Douglas Campbell wrote:
We could probbably do a bit better now, but those numbers imply a large population drop if we go without ferilizer.
If all the nutrients from our excreta could be conserved and returned to the soil that is used to produce our food, it would be enough to produce the food to feed those same humans. This is simply the conservation of matter. Matter in, matter out, matter back in. It is a cycle like the water cycle or the oxygen cycle. Some nitrogen is lost to the air, but some nitrogen is fixed by plants and soil organisms. The rest of the nutrients are largely earth-bound, from what I understand, so they would largely remain in this cycle. Unfortunately your 8 billion humans currently turn almost all of their excreta into waste, breaking the cycle and necessitating artificial fertilisers.
But the original question of this post is intriguing. Is using dilute urine or compost leachate as a fertiliser much the same as using soluble artificial fertilisers? Is the divide really between soluble fertilisers and those that are combined with solid organic matter? Or between organic and artifically produced? Or something else?
My feeling, but I think it's just a hunch, is that soil containing plenty of healthy organic matter and soil organisms can handle, store and help plants utilise inputs of soluble fertiliser. But I don't know how true this really is.