Quite a few papers claim humic acid is an inhibitor of PCR reactions. I understand this is true when working with soil microbes, but how does it qualify to be a PCR inhibitor in general (i.e when not working with soil micorbes)?
In general, when not working with samples from soil or natural waters, you propbably won't encounter high enough concentrations of humic substances to inhibit your PCRs.
If you're just asking about mechanisms of inhibition, humic acids can interact with polymerase enzymes, with nucleic acids (altering primer annealing and template melting temperatures), and they can partially quench fluorescence in the case of real-time PCR (see Overcoming Inhibition in Real-Time Diagnostic PCR). There are numerous sources in this reference if you want to read more about how these different interactions were characterized in the lab.