Re: There is plenty of both.... we just have to get over the..
"Instead of sending it through the sewer system into the lakes and rivers, it needs to get sprayed on the fields along with the animal manure. Rain runoff can be minimized by proper tilling of the land with an eye to the direction of the furrows.
Voila, ALL the NATURAL ORGANIC fertilizer one could ever want and Dupont and Monsanto (or other chemical companies) aren't involved. No anhydrous ammonia, no phosphate mining."
Mammals do not fix nitrogen. We just don't. Not pigs, not cows, not humans.
All the nitrogen in our poo comes from nitrogen in our diet. We do not fix nitrogen. We just pass it along through our digestive tracts. Animal manure, human or otherwise, is not a SOURCE of nitrogen, it's a CARRIER of nitrogen.
We, like cows and pigs and any other mammals, have nitrogen in our waste from the nitrogen we ingest in our food that has been fixed somewhere else. That "somewhere else" is either chemical fertilizer or from rhizomes, filamentous bacteria, or to a lesser extent some other bacteria.
I've read at least one report that states the total amount of crop-available biological nitrogen fixation on earth does not meet the total amount of nitrogen we need to grow food for the entire population. That leaves chemical fertilizers.
If we apply chemical fertilizers to plants that are grown for animal consumption, then feed those plants to the animals and use the animal's poo to grow other crops, we've done what one bloke I know calls "nitrogen laundering"--but I trust you can understand why getting your nitrogen to grow animal feed from animal poo doesn't actually work, given that animals do not actually fix nitrogen.