[SkeuomorphicBee]

Nitrogen can come from the air by using nitrogen fixing crops in a rotation, so it can never run out (N2 is the most abundant gas on the atmosphere). The problem is that growing nitrogen fixers take a lot of land time that could otherwise be growing crops. Also not as nitrogen rich as fossil fertilizers (gas N2 is quite stable, difficult to fixers to extract in high quantities).

I know nothing about P and K though.

[raxxorraxor]

P is mainly manure I believe. It is residual in rocks but I don't know if plants can use that.

Ash is a good source for K if there are no other K-rich fermented dead plants available.

You get a lot of both from coffee grounds so someone might have to drink a lot of coffee.

[shanusmagnus]

Number me in the coalition of the willing.