resist sudden fluctuations in soil solution pH (either way, up or down) because the humus does act like a 'buffer' or reserve.
So for example, a high CEC soil or medium will tend to require more lime at once but fewer applications, while a medium with a lower CEC (such as peat) requires less lime more frequently.
Humus also contains humic substances, including humic and fulvic acids. These are quite complex substances that range from high molecular weight, carbon rich (HAs) to low molecular weight, oxygen rich (FAs) compounds which can form complexes with ions and increase their availability to the plant; hence they are organic chelating agents. With an appropriate amount of lime and healthy soil with a good humus content pH shouldn't be a concern. I routinely fertigate with solutions which have a pH from 5 to 4, and this isn't a problem.
Only issue with humus is that like clay it consists of very small particles, which can become too densely packed. Too much humus isn't good because it will lead to drainage issues.
