Many farms spread human waste on cropland to fertilize it. In this case, the waste is called "biosolids". It can carry household chemicals that affect important bacteria, and that can hurt soil health.
The government has had a hard time regulating chemicals in biosolids, because the equipment that measured bacterial gases was very expensive.
But a new report from Duke University's school of engineering shows that bacterial reactions to chemicals can be assessed by changes in color. That's a cheaper test to administer.