'Root' of the problem: Soil structure called vital to clean water

OAK HARBOR - For soil health expert James Hoorman, the root of the problem behind agricultural nutrient runoff is just that — roots.

Researchers have traced the primary contributing factors to Lake Erie’s harmful algal blooms to agriculture. Excess nutrients from fertilizer, namely phosphorus, runs off farm fields.

That phosphorus runoff reaches the lake and feeds cyanobacteria, or blue-green algae, which can bloom to a massive size, create toxic microcystin, and have a devastating effect for so many that rely on Lake Erie.

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Hoorman, a soil specialist at the U.S. Department of Agriculture’s Natural Resources Conservation Service, believes that roots have to be at least a part of the solution to keeping that phosphorus on the fields.

“At NRCS, soil health is becoming a major leg of what we’re going to be pushing,” Hoorman said. “We have a lot of cracks in the soil. That’s due to poor soil structure.”

Those cracks are a direct path for water at the surface of a field, which contains high concentrations of phosphorus, to reach the tile drainage system, where it pours into ditches, streams and eventually the lake.

A key step missing from that process, according to Hoorman, is that the water is not infiltrating and working its way through the soil, where nutrients can be absorbed rather than flow into the water.   

“What we don’t want is a direct conduit for that surface water going right down to your tile,” he said. “What we want it to do is go down through that soil and have that soil treat the water.”

The best way to do that, he said, is by getting live roots in the soil, which are provided by cover crops.

“It’s going to loosen that soil up and we’re going to get better water infiltration,” he said. “That’s what cover crops do. Roots also shrink those cracks.”

Hoorman cited numerous other conservation practices aimed at treating excess nutrient runoff, including wetlands, buffers, filter strips and grass waterways.

“Everyone of them has one thing in common — roots, live roots,” he said. “What do live roots absorb? Soluble nitrogen and soluble phosphorus.”

The practice not only benefits the lake, but could also benefit farmers by allowing nutrients to be reused. With diversified cover crop species, they are living and dying at different times, creating what Hoorman describes as “pulses” of nutrients being released.

“If we can tailor this to that corn curve or the soybean curve, we’re going to keep those nutrients recycling in the soil,” he said.

Lindsay Pease, of the USDA Agricultural Research Service, said conservation practices with phosphorus make sense economically for farmers.

“It’s really important to follow what your farm needs,” Pease said. “If you’re applying phosphorus fertilizer in your soil, thinking of your soil like a bank, you may not get the phosphorus back in the future because it might be leaving with water.”

Extensive water sampling and monitoring by the National Center for Water Quality Research at Heidelberg University has found that the highest concentrations of phosphorus running off of agricultural lands occur mostly during heavy rainstorms through spring and summer.

“It’s not all just what’s been happening on the fields,” said Laura Johnson, director of the NCWQR. “We’ve been seeing an increase in discharge — water volume.”

The discharge increases began around the early 2000s, which also coincided with an increase in concentrations of dissolved phosphorus runoff, according to Johnson.

“We don’t get half-inch rains anymore. A half-inch rain is not going to cause much of a problem on phosphorus,” Hoorman said. “It’s those 1, 2, 3 and 4-inch rains that really come pounding down — that’s when we’re losing the majority of our phosphorus.”

According to researchers, the amount of nutrients running off increases exponentially as the speed of the water gets faster.

“How do we stop that from happening? You have to slow that water down physically on the land,” he said. “The best way to put a dam up is to just put these cover crops out there, get that water to go down through the soil profile and slow the speed of it down.”

One of the most prominent solutions conservationists have been advocating is practicing the “Four Rs” of nutrient stewardship: right source, right rate, right timing and right placement.

Hoorman taught the Four Rs as an extension educator with The Ohio State University, but he stressed that other steps in addition will need to be taken as a part of solving this problem.

Something important that is not addressed by the Four Rs is nutrient storage, which is in soil organic matter, Hoorman said.

Over about the last century, he said organic matter in field soil is down about 50 percent.

“We have poor soil structure because we don’t have the organic matter left in the soil,” he said. “Live plants have to be part of the solution.”

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