How are you currently populating Person Source in Marketo right now?
At my previous company, our Person Source was the same as our UTM Source, and it was just set on form fill as a hidden field. (At my current company, our Person Source strategy is a little different...it's based on the form they fill out, rather than the channel that pushed them there.)
To date, we've been attributing the traffic to "Web" generically.
If anyone has used HubSpot, then outcome we're looking for should sound familiar: We want person source when the lead is created to be attributed to the organic web channel as the source and not the form or content. We can control other person sources such as paid media or webinars because we control the entry point (specific landing page, form, etc.), but organic traffic is not the easiest to track to the lead source as elegantly with Marketo as it was in HubSpot.