How to?
Go to your ‘farmer’s market’, select the bushel of the crop you are going to harvest accept the terms and start harvesting. With every plot you harvest you will receive an extra XP of that crop!
However, there is a catch-the time span for getting an extra XP is only two hours from the time you use the bushel! i.e. - you have to harvest that crop within two hours from the time you have used the bushel to get that extra XP.
