Unlike Fair Lily-of-the-Valley Crops that have a crit chance to yield Lily-of-the-Valley Leaf (got 7 leaves from 15 fair crops), the Sweet Lobelia doesn't seem to have a rare item attached to it (got nothing from 15 fair crops). That being said, it appears Rivendale Soil simply gives Sweet Lobelia a better chance of yielding fair crops, but that's it, no rare leaf. I could be wrong, but this has been my experience.