https://phys.org/news/2018-10-fertilizers-impact-soil-health.html

In the summer of 2015, they collected soil samples at a variety of depths using a push probe auger. Then they analyzed the samples.

Ozlu and his team concluded that long-term annual application of manure improved most soil quality properties compared to inorganic fertilizer. "Increased electrical conductivity is one of the few negative impacts of manure," Ozlu said.