When a farmer's fields are sown, it means that she's planted seeds in them.
If you sow something, you scatter seeds on it or plant tiny seedlings in it. The adjective sown is the past participle of sow, so you could say, "The field behind the school was sown with wildflower seeds," or "There were so many seeds that couldn't be sown this year because of the drought." The word comes from the Old English sāwan, "to scatter seed upon the ground."