CHAMPAIGN, Ill. — A statewide effort to get to the root of soil phosphorus levels and changes over the past 120-plus years is underway by the University of Illinois and its partners.
Andrew Margenot, University of Illinois assistant professor, and other researchers are involved in a project to resample soil at 450 of the original locations on privately owned farmland in Illinois that were sampled more than a century ago.
The researchers hope to identify and contact landowners and farmers to ask permission to collect soil samples for the project.
The goal of the resampling is to understand changes that have happened in the soil over time.
“Our predecessors had the foresight to keep records of soil sampling locations and to store those samples at the university,” Margenot said at the recent Illinois Nutrient Research and Education Council Investment Insight Live event.
“By resampling soil from these same locations, we will gain unprecedented insight on how these soils have changed over 120 years.
“This work is supported by NREC and also our partners like Illinois Farm Bureau and Illinois Soybean Association have been instrumental in helping us identify sites for samples. We’ve also leveraged the NREC fund to attract over $2.5 million in federal funding like an NSF grant of $2.2 million.”
Ultimately, what Margenot learns will enable improvements in soil fertility management and conservation, including updates to the Illinois Agronomy Handbook.
A map and QR code can be found on the NREC website, www.illinoisnrec.org. Find specific locations of earlier soil testing under “Be a part of the history of soils in Illinois.”
The soil sampling would have negligible impact: a 1 1/2-inch diameter probe will be used to sample soils to a 3-foot depth at three points within a 10- to 15-foot area. All soil data would be provided to the farmer/landowner. The location would be anonymized (county-level) to protect privacy of the specific sampling location.
“One of the applications of a soil archive that goes back 120 years is that we can answer questions uniquely on legacy phosphorus,” Margenot said.
“Legacy phosphorus means phosphorus that has been built up in years past and its agnostic to the duration of that. So, it could have been a year ago, it could have been 100 years ago.
“A lot of this was phosphorus put down in the early days when phosphorus fertilizer became a thing.”
He referred to a report from a 1908 U of I bulletin that recommended about 3,000 pounds of rock phosphate per acre for the first few years of farming, followed by a maintenance rate of 1,000 pounds of rock phosphate every three to five years.
“Now, it’s about 12% P205 (phosphorus pentoxide) but the point is that some of the phosphorus that may have been put down as rock phosphate in the 1910s might still be present,” Margenot noted.
“There are two implications here. One is agronomic. So, this is phosphorus that unlike nitrogen is still there. There’s no gas phase, it doesn’t tend to leach. So, it is a resource for crops. It can be mined.
“Secondly is water quality. This enrichment of phosphorus in surface soils might be susceptible to being lost as dissolved reactive phosphorus from the runoff pathway or by leaching by tiles.
“The problem with legacy P that many people face is it’s really hard to quantify, especially at smaller scales of space and time. Our archive I think will let us get around this problem because we’re able to understand at scales that go state to county and at 100-year time scales what the magnitude of legacy P might be.”
Using the Morrow Plots archived soils, researchers demonstrated potentially large legacy P magnitudes can be accrued in relatively short periods of time.
The Morrow Plots on the U of I campus was established in 1876 and is the oldest experimental field in the nation. The plots were sampled in 1904 and the soils from that sample are in the university archives. The cores were analyzed for P stocks up to 3-feet deep, and cores have more recently been sampled from that same location.
He said what was found in the plots over time was most of the legacy P that was built up today was built up initially in about 1910 to 1920. That was phase two when most rock phosphate was being used — about 3,000 pounds per acre.
“A lot of the positive balance can be accrued in a very short time period. Legacy P does not build up in a specific form or pool. It is simply a number. It’s a positive balance,” Margenot said.
“What we’re finding in the Morrow Plots is that the surplus in these fertilized plots of roughly 1,400 pounds of P per acre, and that’s not P205 that’s P. It’s found mostly in the surface — the top 12 inches.
“This means that all the extra phosphorus that was being put on has transformed and it’s repopulated in different pools that are varying in their availability and in their loss risk. We noticed that there’s a bit of soil P depletion at depth. That’s getting down to about 3 feet and that was a bit unexpected. So, we’ve been mining subsoil P.”
Expanding to a county and statewide level on the magnitude of legacy P, U of I researchers Mark David and Lowell Gentry reported their findings in 2000 that in less than three decades, roughly 5 billion pounds of P was applied in Illinois and not removed by the crop harvest.
“That seems like a lot but on an acre basis, that’s 203 pounds of P per acre across Illinois croplands. Relative to P stocks and 3-foot depth, that’s about a 5% enrichment rate. This points to the absurdly high amount of P that is in native form in our soils. Our soils are naturally rich with phosphorus,” he said.
“We are able to go beyond that mass balance and actually verify the balance at state scale, and that’s where our state is unique and we have a chance to test the how much and the where of legacy P for the Midwest.”
R.S. Smith, then-director of the Illinois State Survey, wrote in 1928 that the soil survey began in 1899 in Adams County. Soil samples were taken in each Illinois county at different layers down to 3 to 3 1/2 or even 5 feet.
Those same soils that were sampled beginning in 1899 are still present. They were salvaged and curated by Ted Peck of U of I in 2000.
“What is valuable about this archive is that someone had the foresight, folks like Dr. Smith, to put labels on the soil sample jars with information on where it was taken, when it was taken, and the soil type,” Margenot said.
“This is valuable because we can now go back and resample those locations to get a delta so we can verify balances by measuring the stock change.
“We also went out to Nebraska because in Nebraska we’ve got the federal soil collection by USDA. We were able to repatriate soils from our state that beginning in 1950 weren’t being kept at U of I,. They were going to the federal soil bank. This is great because we were able to increase the longevity of our chronosequence this time series.”