Requirements
Level 22 Mining / Pickaxe
Getting There
Note; This is currently the fastest way i've found, if there is a quicker way pop a comment and i'll be sure to update the guide.
Teleport to Ammonite crabs from the home teleporter.
From there you'll want to follow the red path to the mushroom tree then select verdant vally
Simply run east to find white ash piles you get (3) per successful mine.
Volcanic ash Uses
Fertile Soil spell ( level 83 magic, lunar spellbook ) This takes 2 volcanic ash from your inventory.
Making ultracompost
This can be done by either using volcanic ash on supercompost, or by adding up to 25 volcanic ash into a compost bin which has super compost in.
You can find more farming tips by checking out SilverNova's guide here;
If i missed anything or have any tips on how to make it more user friendly leave a comment