An agriculturalist is a person — someone who studies farming, or works at a high level in farming, and knows the science of it. It is not the name of the work itself. The work and the science are agriculture; the -ist ending says “the person who does or studies this.” You will also see agriculturist; both are used. The word is formal. A person growing vegetables for the market might just say farmer; an agriculturalist is more often an expert, a writer, or an adviser. You meet it in serious writing about farming, not in casual talk.