tough one...

soul uses alot of the typical chord progressions...I, IV, V  and II, V, I etc...Its the voicings that are really unique.  I am still learning...

alot of use of the tri-tone (because of the gospel influence).  alot of maj 7ths, to get that open, loungy sound.  and lots of b9 and #9's, which I don't well understand.

My advice would be to learn the blues and gospel as much as possible, then borrow what you know from there that sounds soulful.

I've heard gospel piano players "bend" notes on a regular piano that I could not come close to doing.  Even people like Steve Winwood don't seem to be able to get that quite right.

still thinking about this...if anything it might be useful for me

1) I've read that some jazz and soul keyboardists ask themselves, for basically every chord they play is "which 7th will I play"?  By this they mean, will I make this chord a maj6th, and dom7th, or a maj7th.  The idea being that every triad, in almost every situation, can be embellished with one of these three, usually depending on what chord you're moving to.

2) The melody of the song really does determine how you will "color" your chord with your right hand.  In fact, many gospel keyboardists I know do very little interesting stuff in their left hand (just a root and fifth, maybe some walking up and down pentatonically; this is in the slow stuff, in the fast stuff gospel keyboardists burn with the left hand).  But in their right hand, they do all kinds of weird stuff.

So what I ask myself...Lets say the chord is C, but the melody note that "hits on the beat" is an A.   I ask myself, or experiment with: "what chords do I know with an A in it, that are not merely diatonic to that C chord"?  I can experiment with:

the chromatics:
D major = first inversion = D, Gb, A
Gb minor = third inversion = Db, Gb, A
A major = second inversion = Db, E, A

the diatonics:
A minor = second inversion = C, E, A
F major = third inversion = C, F, A
D minor = first inversion = D, F, A

anyway, I will play around with these in the right hand while holding down the bass (C) and perhaps the fifth (G) in the left, until something sounds soulful in the context of the progression.  there are probably rules that govern what will sound good, but for me its still experimentation. 

Seeing as how you bumped it...

1) I've read that some jazz and soul keyboardists ask themselves, for basically every chord they play is "which 7th will I play"?  By this they mean, will I make this chord a maj6th, and dom7th, or a maj7th.  The idea being that every triad, in almost every situation, can be embellished with one of these three, usually depending on what chord you're moving to.

I'm not sure whether you understand this or not, but "major seventh" and "minor seventh" are both types of interval and types of chord. If I add a minor seventh (interval) to a minor triad, I get a minor seventh chord. If I add a major seventh (interval) to a major triad, I end up with a major seventh chord. But "dominant seventh" is not an interval, it refers only to a chord - what you get when you add a minor seventh (interval) to a major triad.

the chromatics:
D major = first inversion = D, Gb, A
Gb minor = third inversion = Db, Gb, A
A major = second inversion = Db, E, A

These chords are all spelt wrong. D major has an F#, not a Gb. A major has a C#, not a Db. Gb minor would have Bbb (double flat), but that would be rare; it would be more common to write it enharmonically as F# minor (F#, A, C#).

Also, I'm not sure you understand inversions correctly.
The first inversion has the third of the chord as the lowest note - so in a D major chord, that would be F#. (If D was lowest, it would be in "root position".)
The second inversion has the fifth of the chord as the lowest note - so in A major, that would be E.

You're joking right, taking me to task for a post I wrote six years ago?

I thought that's why you bumped it, no?

In any case, it may help someone else who finds it, even if it doesn't help you.