If you are unable to fertilize your neighbor's farm, it is normally due to one of the following reasons.
- All the crops on your neighbor's farm have already been fertilized by other neighbors.
- Your neighbor's crops are all already mature and ready for harvest.
- There are no crops planted on the neighbor's farm
If a neighbor has been offline for 3 days or more, you can tap on the horn to call him/her back to the game.
Yellow color horn = neighbor hasn't logged in between 3-14 days.
Red color horn = neighbor hasn't logged in for more than 14 days.