In the past, people used their own manure and that of animals to fertilize fields. Manure contains lots of phosphorus as animals actually absorb only a small pe

In the past, people used their own manure and that of animals to fertilize fields. Manure contains lots of phosphorus as animals actually absorb only a small percentage from nutrition. Urbanization broke this cycle. Sewer systems got established and the phosphorus doesn't end up on the fields anymore. It eventually reaches the rivers and seas and passes into the sediment. What a waste (pun intended). Instead, people started to mine graveyards (for bones) and guano on remote pacific islands... Edit: Urine might work even better because reprocessing it is way simpler than dealing with manure. The nitrate and sodium levels have to be adjusted. Also, excreted pharmaceuticals have to be neutralized. Meanwhile, with manure one has to deal with pathogens, eggs of parasitic worms and a host of other ingredients, which might be difficult to predict in urban settings.