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 water runoff. 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.