If you think your soil loss is tolerable because the revised universal soil loss equation (RUSLE) says so, you might want to think again.
“We are learning that we must have perennial cover in places where water moves, even with no-till,” says Rick Cruse, agronomy professor, Iowa State University (ISU).
Cruse’s opinion is based on field research on 12 small watersheds in Central Iowa. The 1- to 5-acre watersheds are under no-till corn and soybean rotations and farmed on contour strips, an otherwise optimal soil management system. A flume monitors water runoff on each plot, with samples collected for nutrient and sediment loads. The only significant difference between each watershed is the amount of perennial cover, with none having more than 20% and most having 0-10% in areas where water moves.
“In one period between April 1 and June 20, 2008, we had 18,000 lbs. of sediment per acre flow through zero-cover watersheds versus only 400 lbs. with 10% grass cover,” Cruse says. “No-till does great for reducing sheet and rill erosion as measured in the standard RUSLE and RUSLE 2 models, but it doesn’t prevent ephemeral gullies.”
Because the RUSLE models are based on soil loss evaluation of 72.6-ft. strips, ephemeral gullies – temporary, narrow ditches that appear in a field after heavy rainfall – are not accounted for. While accurate enough for sheet and rill erosion in the past, Cruse explains that this run is insufficient for channels to develop. In fact, he says rill erosion and sheet erosion accounted for by the RUSLE models are often less damaging than ephemeral gullies, at least in the short term.
Sheet and rill erosion occur in very thin layers (even a 5-ton loss is estimated at only the thickness of a dime) and moves on a hill slope, Cruse says. “Typically, only about 20% is transported out of the field. However, if ephemeral gullies extend past the edge of the field, almost everything in those gullies will leave the field.”
Ephemeral gullies are stealth erosion factors. In fact, Cruse says water is more effective than an auger at efficiently moving topsoil out of fields, and that is just as true with no-till as with conventional tillage.
“People will argue that they don’t need grass waterways if they have no-till,” Cruse says. “No-till is a great tool, but it’s better if you also have grass waterways, and even better if you have waterways and cover crops.”
These gullies are especially insidious and costly because they occur at the soil surface, removing topsoil and soil-applied nutrients, only to be eventually filled in with surrounding topsoil that is in turn is “augered” away with the next big rain. In Iowa, Cruse credits them with removing an additional 30–40% of topsoil beyond a field’s estimated soil loss. Worldwide, ephemeral gullies are credited with up to 90–95% of estimated soil loss for a given area.
Tolerable soil loss is actually intolerable
In Iowa, allowable or tolerable erosion, the amount of topsoil that can theoretically be replaced, is typically set at 4-5 tons/acre/year. However, Cruse argues, recent studies have shown that only half a ton/acre/year is actually replaced. The idea that most soils in the Midwest have an acceptable replaceable soil loss of 5 tons/acre/year – a concept first suggested in the mid-1900s but never validated – is weak science, he says.
“We are underestimating the rate at which erosion is occurring, and our gold standard of 5 tons per acre doesn’t reflect what today’s science tells us,” Cruse says.
With recent climate changes, the RUSLE models have become even more suspect. Cruse notes that these models use slope, slope length, soil type, crop and management practices, conservation practices and long-term rainfall patterns to estimate soil loss. However, increasingly dramatic rain events don’t fit the model. The Iowa Daily Erosion Project was developed as a response to this reality. It does not evaluate soil loss from ephemeral gullies but does adjust for real-time rainfall and likely erosion, using light detection and ranging (LIDAR) sensing methods to identify field factors and multiple radar systems to verify rainfall.
“If a watershed gets hammered by a 12-inch rainfall, we can show what the impact was on a township or watershed basis,” Cruse says.
Because of ever-larger equipment such as 80-120-foot booms, maintaining grass waterways to prevent ephemeral gully erosion is difficult. Even with no-till practices and precision application technologies, grass waterways are likely to suffer.
“Roundup is a great tool for no-tillers, but there is no Roundup Ready grass for waterways,” Cruse notes. “Auto shutoffs sound good, but it must be used and there is always a delay. At 15-20 mph, auto shutoff isn’t foolproof.”
Cruse suggests that investing more time and care to reduce soil loss around waterways will quickly pay off. He estimates the cost of past topsoil erosion in Iowa to be about $1 billion in potential yield. Those losses come not only from lost nutrients and topsoil but also from poor soil water management.
“The better the infiltration rate, the more water is retained in the soil and the less runoff,” says Andy Lenssen, soybean systems agronomist, ISU. “That is important in wet years as well as dry.”
“Water retention and drainage are really two sides of the same coin,” says Cruse. “Surface soils high in organic matter have better characteristics for retaining and draining water. Soil erosion removes that premier layer, reducing the soil’s ability to retain plant-available water.”
Well-aggregated soil has higher bulk density with small pores to hold water, which works to eliminate or reduce rill, sheet and ephemeral gully erosion and rebuild topsoil. Large pores between the aggregates allow excess water to drain away.
“Loss of topsoil causes a long-term decrease in productivity,” Lenssen says. “Over time, we have had tremendous increases in productivity of our crops, but if we hadn’t lost so much topsoil, the rate of increase would have been even greater. As we’ve lost topsoil and organic matter, a lot of available nutrients, water holding capacity and infiltration have also been lost.”