fertilizer

English fertilizer is simply fertilize plus the agent suffix -er. That -er comes down through Old English -ere and Proto-West Germanic *-ārī from Proto-Germanic *-ārijaz, which itself was borrowed from Latin -ārius, tracing back to Proto-Indo-European *-yósder.

The suffix's job has stayed remarkably steady across all these languages: turning an action or quality into 'the person or thing that does it.' Attached to fertilize, it simply names whatever makes soil fertile.

The -er here shares its deep root with the same ending on countless other English agent nouns, since it descends from the same Latin -ārius line borrowed into Germanic.