soak up vs plunge

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

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

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

Other synonyms of soak up include absorb, engross, engulf, immerse, steep.

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

soak up (verb)	plunge (verb)
devote (oneself) fully to	begin with vigor
take in, also metaphorically	devote (oneself) fully to
	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