[While leading an expedition in the Tsangpo River Valley of Eastern Tibet]Dirt, The Erosion of Civilizations by David R. Montgomery
Looking for outcrops of ancient lakebeds we drove through villages full of chickens, yaks, and pigs. All around the towns, low silt walls trapped soil in fields of barley, peas, and yellow flowers with seeds rich in canola oil.
After a few days it became obvious that corralling dirt was only part of the secret behind ten centuries of farming the lakebed. Following an unsupervised daily rhythm, Tibetan livestock head out to the fields during the day, fend for themselves, and come home at night. Driving back through towns at the end of each day's fieldwork, we saw pigs and cattle waiting patiently to reenter family compounds. These self-propelled manure dispensers were prolific; even a brief rain turned fields and roads to flowing brown muck.
The night after finding the remains of the glacial dam that once impounded the lake, we stayed at a cheap hotel ... The proprietor advised us on our way in that the backyard would serve as our bathroom. That the pigs clean up the yard bothered me during our pork dinner. Still I had to appreciate the efficiency of pigs eating waste and fertilizing the soil, and then people eating both crops and pigs.
Overlooking the obvious public health issues, this system sustained soil fertility.