Modern horse manure, for comparison, is loaded with phosphorous, potassium and nitrogen. The nitrogen is the easiest to lose to groundwater or the air.
Phosphorus, on the other hand, can be held in place by calcium and iron, says Capo, a geologist who did the soil analyses with Michael Rosenmeier and undergraduates Andy Stiff and James Gardiner of the University of Pittsburgh.
"High phosphorous could also indicate human occupation," said Capo, "but that's usually accompanied by other geochemical signatures, which we didn't find in the corral samples."
There was also high sodium concentration in the corral samples, which could be from urine, suggested Olsen.
The real smoking gun, said Olsen, will be if they can detect long-lived molecules of lipids, or fat, in these samples that can be attributed specifically to horses. That analysis is now being arranged.
So what were the Botai doing with those horses? They probably ate them and used them as pack animals, and they may have milked the mares to create a vitamin-rich, mildly alcoholic beverage that's still consumed today in Kazakhstan, said Olsen.
