Driving down rural roads in Iowa last week and this week, you see more soil erosion than usual. Conservation structures such as grassed waterways, terraces, buffer strips and other such practices are helping slow the flow of water running off of fields.
"These permanent practices also settle out sediment and help stem nutrient losses. But as conservation methods, they are even more effective when used in conjunction with other forms of conservation tillage, especially no-till and strip till," says Mahdi Al-Kaisi, Iowa State University Extension soil management specialist.
Rains this spring are coming hard and fast, causing substantial soil erosion in many areas of Iowa. Storms are hitting in late May and into June: a time when soils are most vulnerable. In many situations there's not a lot of last year's crop residue cover left on fields, and there's no crop canopy to slow down the impact of raindrops.
Soil loss can occur despite practices
Complicating the problem of excessive rainfall in late May and into June this year is the fact that the soil profiles in most of Iowa are now filled to capacity with water. Thus, the intensity and amount of rain received has exceeded the soil's capacity to filter water and minimize the amount of surface runoff - even in fields with the most adequate conservation practices.
"These are powerful storms we are having and the ground is already so wet from all the previous rain," observes Mahdi Al-Kaisi, Iowa State University Extension soil management specialist. "The water has nowhere to go, so more of it than usual runs off."
Don't give up on soil conservation practices, he says. Instead, you may need to fine-tune your system. This is a good time to evaluate the effectiveness of your tillage and other conservation practices in protecting the soil.
Good time to evaluate erosion control
"You may need to change some things," says Al-Kaisi. "No single practice will stop erosion. It takes a system of permanent practices such as terraces, grassed waterways, tile, buffer strips, grade stabilization structures, etc.--all working together with conservation tillage or better yet, no-till or strip till, to control erosion."
Why is rain so destructive to bare cropland? What more can farmers and landowners do to help conserve soil? The following answers and information come from Al-Kaisi, and colleague Matt Helmers, an ISU Extension ag engineer.
Raindrops have powerful impact
In a normal rainfall, raindrops range in size from 1 to 7 millimeters in diameter and hit the ground going as fast as 20 miles per hour. The impact of millions of raindrops on the soil surface can be incredible, dislodging soil particles and splashing them 3 to 5 feet away.
A heavy rainstorm may splash as much as 90 tons of soil per acre. However, the majority of the soil splashed is not immediately lost from the field. Most of the splashed soil particles don't leave the field; they clog surface pores which, in turn reduces water infiltration, increases water runoff and increases soil erosion.
Tillage and crop management are critical for reducing the raindrop impact on soil particles. The availability of crop residue to protect the soil surface is important. Excessive tillage can damage soil structure, leading to increased soil sealing and soil erosion. Conservation systems, on the other hand, promote soil aggregates, increased water infiltration into the soil and soil tilth.
Assess the amount of residue cover
Additionally, the improved soil structure of no-till and other conservation tillage systems stands up better against raindrops. A conservation system that includes high amounts of crop residue such as corn or a fall cover crop provides abundant residue cover to protect the soil surface from spring rains.
Farmers are encouraged to assess the residue cover that is remaining since last fall's harvest and ask themselves the following questions:
- Was surface residue enough to prevent soil erosion?
- Is the surface residue cover distributed evenly across the field?
- Is there enough residue cover left after winter decomposition?
If these questions are answered "no", then fall tillage passes and fall manure applications or anhydrous ammonia application need to be considered based on the amount of residue and the residue distribution in the field. Remember that spring is the best time to evaluate conservation systems for their impact on improving soil and water quality since this is generally when we see the most runoff producing rainfall events.
Options to adjust your system
With wetter-than-normal spring weather and the most susceptible field conditions for water erosion occurring, what options should you consider in future years?
Farmers should consider the effect of any additional tillage on remaining crop residue. If residue cover should fall below 30%, adjust your field operations to minimize potential soil erosion due to early spring rain.
Options for steep slope areas include cover crops, permanent vegetation, strip cropping and planting on the contour, all of which can reduce the speed of water runoff and slow soil erosion. If crusting occurs, use a rotary hoe to allow seedling emergence to occur unrestricted. The faster a crop is growing, the sooner the crop canopy will develop--a partial canopy is better than none at all.
Conservation structures such as terraces, grassed waterways and field buffers are good parts of a conservation system. They help in slowing water flow, settling out sediments and directing water away from the field to a suitable outlet. "Remember, what you see in your fields in spring can help in developing a more comprehensive conservation plan that greatly improves soil and water quality," says Al-Kaisi.