Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/29/2007
Publication Date: 3/1/2008
Citation: Hawkins, C.L., Rutledge, E.M., Savin, M.C., Shipitalo, M.J., Brye, K.R. 2008. A sand layer deters burrowing by Lumbricus terrestris L. Soil Science. 173(3):186-194. Interpretive Summary: Most homes in rural areas use septic systems to process domestic waste water. A system of pipes and gravel-filled trenches referred to as the filter field is an important component of a septic system that disperses the waste water and allows it to infiltrate into the soil. If infiltration is too fast the waste water can move to ground water before it is adequately renovated by natural processes in the soil. Deep burrowing species of earthworms can contribute to this concern if their burrows allow the effluent to rapidly move through the soil. In this laboratory experiment we investigated whether a layer of sand placed at the bottom of the trenches would discourage a deep burrowing earthworm species that goes by the scientific name of Lumbricus terrestris from burrowing in the soil. We placed the earthworms and a sand layer at various depths in soil columns and tested several different type of commercially available sand. We found that a sand layer deterred burrowing, but did not prevent it if earthworm survival depended on them digging through the sand layer. Thus, sand layers may help to improve septic system performance, but field experiments are needed to confirm this finding and to determine the type and thickness of sand most effective for usage.
Technical Abstract: The effectiveness of septic system filter fields can be reduced by preferential flow through soil macropores. Burrows of anecic earthworms may contribute to this concern by penetrating the infiltrative surface of soil-wastewater-treatment-system trenches. Adding a layer of sand on the bottom of the trench, however, may discourage earthworm burrowing. In this laboratory study, we investigated if the particle-size distribution and position of a sand layer would inhibit burrowing by the anecic earthworm Lumbricus terrestris L. Three earthworms were added to the top of soil columns containing a 3-cm-thick sand layer (various particle size distributions tested) covered with 2 mm of loamy B-horizon material and underlain by 9 cm of the same material. We repeated this experiment with earthworms placed below the sand layer. In another experiment, the sand layer was covered by 11.3 cm of soil material to investigate whether worms added to the surface would avoid crossing a sand layer embedded within the columns. No treatments in the surface sand layer experiments prevented all worms from establishing burrows. The worms added to the surface, however, took an average of 1.7 to 5 times longer (P ' 0.05) to establish burrows and earthworm mortality was greater in columns with a surface sand layer compared to the no-sand-layer controls. In the embedded-sand-layer experiment, no worms burrowed past the soil-sand interface, whereas all worms in the controls burrowed past the equivalent depth. Within the range of materials investigated, the particle-size distribution of the sand did not significantly effect earthworm burrowing. The results suggest that adding a layer of sand to the bottom of soil-treatment-system trenches may deter earthworms, but will not prevent them from burrowing if their survival depends on it.