Sack
An Empty Sand Bag can be filled with dirt to create a Dirt Bag. A dirt bag is used to spread dirt over a tile, allowing the tile to be tilled for farming. A sand bag can be filled by right-clicking a patch of grass with both the Empty Sand Bag and a Shovel in the player's inventory, and using the action "Take Dirt > Empty Sand Bag".
There seems to be some difficulty with this function as several players are reporting it doesn't work.