undeveloped vs fallow

undeveloped is not a noun while fallow is a noun.

Word	Noun	Adjective	Verb	Adverb
undeveloped	No	Yes	No	No
fallow	Yes	Yes	No	No

undeveloped (noun)	fallow (noun)
	cultivated land that is not seeded for one or more growing seasons

undeveloped (adjective)	fallow (adjective)
not developed, mature, or fully formulated	undeveloped but potentially useful
not developed, improved, exploited or used	left unplowed and unseeded during a growing season