I was improvising soul music this week. That was interesting.
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