fertilizer vs fertiliser

fertilizer and fertiliser both are nouns.

Word	Noun	Adjective	Verb	Adverb
fertilizer	Yes	No	No	No
fertiliser	Yes	No	No	No

As nouns, fertilizer and fertiliser are synonyms defined as:

fertilizer and fertiliser: any substance such as manure or a mixture of nitrates used to make soil more fertile

Other synonyms of fertilizer include plant food.

fertilizer (noun)	fertiliser (noun)
any substance such as manure or a mixture of nitrates used to make soil more fertile	any substance such as manure or a mixture of nitrates used to make soil more fertile