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Study: Variability in Subsurface P Loss Across Crop Fields
Illinois Ag Connection - 08/12/2020

Conventional agriculture relies on the application of phosphorus from fertilizers or animal manure to deliver adequate amounts of the essential element to growing crops for maximum productivity. However, unintended losses of phosphorus from fields to water bodies can impact aquatic ecosystems.

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. 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.

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 and the 4R Research Fund.


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