The bottomless compost bucket is an item obtained as a drop from the Hespori. It can be filled with compost, supercompost, or ultracompost. It can be stored at any Tool Leprechaun.
The bottomless bucket can be filled with up to 10,000 uses of one type of compost. It can be filled from a compost bin or from other buckets of compost (noted ones work as well). Each bucket of compost added to the bottomless bucket counts as two uses, so 5,000 buckets of compost are required to fill the bottomless bucket to maximum. This effectively doubles the use of each compost that is added to the bucket. This means that, if used exclusively for ultracompost, the bottomless compost bucket will pay for itself after fertilizing 358 farming patches.
The bottomless compost bucket is tradeable only when empty. Players may use a right-click option to empty the bucket, though this will cause all the compost stored in it to be lost.
For an exhaustive list of all known sources for this item, see here