Climate Change Impacts Phosphorus Loss From Tile-Drained Farm Fields

What is tile drainage and why do farmers use it? Tile drains are perforated pipes buried three to four feet deep to drain water from cultivated land to a main ditch. In the past, farmers used clay tile for drains, but today they use plastic pipes buried in rows about 25 feet apart. Tile drainage lowers saturated soil to below root depth. Roots then grow deeper, become healthier, and can reach water during dry summers. 

Tile drainage improves crop production and decreases year-to-year variability in harvests. In Ohio, corn and soybean yield increased 30 percent over 25 years with installation of tile drains. Well-drained land allows farmers to work fields sooner in spring and later in fall, which means they can plant earlier and harvest later in the season.

Climate change has resulted in increased precipitation in the northeastern United States over the past 100 years. In just the last 30 years, annual precipitation has risen by nine inches in Vermont’s Northeast Kingdom and by seven inches in western areas of the state. 

“Excess moisture is the biggest reason for crop failure in Vermont, and tile drainage is the most important tool in the farmer’s toolbox to adapt to increasing precipitation,” said Joshua Faulkner, a researcher at University of Vermont (UVM). “Even historically, managing drainage was important in the wet climate of the state.”

Although tile drainage reduces surface stormwater runoff and soil erosion during precipitation events, its increasing use has raised concern about the potential for high phosphorus export from the tiles—an issue also gaining widespread attention in Ohio. Farmers apply phosphorus and other nutrients, often in the form of manure, to croplands to increase soil fertility, but phosphorus is easily transported from soil to surface waters in stormwater runoff, which results in pollution and reduced water quality.

In the Lake Champlain basin, 38 percent of phosphorus loading to the lake is attributed to agriculture, and rainfall and climate trends suggest more extreme periods of wetness and drought in the future. Tile drainage decreases surface runoff and erosion, which is beneficial, but what impact does it have on phosphorus export from farm fields in Vermont? 

Farm field study

With research support from Lake Champlain Sea Grant, University of Vermont (UVM) graduate student Ryan Ruggiero and faculty Donald Ross and Joshua Faulkner published a study in 2022 that investigated the impacts of tile drainage on phosphorus export in Addison County, Vermont. Researchers measured rainfall, surface water and tile drainage flow, and nutrient concentration from two intensive corn silage production fields year-round from 2019 through 2021.

Tile drainage was the dominant phosphorus export pathway at both sites, while soil surface runoff did not contribute to significant phosphorus loss. Phosphorus export from the tile drains was greatest during the non-growing seasons in early spring and later fall when there were more and heavier rain events. 

Of the 60 to 100 pounds per acre of phosphorus that farmers apply to their fields, 1 to 3 percent usually leaves the fields in stormwater runoff or drainage. In this study, researchers found phosphorus export up to 1.4 pounds per acre per year, which matches the 1 to 3 percent exported from other farms. Almost all—96 percent—of the total phosphorus load coming out of the drains occurred during rain events. 

Peak phosphorus concentrations in the tile drainage occurred during rain events directly following manure injection in the fall, after herbicide application to remove cover crops and after fertilizer application at the time of corn planting in the spring. During drier periods, the low stormwater flow moved insignificant phosphorus. 

The researchers confirmed that flow rate, not phosphorus concentrations in the soil, is what drives overall phosphorus losses from the fields in conjunction with phosphorus application timing. When farmers inject manure into their soils during drought periods, storm events may then cause increased phosphorus losses. 

Legacy phosphorus, or phosphorus that has accumulated in the soil from long-term application of fertilizers and manure, increases the difficulty of managing agricultural runoff. Despite this, manure application continues to be the most widespread method to improve soil health by building soil organic matter and increasing nitrogen, a nutrient necessary for crop production.

“Our findings, which provide a better understanding of phosphorus transport to tile drainage in the Lake Champlain basin, will help farmers adapt their nutrient management decisions to the impacts of climate change,” said Faulkner, a research assistant professor in UVM Extension, who connects farmers with research findings on soil, water, and nutrient related issues across Vermont. “Manure injection on cracking clay soils during very dry periods could be risky if the injection is followed by a storm event.”