How and why to keep phosphorus on no-till fields
MARCH 9, 2020
ADA, Ohio—Left untilled, fields gain organic matter and maintain high yields, but there’s a tradeoff to consider when deciding not to till.
Fields that aren’t tilled are less likely to erode, sending soil and the components of fertilizer, including phosphorus, downstream, a threat to water quality.
However, when rain runs off a field that’s not been tilled, that rain is more likely to carry with it phosphorus in a form that can be readily available to produce algal blooms downstream, said Warren Dick, a retired soil scientist from The Ohio State University College of Food, Agricultural, and Environmental Sciences (CFAES).
That’s because fertilizer in a no-till system is not mixed into the soil and often stays at the surface, making it more vulnerable to running off with rainwater.
A major cause of the harmful algal blooms plaguing Lake Erie every summer is phosphorus that runs off farm fields with rainwater. Phosphorus is most harmful to water quality when it’s in its dissolved form—not when it’s attached to soil particles such as clay. In its soluble form, phosphorus can be consumed by algae, which encourages their growth.
“We have reduced the amount of soil-bound phosphorus in our streams, rivers, and lakes by not tilling fields, but we have also seen an increase in our soluble phosphorus going into those waterways,” Dick said.
Dick was among the speakers at the Conservation Tillage and Technology Conference March 3–4, an annual event sponsored in part by CFAES at Ohio Northern University in Ada.
If fertilizer is tilled under the surface, the phosphorus in that fertilizer has a chance to bind with soil particles, so that phosphorus is not as likely to be available to algae downstream. As a result, some experts recommend shallow tillage of fields so that the phosphorus applied to those fields can bind to soil particles. But that destroys the soil quality benefits of not tilling a field.
Another way to keep more phosphorus on fields, especially those that are not tilled, is to apply gypsum to the field, which binds with phosphorus in the soil and keeps the phosphorus from running off, Dick said.
“It’s a tool,” Dick said of gypsum. “But like anything else, it has to be properly used. There’s no one technology that’s going to solve the issue of phosphorus runoff.”
Besides using gypsum on a no-till field, farmers can try injecting their fertilizer a couple of inches below the surface rather than at the surface. Planting cover crops also can help because cover crops take in rainwater and keep some of it from running off a field, Dick said.
“Sometimes one of those or all three of them working together is the best option,” Dick said.
Even if not tilling a field might put it at risk of soluble phosphorus running off that field, no-till fields can require less phosphorus fertilizer because not tilling a field vastly improves the health of the soil, said Steve Culman, a soil fertility specialist with CFAES.
“We’re trying to build soil fertility. We believe, ultimately, no-till is going to reduce the need for fertilizer,” said Culman, who also was a speaker at the conference.
“We have to understand that sometimes tradeoffs exist. No-till brings a long list of environmental benefits, and a single tillage event can undo what’s taken years or even decades to build.”
Every time a field is tilled, carbon in the soil gets released as a gas into the environment. By leaving a field untilled, more carbon stays in the soil, improving soil health and decreasing the need for fertilizer.
How a fertilizer is applied to a field, rather than how much fertilizer is applied, might have greater bearing on whether the components of that fertilizer run off the field with rainstorms, Culman said.
In Ohio, fertilizer is primarily applied at the surface rather than incorporated into the soil, he said.
“And we know that sets us up for greater risk of phosphorus running off fields.”
Applying fertilizer with a planter that deposits the fertilizer under the surface can go a long way toward keeping the components of that fertilizer on the field and out of waterways, Culman said.
“That’s one thing we feel pretty sure about, in terms of what would make a big impact on improving water quality.”
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