Fertility is a mechanic in the game which imcreases/decreases crop yield. The higher the fertility of a farm, the higher the yield of the crops planted there. Fertility can be improved by planting Green Manure, by natural replenishment, or by using a fertilizer. Natural replenishment basically brings the fertility back to 100% when no crop is planted, it only needs time. The natural replenishment is positive when the fertility is below 100% and negative when above 100%. Fertility is being used when growing crops. The amount of fertility used depends on the crop. Sugar Cane uses the most fertility and Canola the least (per minute). The fertility Equilibrium is the value at which the natural replenishment and the fertility usage are equal. The value depends on the crop that is currently growing, that means that in a crop rotation, the value can vary from crop to crop. For example, for Corn the fertility equilibrium is 57,9%, so when not using fertilizer, the fertility will slowly go to 57,9%
Fertility can only be above 100% when using either Fertilizer I or Fertilizer II. The maximum fertility for Fertilizer (Organic) is 100%, for Fertilizer I 120% and for Fertilizer II 140%. In order to actually use the fertilizer, the green slider in the menu of the farm needs to be slided to the desired fertility. The amount of fertilizer needed depends on the fertility used. For example, potatoes use 10,67% fertility per minute, to replenish this, 10,67 organic fertilizer or 5,34 fertilizer I or II is needed per minute. Also when having a desired fertility above 100% the natural replenishment of the farm needs to be compensated with fertilizer. This costs even more fertilizer.
Below are all the recipes, which contain this item. Note, that some of them may be locked behind Research, and not immediately available in your game.