Though Farms can be built in any region regardless of fertility, they will only visually appear on the Campaign Map in regions marked as fertile. The composition of the regions' terrain can be determined by holding the right mouse button over the terrain in question. A handful of regions have no fertility terrain whatsoever- though farms may be constructed within these regions, they will not be visually present and offer minimal agricultural income.
Fertility has three levels of increasing value.
Low - the state of most Desert or Volcanic regions, low fertility offers minimal agricultural income, which can be increased by building higher-tier farms.
Medium - The most common level of Fertility, most of Hyrule's woodlands and plains are Medium Fertility. Medium offers a moderate amount of agricultural income, which can by increased by building higher-tier farms.