Composting

Ultracompost is the best compost; once supercompost is ready to harvest, add volcanic ash (mined on Fossil Island) to the opened compost bin (after produce has matured into supercompost) to convert the whole bin to ultracompost. The Farming Guild has a big compost bin which takes 50 volcanic ashes to convert, a single supercompost bucket needs 2 volcanic ash to become ultracompost--converting whole bins is more effective.

Bushes, once grown, continue to yield berries which you can pick repeatedly. This is a convenient source of super compostable produce; one convenient bush patch is by the Ardougne Monastery--the Ardougne cape has a teleport to there and the Ardougne farm patch, for example. Idea, start farm runs by getting white berries and then fill compost bins along the way. The Farming Guild has a bush patch right next to a compost bin. White berries are supercompostable and also the secondary ingredient for cadantine potions to make super defense potions.