Study focuses on phosphorus loss, options

CUBA, Ill. — Researchers at the Metropolitan Water Reclamation District site in Fulton County are seeking answers to how much phosphorous is lost by soil erosion and alternative solutions.

University of Illinois Soil Science Professor Andrew Margenot led a tour of the research plots as part of a June 7 Nutrient Stewardship Field Day.

“We don’t have great numbers on erosion’s contributions to phosphorous losses in the state. It’s really hard to get good data,” Margenot said.

“This is one of three sites across the state. We have a fourth one in Cook County with the support of Illinois Farm Bureau and that’s going to give us a four-way comparison of how much soil erodes, what’s the phosphorous load of the soil, and how does that compare to monoammonium phosphate (MAP).

“We might be losing, say, one-half pound per acre as runoff from MAP. That’s a typical number from our neighboring states. But is erosion 2 pounds, is it 10 pounds, is it one-half pound? I feel that having context on the non-fertilizer losses is important to give some context of what the fertilizer versus the non-fertilizer losses are.”

The trials feature numerous plots on a hill with treatments of struvite, MAP, diammonium phosphate (DAP) and a mix of treatments. Water and sediment runoff from each plot is captured in 55-gallon drums in the ground at the base of the hill.

“We’re able to capture the sediment that’s going down a funnel into the bucket. We want to quantify how much phosphorous is contained in the sediments by erosion, because obviously it’s still a non-point source loss,” Margenot said.

“We’re doing comparisons of the timing of application. We do fall and spring application of struvite versus MAP, and blends of struvite and MAP.”

The trials will also monitor crop yields and its relationship to nutrient losses.

“There’s a second angle here, though, and this gets into what my lab does with Illinois Nutrient Research and Education Council, is what’s the role of erosion, because erosion is a non-fertilizer source of P losses, but it can sometimes be counted incorrectly as a fertilizer P loss from non-point sources.”

Struvite

A potential phosphorous loss reduction strategy is utilizing slow-release P fertilizer in the form of struvite — magnesium ammonium phosphate — that can not only reduce nutrient losses from non-point sources such as farmland, but also aid water treatment facility point sources.

Struvite crystallization in the pipeline and its accumulation cause significant environmental and industrial issues such as pollution problems like eutrophication, odor formation, clogging in the pipelines, damage in the equipment and reduction in the process efficiency.

Adding magnesium to wastewater creates a chemical reaction that crystallizes the struvite for extraction and use as a phosphorous source in agriculture production.

Adapting struvite crystallization under controlled conditions not only eliminates these problems, but also enables production of a high-value added fertilizer product.

“There’s been a big movement in Europe and now in the U.S. from private sectors to capitalize on the capture of phosphate from point sources and then thinking about can we reuse that material as a fertilizer,” Margenot said.

“It also a way for non-point sources to lead the charge on our nutrient loss reductions. The reason for that is you’ve got to do something with all of that struvite. By offering these recipient fields to use the struvite, agriculture can play a part in this integrated point source and non-point source partnership.

“What we’re doing as soil scientists is trying to figure out how much struvite can we use. Can we fully swap out MAP or DAP for struvite and still maintain yields and expect loss reduction?”

Struvite is 3% water soluble compared to MAP or DAP that are 90% water soluble.

“It does not dissolve. So, if you apply it on the surface there should be minimal dissolution of those nutrients. But there’s also a tension because if it’s too insoluble then the plant can’t get to it in time, especially in early stage crop growth like V6-V9 for corn,” Margenot said.

“So, that tension might present a tradeoff and what we’re doing with NREC support is quantifying what’s the right ratio of struvite to use to support crop yields, to enable the recycling of phosphate from point sources, but also to avoid any yield drags.

“What we found through work with Dave Isermann in La Salle County is there was no real yield difference for the two years we did the study and in the years after either. That’s good news. If we at least maintain yields we can tap into the reduced runoff losses.”

Nutrient Availability

Despite is low water solubility, struvite is more citrate soluble than MAP or DAP, making it available for the plants

“Citrate is produced by crop roots through exudation. If you get a rainfall event there’s going to be minimum dissolution — 3% of struvite. MAP is going to be 90%-plus dissolved. But if you have a root present, the root is going to be putting out small acids like citrate, malic and others, and those small acids will drive the dissolution,” Margenot noted.

“There’s a natural mechanism to synchronize the dissolution when there’s a crop root present. In terms of the 4Rs — right source, rate, time and place — you couldn’t ask for a better mechanism which is driven by the crop root.”