Use of Sorbent-Amended Compost Filter Socks in Grassed Waterways to Reduce Nutrient Losses in Surface Runoff.

Surface runoff from fields used to grow row crops frequently has high concentrations of sediment, nutrients, and herbicides, particularly in the first few events after tillage and planting. Compost filter socks placed in grassed waterways can further reduce sediment concentration as this runoff is transmitted offsite, but are generally ineffective in removing dissolved agrochemicals. In this study, we routed the runoff from one tilled and one no-till watershed (~1.0 ha) planted to corn into parallel, 30-m long, grassed waterways designed by the NRCS. Three, 46-cm dia., mesh bags (filter socks) filled with composted bark and wood chips were placed 5 m apart across the upper half of one waterway and three socks were similarly placed in the lower portion of the other waterway to determine if they increased removal of sediment and dissolved chemicals. A proprietary sorbent, Nutriloxx, was added to the filter socks to increase nutrient retention. Automated samplers were used to obtain samples above and below the treated segments of the waterways for two crop years. The effectiveness of the grassed waterways and filter socks was highly dependent on the timing and size of the runoff events and tillage treatment. In 2009 there were no sizable runoff events during the growing season. Consequently, sediment loses were minimal and no significant effects on sediment concentration were detected. Similarly, nitrate and ammonium nitrogen losses were inconsequential and not affected by filter sock installation.  Averaged for both watersheds, however, the amended filter socks contributed to a significant additional 27% reduction in dissolved phosphorus concentration (range + 59% to -2%) compared to waterways without filter socks. The filter socks significantly increased sulfate concentrations up to 22-fold in the first sampled event, but sulfate concentrations declined rapidly with subsequently runoff events.