Placing dirt/rock needs at least 3 units. Below you get the message "Not enough XY".
This check triggers also on any stack in inventory no matter how much dirt or earth is equiped. It's really confusing since the stack actually "in use" does have enough units. The check also makes no difference between dirt and rock so even when I'm placing rock a stack of 2 dirt _somewhere_ in the inventory will prevent this.
Steps to reproduce:
* Equip a stack of dirt to slow 1
* Add a stack with just 2 units of dirt or rock somewhere else in the inventory
* Select key 1 and try to place dirt
* Message informs you not having enough whatsoever
* Dump the stack with just 2 units from inventory
* Now you can proceed placing dirt