Resource nodes
Fertile land
Fertile land yields food every turn while it sits inside a city's region.
Food finances two things: unit upkeep and population growth (Economy). A city that takes fertile land into its region grows on its own; a city without it needs a Farm to grow at all.
A node only works inside a region, and a region is not fixed: it widens with population and can be extended with gold (Territory and score). Founding a city next to a node is no different from founding it on top, and is usually more flexible — and growing a border toward a node you cannot reach is an option too.
The numbers on this page are read from the game’s balance tables.