A little over a year ago, the Minnesota agriculture commissioner signed the Groundwater Protection Rule into law, with the goal of minimizing potential sources of nitrate pollution to Minnesota groundwater.
One impact of this rule for farmers and agronomists is a restriction on the application of nitrogen fertilizer in the fall and on frozen soils in certain areas of the state starting Sept. 1.
These areas are mostly in the north-central, southeast and southwest parts of the state. There also are smaller areas in other locations. Roughly the northern third of the state is excluded from restrictions on fall nitrogen fertilizer applications.
If you farm or advise farmers with cropland in these areas, you can find more details online with this interactive map. Note on the map that there are fields covered by both purple and green shading. In both areas, applying nitrogen fertilizers in the fall and to frozen soils will be restricted.
The purple-shaded areas have characteristics that allow nitrate to easily enter groundwater, typically because the soils are sandy or gravelly, or have shallow or fractured bedrock known as karst geology.
The green-shaded area is a Drinking Water Supply Management Area (DWSMA), a land base that contributes water to a public drinking water well. The DWSMAs labeled in the green on the interactive map have had nitrate levels at or above 5.4 milligrams per liter within the past 10 years, which is the reason for the fall nitrogen fertilizer restriction. The green DWSMA areas will be updated annually as nitrate levels in public water supply wells increase or decrease.
Some frequently asked questions about the new restrictions include:
Are all nitrogen-containing fertilizers banned from being applied in the fall in these areas? No. There are specific exceptions that allow fall application of U-M recommended rates to establish winter grains or perennial crops, and when applying MAP, DAP or micronutrient formulations, up to a field-average rate of 40 pounds per acre of nitrogen in the fall (roughly 200 to 400 pounds per acre of total product). The 40-pound rate does not apply to fields with “low” or “very low” soil test phosphorus values. More exceptions can be found on the Minnesota Department of Agriculture website.
Is manure use in the fall restricted? No, the fall nitrogen fertilizer use restrictions in place under this rule apply only to commercial nitrogen fertilizers.
Under the Groundwater Protection Rule, are there other potential impacts farmers and agricultural professionals should be aware of? Yes, but only if you farm within one of the green DWSMA areas on the map. The Groundwater Protection Rule includes additional considerations for nitrogen management practices in DWSMAs with the goal of preventing public water supply well nitrate levels from exceeding 10 mg/L. These include the promotion of nitrogen fertilizer best management practices. Additional information can be found under part 2 of the rule on the MDA website.
Regulations that mandate nitrogen fertilizer BMPs or other practices are possible in these areas. However, these are long-term considerations directly tied to local water quality trends and how widely BMPs are being used.
To learn more about the fall nitrogen restrictions, you can view a pre-recorded video online or join a live hour-long webinar hosted by MDA on Aug. 12, beginning at 10 a.m.
For more in-depth reading on the details of the rule, view the full text of the Groundwater Protection Rule or the frequently asked questions page on the MDA website.
Becker and Klinger are University of Minnesota Extension educators.