PLAINFIELD — Farmers are losing more soil than they think, and erosion is costing yield, said Iowa State University agronomy professor Rick Cruse.

"Soil erosion is a losing proposition," Cruse said during last week's Iowa Learning Farms/Practical Farmers of Iowa cover crop field day at the Rick and Jane Juchems farm near Plainfield.

Farmers with NRCS conservation plans must keep soil erosion under five tons per acre per year in order to receive cost-share.

"Soil loss means soil movement," Cruse said. "Some of that soil is deposited in other places in fields, and some leaves the field."

Tolerable soil loss, or T, was determined in the 1940s by USDA officials.

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"Their observations determined that we could lose five tons of soil per year and still maintain productivity indefinitely," Cruse said.

The model only estimates sheet and rill erosion or what is moving in a thin layer on the soil surface.

"The erosion from 2013 rains, such as all the ephemeral field gullies — that is not included in the calculations," Cruse said. "Soil erosion in your fields is way underestimated."

The rate of soil formation is a half ton per acre annually.

"If we're losing five tons per acre per year, and we're not even inducing all the erosion that is occurring, and we're regenerating soil at a rate of a half ton, what does this mean?" Cruse said.

Research shows that as the thickness of top soil decreases, productivity goes down. In areas where soil already is thin and it becomes more eroded, yields drop a lot.

Farmers fix ephemeral gullies by disking and filling them with top soil, Cruse said. When a heavy rain occurs, that soil washes away.

Cruse said people tell him that yields are continuing to go up, but those increases are because of technology, which masks the declines because of erosion.

"Any practice that gives you yield drag, you abandon it," Cruse said. "Soil erosion is giving you yield drag."

The Iowa Daily Erosion Project, which estimates erosion by township, showed that in 2007, multiple townships had more than 50 tons of soil erosion per year, Cruse said. Iowa had six million acres that were eroding at greater than 10 tons per acre in 2007. That's 25 percent of Iowa's corn and soybean ground. The calculation doesn't include gully erosion.

"Cover crops are a part, and a critical part, of the answer, but they're not the whole answer to Iowa's soil loss and water quality problems," Cruse said. "Cover on the soil surface when there is nothing growing will be a requirement if we are going to keep yields where we want them to be."

Well-designed and functioning grassed waterways also are needed to stop ephemeral gully erosion.

Iowa's Nutrient Reduction Strategy, which has goals for the amount of nitrogen and phosphorus going into rivers and streams, is encouraging farmers to implement practices to improve nutrient management and keep nutrients and soil in place.

"My dad told me that in a competitive game with no rules the most aggressive and often least ethical individual determines how the game is going to be played," Cruse said. "If you have selected individuals driving tractors that will farm row crop up and down a goat pasture that should be used for pasture only, an individual that wants to do the right thing can't compete."

Cruse said it will take patience to clean up rivers and streams.

"Even if everyone in here did absolutely everything right, the Cedar or the Wapsie or the Shell Rock wouldn't get clean right away," Cruse said. "We'll see water quality problems for a while."

Matt Helmers, Iowa Learning Farms program co-manager and ISU ag engineer, said cover crops not only help with soil erosion and holding phosphorus, they also scavenge nitrogen. Research at ISU's Northeast Research Farm at Nashua shows that a rye cover crop drilled after harvest on corn and soybean ground decreases nitrate concentrations in tile and groundwater. Flow weighted annual nitrate concentration was reduced by 25 percent at Nashua. A site near Pocahontas showed a 15 to 20 percent reduction. USDA-ARS researcher Tom Kaspar found reductions of 50 to 60 percent at Ames.

"We're continuing to collect water quality data," Helmers said.