plural form vs grass roots

plural form and grass roots both are nouns.

Word	Noun	Adjective	Verb	Adverb
plural form	Yes	No	No	No
grass roots	Yes	No	No	No

plural form (noun)	grass roots (noun)
the form of a word that is used to denote more than one	the common people at a local level (as distinguished from the centers of political activity)
	the essential foundation or source