what are huge fields with only one crop called?

It's called "Monoculture".

Monoculture is defined as "the cultivation or growth of a single crop or organism especially on agricultural or forest land".

Monocultures, the agricultural practice of producing or growing genetically similar, or essentially identical plants, over a large areas (stands), year after year, is widely used in modern industrial agriculture.

It is often argued that monoculture produces greater yields by utilizing plants' abilities to maximize growth under less pressure from other species and more uniform plant structure.

Learn more about Agriculture and Biodiversity at Center for Health and the Global Environment.

See a beginner’s guide to careers in agriculture and farming at Farmers Weekly.