Fertilizing crops with inorganic nitrogen and phosphorus can greatly improve yields, but a University of Nebraska-Lincoln study released this week indicates that while the extended use of fertilizers may be good for the crops, it may adversely affect soil stability and lead to problems like soil instability and erosion.

The study, published in the May-June issue of the Journal of Environmental Quality, is the handiwork of lead author Dr. Humberto Blanco, assistant professor of soil physics at the University of Nebraska-Lincoln, and agronomist Alan J. Schlegel, Southwest Research-Extension Center, Kansas State University.

The study is based on a 50-year chronicle of inorganic fertilization of corn in western Kansas that was irrigated and conventionally tilled.

“While inorganic fertilization did increase soil organic carbon stocks in the long term experiment, it seemingly failed to enhance soil aggregate stability,” according to the study. Aggregate stability is a key indicator of soil structural quality and resistance to erosion, which depends on how water moves through soil.

Blanco reports the result of the study “were somewhat surprising.

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 “Fertilization typically leaves behind more crop residues in fields,” he said, “which in turn can boost soil organic carbon levels, but we didn’t see improvement in soil aggregate stability even though soil organic carbon concentration increased.”

He noted that soil particles usually bind together more strongly in aggregates as soil organic carbon concentrations rise.

“More research is needed over a wider range of management and climate conditions,” Blanco added, citing a shortage of long-term studies about fertilizer impacts. "Definitely, the effects of inorganic fertilizer application on soil properties will depend on tillage and cropping systems, so we need to look at this in other long-term experiments."

Treatment at Southwest Research Center plots

In the present study, he and co-author Alan Schlegel scrutinized a randomized and replicated experiment that was originally set up in 1961 at Kansas State University's Southwest Research-Extension Center in Tribune. The experimental plots of irrigated and disk-tilled continuous corn received six different rates of ammonium nitrate fertilizer (range 0 to 200 pounds per acre) for 50 years. The plots also received two rates of triple superphosphate fertilizer (0 and 18 pounds per acre) for 50 years, and a higher phosphorus rate (36 pounds per acre) for 19 years.

The study noted that growing corn continuously under conventional tillage and with high inputs of water and fertilizer may seem outmoded, but it points out that this management system is "not uncommon," as demand for corn grain and crop residues grow.

When Blanco and Schlegel tested soils from the experimental plots, they saw soil organic carbon concentrations rise gradually with increases in nitrogen fertilization at soil depths from 0 to 6 inches. Also, phosphorus fertilization increased soil organic carbon at depths of 0 to 3 inches and 6 to 12 inches.

But researchers observed a different trend in soil aggregate stability, especially when nitrogen and phosphorus were applied together at high rates. The study indicates that at a depth of three to 12 inches, adding more than 80 pounds of nitrogen per acre reduced the number of stable soil aggregates by 1.5 times when no phosphorus was applied, by 2.1 times at 18 pounds of phosphorus per acre, and by 2.5 times at 36 pounds of phosphorus per acre.

Blanco says he doesn’t know for certain why this occurred, but he has some hypotheses. He says some studies suggest that adding fertilizers rich in ammonium ions may cause soil particles to disperse rather than aggregate, thereby offsetting any positive effects of increased soil organic carbon content. Because tillage periodically disturbs the soil, it may also negate any benefits of fertilization.

Blanco is now testing these hypotheses in three additional long-term experiments in Nebraska that encompass a wider range of tillage practices and cropping systems.

"It's clear that we need inorganic fertilizers to meet the increasing demands for food production, so it's important to look at how the extensive use of inorganic fertilizers affects soil properties in the long term," Blanco concludes. "The hypothesis is that inorganic fertilization combined with conservation tillage—strip till, no-till, and others—may improve soil structural properties relative to conventional tillage systems."

The results of the study were highlighted this week by Phys.org and ScienceDaily.com. For the full study report, visit the May-June issue of the Journal of Environmental Quality.

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