Go to "Nolan's Fly Shop" outpost in Faith's region (east of Hope County Jail). There is a tractor in that outpost. Enter it and then run over five enemies to get the "Fertilizing The Land" achievement. Set the difficulty to Easy and bring a few medkits so you can heal yourself when the enemies shoot at you. You do not have to kill five enemies in one hit, just five total enemies. If you have already captured this outpost, you can reset it by going to the pause menu and clicking "Outpost Master", but you must have captured all other outposts for it to work.