Genesis 4:12

When thou tillest the ground, it shall not henceforth yield unto thee her strength; a fugitive and a vagabond shalt thou be in the earth.

1. Cain would no longer be a successful farmer. His efforts to raise crops would prove fruitless.
2. Cain would become a fugitive and a vagabond.
3. Considering the strictness of the later Law of Moses, apparently given by the same LORD, this punishment seems fairly mild, although the term, "fugitive", suggests he would be running from the local sheriff or the FBI.
4. Either the LORD or the earth, or both, have the power to cause crops to be unsuccessful for one farmer and not for others. The language here suggests that nutrients in the soil would for some reason be prevented from being absorbed by the plants, or perhaps Cain had just exhausted the soil through several years of planting without fertilizing, and the LORD knew this was the case.