Pull up "Market." Go to whichever crop you would like to master and hover your cursor over the mastery bar. It should tell you how many crops you have/how many are needed to get to the next mastery level.
this is the experience bar: http://i47.tinypic.com/25zlch2.jpg
and this is when you hover over it: http://i46.tinypic.com/2cr1wtz.jpg
(btw, to see these, i would right-click and do "open in a new window" or "open in a new tab")