For U.S. farmers, China just got a lot closer.

Long a major customer of U.S. farmers, China on Monday took another step toward being a major supplier, as a key U.S. merger authority cleared China National Chemical Corp.’s planned takeover of Syngenta AG, a top seller of crop seeds and pesticides. Syngenta is a Swiss company, but about one-quarter of its sales come from North America.

The...