I believe there are certain combinations of berries (eg. Mago + Iapapa = Pomeg) that have a chance to give you another berry, when they are planted next to each other.
Untill now I've got:
Mago + Iapapa = Pomeg
Persim + Chesto = Kelpsy
Some other combination I found:
Aspear + Leppa = Hondew
Oran + Pecha = Qualot
It seems as if this only gives you the EV berries.
The professor inside the house at the berry field will also tell you which combinations you've found so far.
Source: Experience, http://gaming.stackexchange.com/questions/135982/how-do-berry-mutations-work