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 A University of Illinois study reveals soil phosphorus
concentrations can vary greatly across a single field
translating to sub-field variability of dissolved phosphorus, a
readily available form promoting algal production, in tile
(artificial subsurface drainage) water.
“We monitored nutrient concentrations in water of 36 parallel
tile laterals at our on-farm study site and found that those
laterals located under large closed topographic depressions (low
spots in fields that are prone to ponding) generally transport
water with higher dissolved phosphorus concentrations,” says
Luis Andino, graduate student in the Department of Natural
Resources and Environmental Sciences (NRES) at Illinois and lead
author on the study.
Andino says closed depressions accumulate soil phosphorus in
excess of the level needed for optimal corn and soybean
production in Central Illinois.
“Closed depressions seem to accumulate soil-bound phosphorus
because heavy rains transport surface soil particles to these
low areas through erosion,” Andino says. “On top of that, crop
removal of phosphorus can be greatly reduced in these areas.
Often, the crop is stunted and sometimes completely drowned out
by standing water, limiting their ability to take up phosphorus.
“Some farmers install surface inlets, but that would likely lead
to even more phosphorus in tile water because these devices
create a direct pathway between the soil surface and tile
drains, allowing water and phosphorus to bypass the soil
matrix,” he adds. “We have not yet evaluated the impact of
surface inlets on water quality in our study site.”
The team used digital terrain models to investigate the
influence of microtopography on soil nutrient accumulation and
associated impacts on tile water quality.
“We found a statistically significant relationship between
depressions and tile phosphorus concentrations in the
non-growing season. Although we assume preferential flow of
dissolved phosphorus through the soil is driving this pattern,
anaerobic conditions in these temporary ponds may also help to
liberate more dissolved phosphorus, further exacerbating the
problem,” says Jennifer Fraterrigo, associate professor in the
Department of NRES and lead scientist on the project.
Lowell Gentry, principal research specialist in agriculture in
NRES and study site manager, says, “The biggest surprise of this
study was finding an area of the field that had extremely high
soil phosphorus concentrations (up to 347 pounds per acre) and
realizing that this area had been an old farmstead years ago. We
found an aerial view of the old farmstead from 1940 showing a
house, three barns, and a pasture, so we sampled the soil every
20 feet across the old farmstead to understand the extent of the
elevated soil phosphorus.”
The team found the highest soil phosphorus was near the three
barns, suggesting animal manure was the most likely source. They
also found a property map from 1950 showing the farm was gone by
that time.
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Collectively, this information explained why the
three tiles installed across the old farmstead had the highest
phosphorus in the study.
“The tiles were installed in late 1970, so without
any additional manure since at least 1950, we can conclude that the
phosphorus from this hotspot will continue to be slowly released to
the tiles for years to come,” Gentry adds.
The good news from the study is that tile phosphorus loads were
quite small from this farm, with losses on the order of 0.1 pound of
dissolved phosphorus per acre per year. When averaged across the 36
tiles and accounting for the differences in drainage area, dissolved
phosphorus in tiles represented a contribution of about 14% to the
load of dissolved phosphorus in the nearby Embarras River.
“Our study highlights the need to identify areas with the greatest
risk of subsurface phosphorus losses to implement sub‐field-scale
nutrient management practices such as variable-rate phosphorus
application,” Gentry says. “Thus, accurate soil phosphorus mapping
across a given field is critical for the efficient use of
fertilizers and to avoid overapplication, especially when relying on
phosphorus from animal manure.”
The Illinois Agronomy Handbook recommends farmers test their soil’s
phosphorus levels regularly – at least every 4 years – to determine
how much of the fertilizer is needed to build up and maintain a
level of sufficiency, with annual additions based on the amount of
phosphorus removed from the field with grain harvest.
The article, “Closed depressions and soil phosphorus influence
subsurface phosphorus losses in a tile‐drained field in Illinois,”
is published in the Journal of Environmental Quality [DOI:
10.1002/jeq2.20120], and is available online through open access.
Authors include Luis Andino, Lowell Gentry, and Jennifer Fraterrigo.
The research is funded by the Illinois Nutrient Research and
Education Council, which is supported by a research fee based on
tonnage of fertilizer sold annually. Also, partial support was
provided by the Foundation for Food and Agriculture Research (grant
number 534655) and the 4R Research Fund (IPNI-2017-USA-4RF01).
The Department of Natural Resources and Environmental Sciences is in
the College of Agricultural, Consumer and Environmental Sciences at
the University of Illinois.
[Sources: Luis Andino, Jennifer
Fraterrigo,
Lowell Gentry]
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