As Earth's population creeps toward 8 billion, the amount of arable land on the planet becomes more crucial for the survival of its growing population. Many factors contribute to the crop-worthiness of land, including local climate, soil type, annual rainfall and access to water. But just because a swath of land is arable, doesn't mean it can grow any crop that is needed or desired. The Midwest may be great for corn and soybeans, but the climate doesn't allow the growing of citrus trees. Parts of the South don't have the rich topsoil that can grow wheat, but tobacco does very well.

Total Landmass of Earth

Ocean waves

Earth is a watery place, with around 70 percent of its surface covered in water, the vast majority of which is salt water. This leaves around 30 percent of the planet's surface as potential farmland, totaling just over 57 million square miles. That's a lot of land, but much of it is not even remotely arable, as 20 percent of Earth's land mass consists of desert, and even more is devoted to alpine and arctic tundra, neither of which permits crop growth. According to the National Geographic News, this leaves a little less than 40 percent of Earth's total landmass suitable for agriculture.

Land Used for Crops

Ears of wheat in field, close-up

Of the land used for agriculture, less than one-third is devoted to growing crops. Wheat covers more of the Earth's land area than any other crop, while corn, rice, potatoes and cassava round out the top five most-grown food crops in the world. These crops vary greatly in their input needs and growing ranges. Rice, for example, needs more water as an input than most other crops, while wheat grows successfully over the widest range, as it can be grown even in cold climates with little water.

Land Used for Livestock

Piglets in barnyard

Around 26 percent of Earth's agricultural land is available for grazing livestock. But that's not the whole story as far as animal agriculture is concerned, since roughly one-third of crop land is also devoted to growing crops that are used to produce feed for the world's livestock. Massive amounts of feed and water are required for livestock. In fact, it's estimated that around 20 percent of the world's land-based animal biomass is domesticated animals, with pigs and chickens most numerous globally.

Unused But Usable Farmland

Lush forest

In addition to the land that is already devoted worldwide to growing crops -- about 11 percent of Earth's surface -- experts estimate that amount could be doubled. The issue, of course, is that much of the Earth's potential farmland is currently forested or protected. Future increases in yields necessary to feed the planet's growing population will likely need to come from higher-yielding crops rather than an expansion of farmland.