Leaching
Leaching is wearing down materials into finer materials by constant water flow.
How to leach
You can leach nodes by letting water flow on top of them for long periods of time. You can test if it is working by pummeling the top of the node. If particle effects appear and the node stays the same, it is most likely working.
Recipes
Raked Humus -> Dirt (Faster recipe: Let Grass spread to the Humus then remove the Grass from the Humus)