RE: problems with my rape plot
No ideal what exactly it is, but can give you some posibilities. First would be fertility, but I don' t think it is. Second could be disease or insect buid up which could effect the growth. Third, is that the plant itself produces some type of toxic which inhibits its growth in future years.
Most likely it' s number two. Try to rotate the plot with a legume or a grain crop which could break a disease or insect cycle. I am sure others on this site will have more specific information.