Fertile Soil is a Lunar spell that can treat a target farming patch with compost. By default, this spell applies Supercompost to the farming patch. By casting the spell with 2 Volcanic ash in your inventory, the spell will instead apply Ultracompost. In order to apply Ultracompost, the player has to have read the Ash covered tome which can be bought from Petrified Pete's Ore Shop for 40,000 points.

This spell can be applied to an empty patch or one that is currently growing - but not a fully grown patch. It is best to use this immediately before or after planting to gain the best results.

A Magic level of 83 is required to cast this spell, giving 87 experience. It requires 2 Nature runes, 3 Astral runes, and 15 Earth runes to cast.