A Crux is a block that is used to allow to grow. Each tier 6 crop has its own Crux.
Place the Crux below the farmland that the crop is planted on.
Nether Ores
End Ores
Tier 6 Crops
Cruxes
Insanium Essence
Essence Coal
Essence Apples
Essence Paxels
Withering Soul
Dragon Scale