Who put the P in the Pecatonica? Just a handful of people, it turns out. A large share of the phosphorus that’s contaminating this picturesque southwestern Wisconsin stream is likely coming from a relatively small number of farm fields, a recent analysis finds.

A study of one sub-watershed suggests that about 60 percent of the phosphorus flowing into the river comes from just 12 percent of the surrounding farmland. Ten of the area’s 61 farms account for most of the problem spots, leading researchers to think that changing practices in a small number of places could make a big difference in water quality.

That approach is being put to the test in the Pecatonica by a partnership that includes university researchers, public agencies, farm groups and The Nature Conservancy. The strategy was outlined in a 2005 report by a policy advisory group called the Wisconsin Buffer Initiative, which advised using UW-Madison-derived models to identify areas that had the greatest likelihood of nutrient runoff—and the best chance of fixing the problem.

“We’re testing the idea that if you go into a watershed, identify the high phosphorus-loss fields and change management on those fields, you can decrease phosphorus at the mouth of the watershed,” says Laura Good, a CALS soil scientist who rated fields for their potential to shed nutrients into the river.

With the problem fields identified, a team of farm-management specialists and conservationists is beginning to work with farmers to modify cropping practices. “We’re trying to bring a whole-farm management perspective, looking at the impact of implementing these best management practices and trying to work it all the way through ,” says Tom Cox, a professor of agricultural and applied economics.

These farmers aren’t bad actors, says Pete Nowak, a professor in UW-Madison’s Nelson Institute for Environmental Studies who directs the buffer initiative. In most cases they are already employing many sound conservation practices, but the choices aren’t always appropriate on vulnerable fields, he says.

“The traditional approach is to come in with a technical manual and say you have to do this,” says Nowak. “Our approach is to ask, ‘Why are you doing this?’ and then work with them.”

Usually the reasons farmers cite for particular practices are economic. A farmer might grow corn silage, a crop that leaves the field bare at harvest, because it’s needed for dairy rations.

“We’re looking for the win-win in terms of environmental and economic performance,” says Cox. “We think it’s possible, but it’s not a cookie-cutter solution. This won’t work without working through the potential spillover aspects of the change onto the whole farm operation.”