I play on component level, and I can't say that fertilizer has been one of my issues, at least not that I've noticed. Caravans do generate small amounts of fertilizer, and I always build a brahmin feeding tub to help attract brahmin into all of my settlements (that have farms, anyway). Between those and my organics, I have been okay. There is an industrial conversion plot, the Coal Pit, that converts wood into fertilizer, if you have an abundance of wood (I normally only have that much wood if I have used the recreational plots to raise all of the relevant stats in my settlers to 10 before placing Mark 1 beacons).
