When runoff flows from farm fields into the Lake Erie Basin, phosphorus in that runoff contributes to the algal blooms that can contaminate drinking water supplies. Surface runoff is generally considered to be the most significant source of that phosphorus. But studies by two U.S. Department of Agriculture (USDA) scientists show that underground tile drains—commonly used by Midwestern farmers to drain excess water from crop fields—are also major contributors of phosphorus.
Since 2008, Doug Smith, a soil scientist at the Agricultural Research Service (ARS) Grassland, Soil and Water Research Laboratory in Temple, Texas, has been monitoring phosphorus in surface runoff and tile drainage from farm fields in the St. Joseph River Watershed in northeast Indiana, which is part of the larger Lake Erie Watershed. Between 2008 and 2013, he found that 49 percent of dissolved phosphorus and 48 percent of total phosphorus in the watershed was discharged via tile drains.
From 2005 to 2012, Kevin King, an agricultural engineer at the ARS Soil Drainage Research Unit in Columbus, Ohio, monitored phosphorus levels in the discharge from six tile drains and the outlet of a headwater watershed in central Ohio. He found that tile drains contributed 47 percent of the phosphorus discharge.
Farmers in the region are generally careful to apply only as much fertilizer as needed, and King’s measurements indicated that only around 2 percent of that phosphorus was lost through runoff. But phosphorus concentrations in the tile drainage and the watershed discharge often exceed concentrations recommended for preventing algal blooms, the researchers say. King’s team concluded that reducing phosphorus losses will require practices that mitigate losses via tile drainage in the late fall, winter and early spring, when most of the phosphorus loading occurs
The findings support efforts to reduce phosphorus levels in the Lake Erie Watershed by highlighting the importance of managing nutrient losses in both surface runoff and the amounts transported to tile drains. The U.S. Environmental Protection Agency has announced a goal of reducing phosphorus fertilizer runoff into the Great Lakes by more than 1,400 tons by 2019.
Read more about this work in the January 2015 issue of Agricultural Research magazine.