They pee on it.

http://fertsmart.dairyingfortomorrow.com.au/wp-content/uploa...

(Image hotlinked from https://fertsmart.dairyingfortomorrow.com.au/dairy-soils-and...)

Yeah but where is that nitrogen coming from? It's coming from plants.

Yeah, hence the “Nitrogen Cycle”.

Yes but they do not actually add nitrogen to the soil

Sure they do, they add nitrogen back to the soil that the plants extracted from the soil.

Poop and pee.

Nitrogen that they get from plants