Re: There is plenty of both.... we just have to get over the..
You're right that we don't effectively recycle nightsoil. In typical sewage treatment we allow a succession of aerobic microbes to convert all the nitrogenous compounds into elemental nitrogen that is lost into the atmosphere. Pure water comes out of our predominantly aerobic sewage treatment plants, separated from 'sewage sludge' that contains a variety of simple minerals, minus the nitrogen. Sewage sludge may get sprayed on fields to replace other plant nutrients including phosphorus, but it doesn't contribute nitrogen. Unfortunately it tends to contain toxic chemicals, particularly 'heavy metals', because we put more than shit and piss in our sewers. Unsurprisingly farmers, and their advisors, are reluctant to use sewage sludge.
In an ideal world we'd collect shit and piss separately and treat it in a way that killed all the pathogens*, then spray it on our fields with the nitrogenous compounds intact. However, soil microbes would still release some of the nitrogen before our crops could use it. So we'd still need to use the Haber process (invented pre-1914). Of course, we could ditch agriculture and go back to a sustainable population of hunter gatherers. That's probably about 10 million people:) If we want to keep feeding the current, growing population, in a habitable world, we need to discover how to manufacture synthetic food, without using biological organisms. That's because we're not maufacturing any more land for agriculture. Maybe we could get the energy input well below 10:1 for food calories.
*AFAIK I've only caught Ascaris lumbricoides from nightsoil. YMMV^H^H^Hour infection might be worse.