Potty would be a small pot, for keeping the lentils. Crib is a mistranslation, I don't know German well enough to know if it's even a possible translation but the word means small crop. The ashes go into the crop because they are fertilizer. Both words, pot and crop, are in diminutive form which makes the sentence have a sing-song feel and memorable.

Crop as in "bird's throat", I think. Good goes in the kitchen, bad goes in the pigeon.

Good goes in the pot, bad goes in the crop. (i.e. keep the good, toss the bad)