In the battle against weeds, tillage is one of the strongest weapons organic or ecologically based farmers have. But, a new study shows that, depending on when it is used, tillage can also be a strong driver of nitrogen losses that contribute to groundwater pollution.
“We know that organic farming relies a good deal on tillage to manage weeds and to incorporate manure and cover crops into soils, and our research shows that this practice can pose environmental tradeoffs,” says Denise Finney, a postdoctoral scholar at Penn State.
“Although it helps to reduce the use of chemicals, tillage—especially fall tillage—is an important driver of nitrogen dynamics and has potential environmental implications.”
In a study that spanned five years, researchers conducted intensive nitrogen monitoring in four different cropping systems designed to allow comparisons of soil nitrogen levels over time and under different organic management practices. The findings may help growers make decisions that will reduce their nitrogen losses.
Their results will be published in the December 2015 issue of Ecological Applications, and may help growers make decisions that will reduce their nitrogen losses.
“Nitrogen is complicated,” Finney says. “It’s affected by variables we can’t control, like temperature and moisture, and by management decisions that we make. Though we know a good deal about the impacts of these different variables on nitrogen availability and potential losses singularly, we wanted to understand how they interact in the field, particularly looking at organically managed systems.”
The researchers carried out their field-based experiment by implementing four cropping systems designed to replicate typical Pennsylvania organic feed and forage production systems. They differed from each other in terms of the cash crops and cover crops—unnharvested crops planted to provide benefits such as improved soil quality and weed control—that were grown, the sequence in which they were grown, the timing and intensity of tillage operations and manure inputs.
When to Till
The researchers planted the first system with a sequence of cover crops during the first growing season and this system was conventionally tilled between each planting. For the next two years, they kept this system in a minimally tilled alfalfa cash crop.
The second cropping system received a manure application and then a summer planting of sudangrass—a cover crop—that was tilled and followed by a fall fallow period. Like the first system, this one also was in minimally tilled alfalfa for the next two years.
