I know that farmland is supposed to decay into dirt if it is dry for too long but it should not decay if there is something planted in it. The only effect should be that the crop grows slower.

This is what the wiki says when the farm land should decay.
"If the farmland block is dehydrated and nothing is planted on it for too long."

http://minecraft.gamepedia.com/Hydrated#Hydrated_farmland_tiles

But I do have something planted in it.