SIMPLE PRACTICES… BIG IMPROVEMENT – Page 2
These simple practices can be highly effective. A review of published studies found that properly designed and placed buffers reduce the speed and volume of runoff and trap, assimilate or transform pollutants that would otherwise end up in streams, lakes and rivers.33 The review found that buffers trapped:
- 41 to 100 percent of the sediment,
- 9 to 100 percent of the runoff water,
- 27 to 96 percent of the phosphorus, and
- 7 to 100 percent of the nitrate.
A project currently underway in Iowa called STRIPS provides additional, compelling and very encouraging evidence that small amounts of cropland devoted to strategically placed strips would make a big difference.34 The project puts 10 percent of the cropland in a small watershed into strategically placed grass strips and compares the amount of water, sediment, phosphorus and nitrogen coming off the stripped fields to the amount coming off corn and soybean fields without strips.
Two different arrangements are being tested. One places 10 percent of the cropland into a grass strip located at the bottom of sloping fields. The second puts 10 percent of the cropland into contour grass strips.
The first two years of results were stunning.
In the first year (2008), rainfall was more than three times the normal amount, and Iowa suffered devastating floods, with 85 of Iowa’s 99 counties declared disaster areas.35 But even in such a dangerous year, the strips provided an encouraging amount of protection:
- Strips at the bottom of sloping fields reduced runoff by 64 percent, sediment by 63 percent, total phosphorus loss by 93 percent and total nitrogen loss by 90 percent.
- Contour strips reduced runoff by 20 percent, sediment loss by 40 percent, total phosphorus loss by 90 percent and total nitrogen loss by 85 percent.
In the second year (2009), rainfall was about normal. That year:
- Strips at the bottom of sloping fields reduced runoff by 69 percent, sediment loss by 97 percent, total phosphorus loss by 94 percent and total nitrogen loss by 92 percent.
- Contour strips reduced runoff by 45 percent, sediment loss by 96 percent, total phosphorus loss by 93 percent and total nitrogen loss by 85 percent.
Grass or forest strips planted along stream banks have been shown to not only filter out pollutants but also reduce pollution caused by collapse or erosion of the stream bank itself. A Minnesota study found that slumping stream banks contributed 31 to 44 percent of total sediment dissolved in the Blue Earth River in Minnesota.36 Two studies of Walnut Creek in Iowa found that eroding stream banks contributed 50 to 80 percent of the sediment load.37&38
A long-term project in Iowa’s Bear Creek found that stream banks bordered by row crops — a common sight — suffered the most stream bank erosion and total soil loss. Buffering the stream banks with strips of grass and/or trees cut stream bank erosion by 80 percent.39
Reducing the number of times a crop field is tilled and leaving more crop residue on the soil has been promoted for decades. An exhaustive review of the scientific literature, completed in 2006, found that no-till practices that leave the maximum amount of residue on the field can reduce erosion by as much as 100 percent and runoff by as much as 99 percent, depending on the site and the amount of soil covered by residue.40