December 2020 - 22

How and why to keep phosphorus on no-till fields
By Alayna DeMartini
Ohio State University
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 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 soilbound
phosphorus in our streams, rivers,
and lakes by not tilling fields, but we
have also seen an increase in our soluble
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 Ohio State University
College of Food, Agricultural, and Environmental Sciences. Photo: Ohio State University
phosphorus going into those waterways, "
Dick said.
Dick was among the speakers at the
Conservation Tillage and Technology
Conference, 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. " VGN
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December 2020

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