Soul has its origins in Gospel, Blues and Jazz.
We played in the Dorian mode. This is built off the second degree in a scale. Don't glaze over - this stuff is not that difficult. Soul chord progressions can be simple - mainly ii, V (so - in the key of C - the chords would be likely to be Dm7, G7 (dominant chord). The Dorian mode would run D - D (ie from the second degree of C) and uses the notes of C major scale. It works the same for any key.
That's just the start though. We also used the blues scale here and there, and if the chords are Dominants +9 you can use mixolydian (basically a flattened 7th).
Then there is the rythmns and voicings. This takes a bit of research and there are loads of great soul artists out there to listen to. Otis Redding, Marvin Gaye, Diana Ross and the Supremes. More on that later