Salem

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

put worms with higher purity in your bins.

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.

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.