[Dave Wright]

Question: Where does the nitrogen in manure come from? Answer: From the grain and feedstock the animals eat.
Question: Where does the nitrogen in grain and feedstock come from?
Answer: From chemical fertilizers that make agriculture abundant and economical enough to support billions of humans and domesticated animals.

[Mr. Lucky]

Grain isn't a source of nitrogen; Leguminous plants can be, but the air an animal breathes is the biggest source.

[cymbeline]

“From chemical fertilizers that make agriculture abundant and economical enough to ...”

All’s good as long as we’re not dependent on faraway countries for the petroleum used to make the fertilizers.

I’d say further that our farms don’t need to be supplying the rest of the world with their products.