Compost Bin

[Limrog]

Can anyone explain me how to raise the purity of my humus?

[DarkNacht]

put worms with higher purity in your bins.

[Lusewing]

and in order to raise the purity of your worms you will need to put them in a box with 'any plant'. Over time they will eat the food and gain 10% of its purity.

[lachlaan]

As a more long-run explanation as to the purity mechanics, your best bet at continuous un-stunted growth for plants is cabbage. FIelds are the only things with a random factor involved in the outcome, so only cabbage can be fed to worms that can also increase past any old max values pots might've had.