Try putting ­ where you want soft hyphens.

Using soft hyphens is not recommended:
1. The spec doesn't even mention them, how it is handled
   is implementation defined.
2. FOP does not implement the expected behaviour
   (you get always some character, whether at the end of a
   line or not).
Use zero width spaces with FOP 0.20.3 or 0.20.5cvs (0.20.4
has a bug in this area).