Saltpeter is made in a basic tub or a sturdy tub. Each tub can hold a maximum of 500 dung to be changed into saltpeter. Dung converts to saltpeter at a 5:1 ratio. Saltpeter is used in a few different capacities. Saltpeter can be used to fertilize trees to halve the time it takes wood to respawn on that tree. You would need to reapply fertilizer after every time you collected wood from that tree to have any effect. Also saltpeter is used to fertilize palm trees to produce dates. Finally, saltpeter is used at a pyrotechnics lab as an oxidizer when making gunpowder and fireworks.