Wither Roses cannot be placed on Soul Soul blocks. They can be placed on Soul Sand but not Soul Soil. This is an inconsistency, and should be fixed.

Steps to Reproduce:

1. Place a Soul Soil block
2. Attempt to place a Wither Rose on it

Observed Results:
Wither Roses cannot be placed on Soul Soil.

Expected Results:
Wither Roses should be able to be placed on Soul Soil.