plunge vs soak up

plunge is a noun but soak up is not a noun.

Word	Noun	Adjective	Verb	Adverb
plunge	Yes	No	Yes	No
soak up	No	No	Yes	No

As verbs, plunge and soak up are synonyms defined as:

Other synonyms of plunge include absorb, engross, engulf, immerse, steep.

plunge (noun)	soak up (noun)
a brief swim in water
a steep and rapid fall

plunge (verb)	soak up (verb)
begin with vigor	devote (oneself) fully to
devote (oneself) fully to	take in, also metaphorically
cause to be immersed
immerse briefly into a liquid so as to wet, coat, or saturate
thrust or throw into
fall abruptly
drop steeply
dash violently or with great speed or impetuosity