Fertilizers are used to add nutrients to crops. Right Click with a fertilizer in your hand on some Farmland or a Crop to add the nutrients. Particles should appear, indicating the fertilizer was added.
Compost is the product of the Composter.
Bonemeal is made of crushed bones.
Saltpeter is made from its ore.
Guano is found deep underground and on gravelly shores.
Wood ash is produced by breaking firepits. Throwing a torch item into water also has a chance to produce ash.
Sylvite is made from its ore.