AMES, Iowa – A newly-released study conducted by Iowa State University soil scientists is providing new insight into phosphorus loss from farmland, as well as evidence showing phosphorus runoff is often underestimated.

“Conventional views of phosphorus loss are that most of it is tightly bound to soil particles, so phosphorus is not a problem as long as erosion is controlled,” said Antonio Mallarino, Iowa State University professor of agronomy, and extension soil fertility and nutrient management specialist.

“Our study shows this is not necessarily the case,” said Mallarino, who led the project. He worked with Mazhar U. Haq, Iowa State agronomy research specialist, and former Iowa State graduate student John D. Jones Jr.

Conducted over three years with funding from the university’s Iowa Nutrient Research Center, one of the project’s primary objectives was to determine how much dissolved phosphorus in surface runoff may not be accurately measured by common methods.

Mallarino’s team used soil and surface runoff samples from several Iowa field experiments where they measured phosphorus loss from corn and soybean fields that had different soils, phosphorus sources, tillage management and conservation practices.

Their findings showed more phosphorus leaves fields dissolved in runoff than expected, and that some conservation practices may not reduce – or may even increase – dissolved phosphorus loss.

“All the phosphorus that leaves the field is potentially important,” Mallarino said. “However, dissolved phosphorus is in a form that is highly bioavailable, so it is of particular concern for its contribution to eutrophication (excessive richness of nutrients in a lake or other body of water, frequently due to runoff from the land) of freshwater and coastal waters, and harmful algal blooms.”

According to the study, phosphorus is characterized in a number of ways, and there are different approaches to monitoring where and how it shows up in the environment.

Another major objective of the study has been to refine knowledge and guidelines for best management practices to reduce dissolved phosphorus loss with runoff. The researchers looked at the influence of different tillage systems and phosphorus sources, as well as potential benefits from cover crops and use of two soil amendments, alum and gypsum, thought to reduce phosphorus loss.

The study found applying phosphorus beyond the amount needed by crops drastically increases dissolved phosphorus loss. In addition, no-till management increased the proportion of total phosphorus loss comprised of dissolved and bioavailable forms, reflecting findings from some other long-term research efforts, especially in the Great Lakes region.

With tillage to incorporate the fertilizer or manure, the study said the proportion of dissolved phosphorus in runoff was smallest where solid poultry manure was applied and larger after fertilizer or liquid swine manure.

With no-till, however, the proportion of dissolved phosphorus loss was highest for fields where fertilizer was applied. Where manure was applied to no-till fields, loss did not significantly differ based on the type of manure, probably because swine manure was injected into the soil, Mallarino said.

The study said using a winter cereal rye cover crop reduced all the forms of dissolved phosphorus in runoff – in contrast to a study in the Great Plains that showed an increase in dissolved reactive phosphorus loss where cover crops were used. But the Iowa State research showed benefits from cover crops for fields managed with both tillage and no-till.

Of the two soil amendments tested, only the aluminum sulfate (alum) consistently reduced the loss of dissolved and bioavailable phosphorus.

“This work to better understand the forms of phosphorus loss is critically needed as we work to reduce loss to downstream water bodies,” said Matt Helmers, Iowa Nutrient Research Center director. “It highlights the importance of dissolved phosphorus loss and the need to better understand the impact of conservation practices on dissolved phosphorus.”

Mark Licht, Iowa State University assistant professor of agronomy and extension cropping systems specialist, said concern with phosphorus loss and soil erosion goes back to water quality.

“Keeping soil and thus phosphorus out improves the water quality,” he said. “The dissolved phosphorus makes things a bit more complicated because it is moving with water and not soil. Phosphorus loss is also partly to blame for algal blooms. So, making sure phosphorus is not a pollutant is a priority.”

Virgil Schmitt, Iowa State University field agronomist, said soil erosion is still the largest factor in phosphorus losses from the field because phosphorus tends to bind fairly tightly to soil particles.

“The filling of ponds and lakes with sediment and the need to dredge the Mississippi River to keep it navigable are testaments to the quantity of soil erosion occurring,” he said. “Dissolved phosphorus in runoff and tile drainage is a lesser, but still significant, issue.”

He said some ways farmers can manage or reduce phosphorus loss from their farmland are using cover crops to reduce losses associated with both erosion and runoff, and applying phosphorus fertilizer only to soils with an optimum or lower soil test.

“This makes sense economically, but also reduces phosphorus losses in eroded materials leaving the field and losses of dissolved phosphorus,” he said.

He also advised applying phosphorus fertilizer in the fall with an eye on weather forecasts, selecting a time when runoff events for the next two weeks are unlikely: “This is especially important for no-till situations.”

He said another way is no-till, which reduces erosion but may increase runoff, which is a source of dissolved phosphorus leaving the field unless there is a 14-day-or-more gap between fertilizer application and the runoff event.

In addition, he advised farmers to apply phosphorus fertilizer annually, or apply a two-year application to corn stubble going to soybeans.

“Annual applications at the rate for just the next crop reduces the phosphorus concentration at the surface, which reduces losses of phosphorus attached to soil particles or dissolved,” he said. “For those applying a two-year fertilizer rate every second year, this does double fertilizer application expenses, however.”

He said for those on a corn-soybean rotation, the conventional wisdom is to apply enough fertilizer for the next two crops on soybean stubble going to corn. “In theory, this allows the corn crop to take advantage of the nitrogen in the phosphorus fertilizer,” he said.

However, he said, “If the phosphorus fertilizer is applied at a variable rate, then the nitrogen in it is also applied at a variable rate, and to take advantage of that nitrogen, nitrogen fertilizer also needs to be applied at a variable rate, with lower nitrogen rates being applied to areas where higher phosphorus rates are applied.

“In reality, that seldom happens,” he said. “Because corn residue provides greater protection to the soil, applying phosphorus fertilizer to corn stubble going to soybeans allows the corn residue to help phosphorus attached to soil particles stay in place and also reduces runoff, which reduces losses of dissolved phosphorus.”

Licht said phosphorus fertility (commercial or manure) needs to be managed properly by paying attention to soil test levels and balancing it with crop removal rates, with the highest risk of phosphorus coming with higher soil test levels.

“Cover crops are definitely a way to reduce the risk of phosphorus loss because they actively take up phosphorus and hold it,” he said. “No-tillage is really effective at stopping phosphorus lost through attachment to soil.

“However, dissolved phosphorus has the potential to move more readily because of increased infiltration and soil porosity with a no-tillage system,” he added. “More work needs to be done to fully understand how tillage systems affect dissolved phosphorus movement.