Compost filter socks help reduce pollutants in agricultural runoff

Compost filter socks are mesh tubes filled with composted bark and wood chips. Besides making lovely wedding gifts, they are also used at construction sites to limit the amount of silt in the water that runs off. What was previously unknown, however, was their effectiveness at reducing sediment, herbicides and nutrients in runoff from agricultural fields. Two soil scientists from the United States Department of Agriculture (USDA) have completed a two-year study, measuring just how good a job the socks did when placed in grassed waterways alongside fields. Their conclusion: the socks rock... sort of.

Martin Shipitalo and Lloyd Owens, along with two other partners, tested both tilled and untilled Ohio corn fields. In the case of the untilled fields, the socks reduced the runoff’s sediment concentration by 49 percent. They had no effect with the untilled field, but its sediment flow was already one-fifth that of the tilled field in the first place.

When it came to herbicides used on the tilled field, the socks reduced the concentration of alachlor by 18 percent, and glyphosate (Roundup) by 5 percent. They had little effect on the nutrient content in the runoff from either field. The socks were also observed to introduce certain compounds into the runoff, likely originating in their compost, but the levels were deemed inconsequential.

The scientists reached several conclusions, both regarding the socks and otherwise:

• The practice of leaving fields untilled does help in reducing sediment runoff, but does little to reduce the amount of nutrients and herbicides that end up in the water
• Conservation buffers such as grassed waterways (i.e: ditches) do help absorb sediment and chemicals before they can reach streams, but work best when the runoff is spread evenly across the buffer - unfortunately, the runoff tends to end up concentrated in smaller areas
• Compost filter socks could be a valuable tool in protecting water systems from polluted agricultural runoff

The USDA research team is now working with Filtrexx International, the manufacturer of the socks, to determine what could be added to the wood compost to help it better absorb nutrients and herbicides.