Automating Source and Medium for organic/direct visitors

Hey guys,

I'm trying to plan out a way to automate Source and Medium values for records who are hitting our pages from Organic Search or Direct Visitors. I notice that Marketo is sometimes able to populate utm_source and utm_medium for organic Google visitors, but this seems to be inconsistent. 

Has anyone been able to solve for this? Trying to get some ideas on how best to set this up. We are able to successfully capture utm values for records who come in from advertising or email channels, but still struggle to automate values for organic and direct visitors.

Thanks!