Soul Keeper plants spawn underground near water and in darkness. They can only be moved through Silk Touch, otherwise, breaking them will yield useless items called dark crystals shards, or whatever.
They only grow in darkness and at lower than 58 Y levels, with still water around them (See Video.) However, the deeper it is the more experience it gains, the more still water around it, the faster it grows.
Everything else added by the mod is currently useless, and will be until some kind of add on is released.
It exactly. To fashion has become more useful, I write addons.
History is written to make it more interesting.