Dirt is a simpler way of saying earth or soil. Potioneers use it to create a garden in order to grow plants. Dirt in Potioneer comes in the form of small cubes, coloured and textured brown. If you put enough dirt in one spot, say in a pot or planter cell, you can drop a plant seed in it to make the seed grow into a plant.
In order to provide smooth performance, Potioneer will expire dirt cubes which have been sitting out in the open, not being used, after a period of time. This prevents the player from accidentally triggering a scenario where the computer spends more time thinking about dirt cubes than it does about the rest of the game!
Any dirt cube which has been picked up will reset the decay time.