A filter strip is a conservation practice defined by the U.S. Department of Agriculture Natural Resources Conservation Service as a “strip or area of herbaceous vegetation that removes contaminants from overland flow.” It is used to protect environmentally sensitive water bodies from suspended solids (typically with contaminants bound to them) and dissolved contaminants in natural runoff and excess irrigation. A filter strip functions similarly to other conservation practices including riparian herbaceous and forest buffers to reduce runoff. The latter apply to slowing water, whereas a filter strip also has the additional requirement to trap suspended solids and adsorb dissolved chemicals and nutrients. For any of these to work, runoff entering and passing through them must be more or less uniform in depth and not concentrated in shallow channels or gullies, which must be eliminated when the practice is established and maintained. If this condition is met, the velocity of uniform, shallow runoff entering the filter strip (or buffer) is reduced, and solids tend to settle out of suspension as runoff passes through it. Trapping solids in a filter strip is further enhanced because it has by design no more than half the grade of the field discharging into it and a width of at least 20 feet based on the expected sediment load.
The minimum effective width is strongly affected by the density of vegetation near the soil surface. Therefore, higher seeding rates than normally used for establishing a hay field are recommended. Stiff-stemmed plants that establish quickly, can survive partial burial by sediment and are tolerant of herbicides used in the area are recommended.
Because a filter strip is a distinctly different land use, it increases diversity of the landscape mosaic. It also contributes to wildlife habitat and benefits pollinators. To best meet these requirements, native grasses and other plants should be included in the seeding mix (excluding poisonous species, and if cut for hay or lightly grazed, unpalatable species). Native Louisiana grasses include big and little bluestem, eastern gamma grass, Indian grass and switchgrass, which vary from fair to good forages. Native legumes include herbaceous mimosa and prairie acacia, but the latter is toxic to sheep.
Bunching grasses alone, however, may not be highly effective in reducing the concentration of suspended and dissolved solids because of reduced coverage at ground level. Research has shown that a vegetative barrier of switchgrass at the leading edge of a filter strip improves its performance in removing contaminants. Incorporating native grasses into a filter strip in this way, as well as using other native species in the downslope filter strip, may help achieve both the main and secondary objectives.
Filter strips perform most effectively when combined with conservation practices that keep fields covered. Management systems that incorporate no-till, reduced-till and cover crops minimize soil particle detachment and off-field movement. When the sediment load reaching a filter strip is reduced, the lifespan of the filter strip is increased. Cover crops help to intercept nutrients that would ultimately be dissolved in water running off the fields and cycle them back into the soil for the following cash crop.
Filter strips in general are more effective in reducing sediment loss than reducing dissolved chemicals. Minimizing the loss of chemicals and nutrients dissolved in runoff depends on their adsorption onto organic material and soil in the filter strip, which for soil is greater if the existing adsorbed concentration is low. A case in point is dissolved phosphorus. To minimize soil phosphorus and maximize its retention, plants in the filter strip should be harvested to remove the phosphorus taken up. However, care should be taken in harvest and other management activities to limit damage to the stand and least affect wildlife.
Lewis Gaston is a professor in the School of Plant, Environmental and Soil Sciences.
(This article appears in the fall 2017 issue of Louisiana Agriculture.)
This filter strip of native grass and bermudagrass separates two fields. Filter strips are designed to stop fertilizer and chemical runoff from agricultural fields from getting into waterways. Photo by Kyle Peveto