Ultracompost is a more potent version of supercompost and is used in conjunction with the Farming skill. Ultracompost reduces the chance of a players crops becoming diseased (1:10 ratio) whilst growing, as well as increasing the number of crops that will be received by the player upon harvest, even if those crops are protected. The minimum number of herbs increases from five to six (when compared to supercompost), and in general, the yield will be higher, around 20% higher on average. The combined effects increase expected yield per patch to about 8.6 herbs at high Farming levels.

To make ultracompost, a player must use two volcanic ashes on a bucket of supercompost. Alternatively, 25 volcanic ashes can be used directly on a full compost bin containing supercompost in order to upgrade it to ultracompost. Each bucket of ultracompost collected by the player from a compost bin will grant that player 10 Farming experience. When used on a farming patch, each bucket will grant 36 Farming experience.

The Lunar spell Fertile Soil can be used on a patch to give the same effect as ultracompost after the player has read the ash covered tome, if the player has two volcanic ash in their inventory. Without the volcanic ash upgrade, the spell Fertile Soil normally acts as supercompost.