This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
榡 is a phono-semantic compound with 木 (tree) on the left and 素 on the right, which likely provides the sound. The exact historical development is uncertain.
The left side is a tree 木, and the right side is 素 (element, plain). Picture a plain, unadorned tree with no special meaning—just a simple tree.
For ソ, imagine a plain tree so simple it's just 'so' plain: so -> ソ.