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.
The effectiveness of fertilizers are influenced by the type of Farmland they are used on.
Compost is the product of the Composter.
Bonemeal is made of crushed bones.
Shellfish can be obtained by killing some aquatic invertebrates.
Saltpeter is made from its ore.
Guano is found underground in lush biomes and on rocky coastlines.
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.