Regarding Guest Dave's post on 12/2, the term "John Barleycorn must die" dates back to the 1600's. With barley and corn being the two major grains alcohol was made from, the term arose when you woke up with a hangover (probably not what they called it back then) and cursed the drink you had the night before.

However, the folk song and this thread make it very clear that the lyrics describe the harvest cycle. The grain must be put in the ground (die) to ready for the next cycle.