Filter strips are vegetated areas that are situated between surface water bodies (i.e., wetlands, streams and lakes) and cropland, grazing land, forestland or disturbed land. They are generally in locations where runoff water leaves a field, with the intention that sediment, organic material, nutrients and chemicals can be filtered from the runoff water. Filter strips are also known as vegetative filter strips or buffer strips.
This best management practice reduces sedimentation of surface water bodies and may reduce runoff pollutants such as phosphorus, nitrogen and pesticides. Filter strips can also lessen sheet and rill erosion. Strips slow runoff water leaving a field so that larger particles, including soil and organic material, can settle out. Filter strip vegetation can improve soil aeration, lessen water quality degradation by nutrient removal in the root zone by plant uptake and sorption to soil, and provide wildlife habitat. Filter strips allow the pollutants to infiltrate low and is dispersed across the width of the filter strip. Filter strips also allow the entrapment of sediment.
How Does This Practice Work?
Due to the entrapment of sediment and the establishment of vegetation, phosphorus can be sorbed to the sediment that is deposited and remain on the field landscape instead of entering sensitive water bodies. Conservation buffers improve infiltration and percolation, thus reducing runoff volume. Vegetation in conservation buffers recycles entrapped nutrients in any harvested material and provides permanent habitat for many types of fauna.
Where This Practice Applies and Its Limitations
Filter strips are useful in agricultural areas where both point and nonpoint source pollution occur; in residential and industrial areas such as parks, businesses, schools, urban developments, and water/wastewater treatment facilities; and in areas with potential sediment erosion, leaching and runoff.
Basically, they can be applied anywhere land areas contribute contaminants to a water body. Advantages of filter strips include flood damage prevention, erosion control, aesthetic value, water quality improvement and soluble contaminant flow retardation.
Limitations of filter strip practices include the cost of installation (e.g., grading slopes and vegetation establishment), weed control, maintenance costs, loss of acreage for pasture or crops and the variability of effectiveness due to the uncertainty of runoff rate and frequency. Filter strips are most effective at slopes between 2 and 6 percent because of the increased contact time between the runoff and the filter strip.
In general. filter strip efficacy is determined by trapping efficiency and filtration capacity. These components are impacted by:
- the width and area of the strip as it compares to upslope drainage area, the steepness and length of the slope surrounding and including the filter strip
- soil characteristics, including sorption capacity for various nutrients
- the type of vegetation used and its established density
- correct installation and maintenance of filter strips to minimize sedimentation
- the time between cropland tillage and rainfall events
- the amount of incoming cropland sediment
- the intensity and duration of the rainfall events and its associated runoff velocity
While 10 to 15 ft of buffer width has been reported to remove up to 90% of sediment mass from runoff, NRCS Code 393 sets a minimum buffer strip width at 20 feet (and a maximum at 120 feet) to meet conservation practice standards and receive financial compensation as part of the Conservation Reserve Program. Greater filter strip widths tend to increase the removal of pollutants from runoff water, but filter strips exceeding 60 feet do not appear to significantly decrease nutrient, pesticide, and sediment loads. Additionally, the area ratio of the filter strip to the upslope drainage area may be a significant factor in load reduction, though this is often mediated by other field characteristics. Effectiveness of properly placed and maintained vegetative buffers can be expected to be greater than 70 percent for sediment removal and 50 percent for soluble pollutants (nitrates, phosphorus and pesticides). However, these estimates carry a fair amount of uncertainty, and reviews of filter strip efficacy have reported ranges of sediment removal of 2% to 100%, and similarly wide ranges for runoff volume reductions and nutrient and pesticide reductions.
According to Nebraska Cooperative Extension, filter strips are known to be more effective in removing sediment than nutrients, are more effective when runoff is of a relatively shallow depth, more effective with sod-forming vegetation, less effective when the cropland area drained is increased as compared to the filter strip area, less effective as more sediments and nutrients accumulate in the filter strip, and less effective when the filter strip is not appropriately maintained. According to Cooperative Extension experts, the contract life for grass (vegetative) filter strips is 10-15 years eligible for use under the continuous CRP sign-up.
The ecological benefits of using filter strips include:
- the roots of plants stabilize the soil by increasing soil aggregation
- the shoots of plants protect soil from destructive forces of wind, water and raindrop impact
- vegetation provides shade that impacts soil temperature and thus moisture content
- nutrients are recycled, limiting stress to filter strip plants caused by dry summer winds and cold winter winds that can cause reduced production
Over time, the effectiveness of filter strips may be limited by accumulation of sediment and nutrient saturation of soil. Large rain events or disturbances can release these accumulated nutrients from the filter strip, resulting in the filter strip acting as a source of soluble nutrients, rather than a sink. This dynamic may be a limiting factor on the lifetime and efficacy of filter strips, and producers should consider this dynamic and avoid heavy use of a degraded filter strip.
Cost of Implementing the Practice
The cost of establishing a filter strip will vary according to the equipment, labor costs, grading, seed and fertilizer selected. Potential returns include revenue from harvesting and marketing filter-strip hay. The landowner/farmer may be eligible for CRP and EQIP programs and can receive both technical and financial assistance from federal, state and local levels.
A 2013 study from Iowa State University reported the costs for managing and operating contour filter strips were, on average, $130 per acre per year, though there was significant uncertainty in this estimate. Given that these filter strips are often used to treat an area 10 times larger than the filter strip itself, the cost per acre of treated land was roughly $13 per year. Individual filter strips likely have different costs, but the categories of the costs are similar. These costs include:
- site preparation through tillage and occasional herbicide application
- site establishment and seeding
- other general field maintenance and operating costs
These costs do not directly include opportunity costs for the land used as a filter strip. Opportunity cost is a measure of the value of the land that the producer could otherwise use for a productive, harvested crop. This cost is highly site-specific, and while often approximated using land rental prices, should be evaluated on a case-by-case basis, just like other operational costs. In general, this opportunity cost is usually larger than the operational costs alone. The CRP offers payment to producers adopting filter strips that meet NRCS code 393 standards, and these payments may account for up to 86% of the direct costs associated with operation.
Benefits of utilizing vegetative filter strips are difficult to quantify monetarily, in large part because the benefits of water quality protection are diffuse and do not return directly to the land owner, but are benefits that are enjoyed by society as a whole. The Conservation Reserve Program payments attempt to bridge this gap by using tax dollars to directly compensate producers for the benefits their land management decisions bring to the environment.
Additional factors to consider before installing filter strips include:
- types and concentrations of pollutants for which they are being designed
- soil characteristics, such as clay content, organic material and infiltration rate
- size of contributing area
- previous or existing vegetation
- steepness of slope/irregularity of topography
- dimensions of the watershed that will be draining into the filter strip
- types of vegetation adaptable to the area
- climatic conditions at planting times
- possible combinations of conservation practices to reduce erosion and chemical loss
Operation and Maintenance
The operation and maintenance of this best management practice is critical once the vegetation is established. After establishment, filter strip maintenance may include harvesting and marketing forage, repairing rills and removing accumulation of deposited sediment. Since adsorbed phosphorus is the main portion (75-90 percent) of the phosphorus transported from cultivated land, filter strip use in conjunction with tilled cropland can be a vital component of an overall conservation plan.
Arora, K., Mickelson, S. K., Helmers, M. J., & Baker, J. L. (2010). Review of Pesticide retention processes occurring in buffer strips receiving agricultural runoff1. Journal of the American Water Resources Association, 46(3), 618–647. https://doi.org/10.1111/j.1752-1688.2010.00438.x
Dabney, Seth M., Matthew T. Moore, and Martin A. Locke, 2006. Integrated Management of In-Field, Edge-of-Field, and After-Field Buffers. Journal of the American Water Resources Association (JAWRA) 42(1):15-24. https://www.ars.usda.gov/ARSUserFiles/35278/integrated_management_buffers.pdf
Dorioz, J. M., Wang, D., Poulenard, J., & Trévisan, D. (2006). The effect of grass buffer strips on phosphorus dynamics-A critical review and synthesis as a basis for application in agricultural landscapes in France. Agriculture, Ecosystems and Environment, 117(1), 4–21. https://doi.org/10.1016/j.agee.2006.03.029
Douglas-Mankin, K. R., Helmers, M. J., & Harmel, R. D. (2021). Review of filter strip performance and function for improving water quality from agricultural lands. In Transactions of the ASABE (Vol. 64, Issue 2, pp. 659–674). American Society of Agricultural and Biological Engineers. https://doi.org/10.13031/TRANS.14169
Helmers, M.J., T.M. Isenhart, M.G. Dosskey, S.M. Dabney, & J.S. Strock. 2008., Buffer and vegetative filter strips. Chapter 4 in UMRSHNC (Upper Mississippi River Sub-basin Hypoxia Nutrient Committee), Final Report: Gulf Hypoxia and Local Water Quality Concerns Workshop. St. Joseph, Michigan: American Society of Agricultural and Biological Engineers. Pp. 43-58. https://www.fs.usda.gov/nac/assets/documents/research/publications/2008helmerschapter4.pdf
Leeds, R., L.C. Brown, M.R. Sulc, & L. VanLieshout. 1994. Vegetative filter strips: Application, installation and maintenance. Ohio State University Extension Publication AEX-467-94. http://lshs.tamu.edu/docs/lshs/end-notes/vegetative%20filter%20strips_application,%20installation%20and%20maintenance-1802671180/vegetative%20filter%20strips_application,%20installation%20and%20maintenance.htm
Lorimor, J.C., S. Shouse and W. Miller. September 2002. Solutions: Vegetative Filter Strips for Open Feedlot Runoff Treatment. Iowa State University Extension. PM 1919. NRCS. (2017). Conservation Practice Standard Filter Strip (Code 393). https://www.nrcs.usda.gov/
Sharpley, A.N., R.G. Menzel, S.J. Smith, E.D. Rhoades and A.E. Olness. 1981. The Sorption of Soluble Phosphorus by Soil Material During Transport in Runoff from Cropped and Grassed Watersheds. J. Environ. Qual. 10 (2):211-215.
Smyth, A., L. Wu, R. Munoz-Carpena, & Y. Li. 2018. Vegetative filter strips – A best management practice for controlling nonpoint source pollution. University of Florida Extension Publication #SL432. https://edis.ifas.ufl.edu/publication/SS646
Tyndall, J. C., Schulte, L. A., Liebman, M., & Helmers, M. (2013). Field-level financial assessment of contour prairie strips for enhancement of environmental quality. Environmental Management, 52(3), 736–747. https://doi.org/10.1007/s00267-013-0106-9
Photos courtesy of the Colorado State University Agricultural Water Quality Program
For Further Information
Contact your local soil and water conservation district, USDA-NRCS or Cooperative Extension Service office. To find your local USDA Service Center, visit https://www.nrcs.usda.gov/contact/find-a-service-center.
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Trimarco, T. 2023. Filter Strips. SERA17 Phosphorus Conservation Practices Fact Sheets. https://sera17.wordpress.ncsu.edu/filter-strips/