The following is the second of a three part series on Vegetative Filter Strips (VFS) and their role and use in reducing the potential for nutrient transport from crop and pasture fields due to overland flow. Information was excerpted from "Grassed Filter Strips Can Reduce Losses of Nitrogen and Phosphorus in Runoff", bu D. R. Edwards, P. A. Moore, Jr., and T. C. Daniel. The original article appeared in the Better Crops with Plant Food Journal, Volume 80, (1996, No. 4), pages 8 to 11).
Grassed Filter Strip Studies Provide New Insights: Edwards et al. followed up their first set of studies with an investigation of how effective VFS or grassed filter strips (GFS) were in terms of removing nutrients in runoff water from pastures. Grassed filter strips are simply grassed areas installed down slope of areas where manure or fertilizer has been spread to trap, filter, and purify runoff as it flows across the filter. The primary purpose of these filters is to do just that "filter", i.e., catch and retain nutrients, sediment, and water that might run off agricultural lands and into surface water systems. A number of studies have shown that VFS can be very effective (better than 95%) in removing sediment and nutrients. However, not much effort has gone into studying the role of VFS in conjunction with pastures.
In this study, poultry litter and swine manure was applied to the upper end of a 80 feet long fescue strip. The field was a silt loam soil with 3% slope. Runoff samples were collected at several locations down through the VFS. The results were similar to those from other studies, showing that 90% or more of the incoming N and P (in the manure) was removed by the VFS. Generally, all of the nutrients were removed from the runoff water in the first 30 feet of the VFS.
They followed up this study by applying poultry litter on the upper 20, 40, and 60 feet of the strip and then making the same measurements again, i.e., covering 1/4, 2, or 3/4 of the 80 feet strip with litter. Even for the longest fertilized length,, i.e., 60 feet of the entire 80 feet strip, the remaining 20 feet of VFS were effective at removing 20 to 40% of the N and P. As would be expected, the effectiveness of a VFS decreased with increasing length of the contribution fertilized area and decreasing length of the VFS.
The researchers concluded that the VFS removed N primarily through infiltration and P through infiltration and particle adsorption. Because the soil has a finite infiltration rate, the strips could absorb only a finite amount of water. Based on all of the information Edwards and colleagues gathered, they developed a chart that they propose can be used to determine the length of VFS required to remove a specific amount of N and P from runoff water. In essence, they concluded that the percentage of nutrient removal is a function of two factors: the first is the ratio of soil infiltration rate to intensity of rainfall event; and the second is length of the strip where the manure is applied relative to the length of non fertilized VFS. In other words:
§ If the rate of infiltration into the soil is low relative to the amount and severity of the most extreme rainfall event, then a VFS will need to be as long as the length of the fertilized strip in order to be effective; and then a substantial amount of the N and P might still run off.
§ Conversely, if the rate of infiltration into the soil is high enough to accommodate the amount and severity of the most extreme rainfall event, then a VFS only 1/10th the length of the fertilized area with adequately remove between 90 and 95% of the N and P from the fertilizer.
§ Finally, if the rate of infiltration is more than 50% of the amount and severity of the most extreme rainfall event, then the VFS must range in length from 1/10th to the same as the fertilized strip to effectively remove at least 50% of the N and P in the fertilizer source (inorganic and manure combined).
Categories: Filter Strips, Water Quality
Date: 1997