He'd recorded folk songs at Sun, including "Rock Island Line," made popular when former Southern prisoner Huddie Ledbetter, aka "Leadbelly" or (depending on your preference) "Lead Belly," first recorded it. It wasn't the last time he recorded a Leadbelly song. In 1958 he recorded "Goodnight Irene," a Leadbelly number that became an early 50's hit for the Weavers, though Ledbetter almost certainly picked it up from someone else and adapted it to his own purposes.
Here's a less well-known example of Cash reworking-adapting from Leadbelly. It's from 1943, with harmonica by longtime Brownie McGhee partner Sonny Terry. The song is "On A Monday," another prison number.
This is Cash in on TV in 1959 performing the same song, which he's reworked into "I Got Stripes," with the Tennessee Two on the famous LA (Compton, actually) country and rockabilly TV show Town Hall Party. Sure, his vocal's a bit off-key in places and he's changed the lyrics around, but it's clearly the same song. A former prisoner, Leadbelly had tons of these songs rolling around his head. It's on Johnny Cash at Folsom Prison as well.
Cash's folk roots were significant, and explain a good bit of why he was able to grab a wide audience even in the days before the Folsom album or the very diverse Rick Rubin recordings.