The plant grows from the womb of the land, it's roots deeply embedded in the safety of soil.
Twenty bits are required in order to fill a Plant Bowl. Although they can also be filled using dirt from any farmer's field, plants growing in fertile dirt will receive bonus growth checks throughout their lifetime.
You may find fertile dirt as loot on the following creatures:
Fertile dirt is most easily obtained from NPC vendors called Mystics in the Royal City in Ter Mur for a base price of 2 gold each. It is also occasionally brought to the surface when Green Thorns are planted in normal dirt tiles.
Originally appearing in Ultima VIII: Pagan as a Necromancy reagent, it was to have been used in Ultima Online as a reagent for the (originally canceled) Necromancy system.