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Across much of the Upper Mississippi River Basin, improving water quality will require reducing nutrient runoff from agriculture. Phosphorus is a valuable and scarce nutrient that helps make farmland productive, yet its loss into streams and lakes is a waste of resources that has impaired water quality.
In a recent study, Wisconsin water quality experts told FEWscapes researchers that policymakers need to rethink how nutrient standards for farmland, such as the Wisconsin Phosphorus Index, connect to in-stream nutrient goals. Available data suggest that, even if all farms comply with existing standards, nutrient levels in lakes and streams would likely not meet targets for safe fishing and swimming.
These results suggest that now is the time for a conversation about what policies and practices can meet phosphorus targets in the future.
The FEWscapes study, published in the fall of 2024, explores pathways for addressing this disconnect. Report authors conducted 48 interviews with farmers, government employees, and members of the private sector in northeastern Wisconsin, a dairy production hotspot along the coast of Lake Michigan.
“Reducing phosphorus loss is a very slow-moving environmental goal,” said lead study author Adena Rissman, Professor of the Human Dimensions of Ecosystem Management at UW-Madison.
Runoff from fields today sits in the sediment at the bottom of water bodies for a long time, “affecting the ability to meet phosphorus goals several decades into the future,” Rissman said.
Study participants often pointed to the Phosphorus Index (PI), a metric that estimates phosphorus runoff from farm fields to surface water based on factors such as fertilization, weather patterns, and soil erodibility. Wisconsin PI regulations need revisiting, some said.
Wisconsin administrative code requires farms to keep their average PI below six over the course of each crop rotation, defined as a period of eight years. This amounts to roughly six pounds per acre per year of phosphorus runoff. Farms must also stay below 12 in each individual year of the rotation.
The water quality impact of a given PI value varies based on a variety of factors, such as the distance from a field to a water body as well as a water body’s size. However, higher PI values are more likely to lead to phosphorus runoff, which can contribute to algal blooms.
After the standard of six was adopted in 2010, the Wisconsin Department of Natural Resources conducted “Total Maximum Daily Load” (TMDL) research on its expected water quality impacts. TMDL analysis aims to determine how much pollution a water body can withstand and still meet water quality goals.
The models showed the PI of six may come up short – and a PI of closer to one or two may actually be what is needed to hit water quality goals.
“Now we have more modeling and, in some cases, monitoring data to show that that’s not going to be effective to improve water quality,” one water quality administrator said in the study. “There’s going to be a need to have a discussion about stricter or new targeted performance standards.”
Interviewees, whose names are excluded from the report for anonymity, described two potential approaches to revising the standard.
First, the state could transition to a lower static PI, interviewees said, either statewide or in specific watersheds. Under this approach, once the new PI thresholds are set, they would be expected to stay the same indefinitely.
The second option would replace static targets with continuous improvement, focused on farms in specific areas where runoff exceeds capacity to meet local water quality goals. In these areas, PI requirements would gradually become more stringent.
For example, regulations could decrease the maximum allowable PI value by a set percentage at the start of each crop rotation or each decade. In this case, PI requirements would continue to tighten until water quality goals are met.
However, the political feasibility of these changes may currently be low, since changing administrative rules in Wisconsin is a complex process that requires approval from the state legislature.
Some interviewees also worried about the challenges these changes might create for producers, especially since complying with existing standards is sometimes difficult already.
One challenge stems from periodic updates to the calculation tool used to determine compliance with PI standards, SnapPlus. New versions of SnapPlus are sometimes released to incorporate the latest data and science, and these changes potentially create new obligations for growers, interviewees said.
For example, updates to SnapPlus calculations that reflect a recent increase in heavy rains could result in more estimated runoff from one year to the next, even if a farmer applies the same amount of phosphorus to their land. So, even if a farmer uses the same amount of fertilizer each year, SnapPlus could indicate a higher PI value, taking into account increased rainfall.
If phosphorus standards are revised, producers and policymakers would have to consider what in-field technologies and practices can achieve them.
One route is technological. Scientific advances in manure digestion and nutrient concentration could offer new ways to reduce nutrient pollution, especially in concentrated animal feeding operations.
Nutrient concentration systems – often paired with digesters that produce methane – separate out the nutrients from raw manure into a byproduct that can then be used again as fertilizer. In other words, nutrient concentration systems allow farmers to better manage nutrients and thus divert them from water bodies.
However, adoption of this technology so far has been limited by cost and by the difficulty of selling concentrated nutrients to farmers. Some interviewees in the study were also concerned that only large producers would be capable of investing in digestion systems.
A second route to reducing phosphorus runoff would be for farmers to adopt forms of low-input perennial agriculture, such as rotational grazing systems, that generate less nutrient pollution. Some interviewees said that growing perennial agriculture will require training more technical service providers who can advise farmers on transitions to grazing.
To figure out which of these routes work for the state, Rissman says it’s important to have dialogues about policy options now.
“How can we support farmers by incentivizing the kinds of practices that are going to be beneficial economically, good for farmers’ quality of life, and beneficial for soil and water?” Rissman said.
Read the full report.