Minecraft wiki entry for Bone_meal_farming, associated text: The moss can convert stone in a 7x7 area (minus the 4 corners) up to 5 blocks below the source moss block. It can also convert blocks up to 5 blocks above the source, but only where there is a connection of non-air and non-liquid blocks between the moss block and the stone platform above. In any case, only the highest block within the range can be converted. It is also necessary for the source moss block to have air above it for the bone meal to have any effect.