While the last choice isn't as direct in its relation to Soul as the others, it should be obvious how the connection comes in anyway.
1) John Prine, "Picture Show". In the direction of the last post, who knows why such songs are resonant. This initially seemed a bit perfunctory, and my first assessment of The Missing Years followed suit; thank goodness for the law of multiple listens. The key line, as I see it, is the one about the Mocca man, even if it is relatively out of the blue; as it goes with great lines, it has the right mixture of humour, pathos and magic.
2) Van Morrison, "Caravan". I can't think of a mystic masseur in popular song I trust more than The Man; definitely not when going through side A of Moondance, anyway. This album, and most songs contained within, also never really struck me until recently. But now I find his belief in Soul, whether his own or that of the radio, hugely inspiring; with the latter, I imagine what he's really getting at is us discovering the soul within, and preparing for a journey into the mystic.
3) Cleftones, "Heart and Soul". If pressed, maybe I could think of other instances of covers that do the unthinkable with the original, but why bother? One can only hope to experience a love strong enough for such a song to be the natural outcome. Purposeful and honest, at the moment it seems like one of those tight, perfect songs that were apparently all the rage back in the day.
4) Gram Parsons, "Brass Buttons". If ever melody and lyric came together as perfectly as on "Warm evenings, pale mornings and bottled blues", I don't want to know; nor do I wish to ever experience yearning with the same sadness I sense behind the song. Parson's talents might not have extended to creating a singular album masterpiece, but perhaps his three major contributions to country-rock (I exclude Burrito Deluxe, you understand) are best considered as one long album: in which case, this would still be contender for the best song.