Organic fertilizer is a new item in Elona+ 1.18. It can be used [ ] on a crop which 1) has budded but isn't yet fully grown, and 2) is planted on "field" terrain (trying it on crops planted elsewhere will tell you that there isn't any crop there). This will cause the crop to instantly become full grown, and for vegetables and fruits will increase the quality of the harvested item.
The item can be created by leaving a rottable/rotten piece of food or a certain junk items (dead fish, shit, garbage, straw) on the "compost" terrain found in a farm; it can take a great deal of time before the change happens. Note that the compost terrain cannot be selected from the terrain menu of design mode, nor can be copied by right-clicking on an already existing compost. This means that if you created your farm in a pre-1.18 version and then upgraded to 1.18, or if you replaced all of the compost terrain with other terrain via design mode, the only way to get compost heaps is to reset the farm, which will cause all plants and all items in the farm to disappear.