1 Black Syrup Fertilizer is made from 1 Flower Fertilizer and 1 Khefre's Essence.
1 Black Syrup Fertilizer is used on each fertilization of a flower.
This fertilizer is the best you can get as it seems to equal around 12 normal fertilization hence its cost. (Does anyone know the wait time before you can fertilize again?) --It reported 2 hours to me, so I assume that it works like all the other fertilizers, except harvest elixer, of course. (Pyrite)