From Rolling Stone:
For the follow-up to “Hold On, I’m Comin’,” writer-producers Hayes and Porter decided to tinker with their winning formula: Porter asked singer Sam Moore to give him “the Bobby Bland squall,” guitarist Steve Cropper came up with the licks that set up the familiar blast of the Memphis Horns, and — voila! — another Memphis soul classic was born. “We had no idea how good we were,” Hayes said of the partnership.
Co-author Isaac Hayes found the inspiration for “Soul Man” in the turmoil of the African-American Civil Rights Movement of the 1960s. In July 1967, the 12th Street Riot in Detroit, Michigan occurred. Watching a television newscast of the aftermath of the riots, Hayes noted that black Detroit residents had marked the buildings that had not been destroyed during the riots – most African-American owned and operated institutions – with the word “soul”. Relating this occurrence to the biblical story of the Passover, Hayes and songwriting partner David Porter came up with the idea, in Hayes’ words, of “a story about one’s struggle to rise above his present conditions. It’s almost a tune [where it’s] kind of like boasting ‘I’m a soul man’. It’s a pride thing.”