Mastery bonus

A premium crop is a fully mastered one which conveys a mastery bonus upon harvest. The crop will appear bigger than a normal crop — exactly like a mature, fertilized crop. The bonus given depends upon the crop being harvested and a percentage "bonus chance" assigned to that crop. Each type of seed offers a number of bonus XP per plot between 1 and 8, as well as a percentage chance of receiving that bonus. If the plot isn't converted into a premium crop, then it returns no XP upon harvest, unless it is fertilized. If a premium plot is fertilized, then it receives an additional 1 XP beyond its mastery bonus.

By way of example, both premium wheat and lilac give 6 XP on harvest. However, there's only a 5% chance that a plot of mastered wheat will become a premium wheat, whereas there's a 20% chance that one of lilac will. Thus a farmer would expect — though it is not guaranteed — that a field of mastered lilac would return more XP than one of mastered wheat. As a general rule of thumb, there is an inverse relationship between the amount of the mastery bonus and the bonus chance; that is, the higher the potential XP per plot, the lower the chance of getting it. It is unclear why crops offering the same mastery bonus — like lilac and wheat or carrots and coffee — have different bonus chances.

Although msstery signs seem to indicate that they are somehow connected to premium crops, they are merely decorations. Only mastery itself — and a bit of good luck — are required to receive a mastery bonus.