A (smelly) question?

As a beginner veggie/vegan of 30 years the comments of the last few days about being veggie as being the way forward for the planet raises some interesting questions when taken in consideration of the carbon argument. It is simply this most fertilisers come about now, or so I am given to believe, by chemical means - e.g. oil is involved.
If the chemistry were taken out of it flatulent animals (pointed out by some as a major source of methane etc.) would be needed to fertilise the earth along with plant rotations of peas etc, and plants which tie chemicals and can be used as green fertilisers.
Given that places like Edinboro' and other cities were rich sources of night soil and fed the growing populations of the new developments in the 19th etc. centuries - as in China and other places - how would many regard this as an acceptable answer to future problems? Yorkshire Bounty was very popular, I am told, but was stopped because of issues relating to this.
I am asking because if we have a basic argument, put by Singer and others, that approx. 5 K of corn produces 1 K of meat, we cannot produce cattle purely for fertiliser because the equation is out of balance. How do we meet the long-term needs of the earth? C****P can you answer please given your background?
Peter