Fertilizer
From Captain of Industry Wiki
Fertilizer is a crafted liquid product used to restore 
Fertility in the Farm, Irrigated Farm, Greenhouse, and Greenhouse II. Along with the growing of 
Green Manure, it is the only method of restoring Fertility levels.
In the mid-game, Fertilizer is generally produced using the the Ammonia byproduct from Sour Water stripping, while in later stages of the game it is possible to create it from entirely elemental sources using the output of a Hydrogen Reformer and an Air Separator. Alternatively, Fertilizer can be sustainably produced through green methods using Compost.
