PIERZ, Minn. (AP) — Farmers in parts of Minnesota face new regulations starting Tuesday on how they apply nitrogen fertilizer. The Groundwater Protection Rule, developed by the Minnesota Department of Agriculture, took effect in June to tackle nitrate contamination in the state’s groundwater. But the restrictions on when farmers in specific areas can apply fertilizer began Tuesday. Minnesota Public Radio News reports that the restrictions prohibit farmers from applying nitrogen fertilizer in the fall or on frozen ground, in parts of Minnesota where the groundwater is vulnerable to contamination. The regulation applies to nearly 12% of the state’s cropland, mainly in the central and southeastern parts of the state.