Factors Influencing the Dispersibility of Clay in Worm Casts

Earthworm activity is believed to be beneficial for the maintenance of good soil structure; however, some research suggests that it promotes soil degradation. Factors affecting the contribution of casting activity to aggregate stability were assessed by measuring clay dispersibility in casts produced by Lumbricus terrestris L. and Lumbricus rubellus Hoff. in laboratory cultures when provided alfalfa (Medicago sativa L.), bromegrass (Bromus inermis Leyss.), red colver (Trifolium pratense L.), or corn (Zea mays L.) leaves or no food. Fresh, moist casts were 26 to 41% more dispersible than uningested, moist soil. Aging reduced dispersibility of moist casts produced under alfalfa, red clover, and corn leaf diets and after 32 d casts were 26% more to 16% less dispersible than uningested, moist soil. Casts aged moist and analyzed after air drying were unaffected by aging but were 9% more to 49% less dispersible than uningested, dried soil. The effects of aging and drying increased as cast organic carbon content increased, therefore L. rubellus casts, which contained more incorporated organic matter than those of L. terrestris, were less dispersible than L. terrestris casts for most treatments. The initial dispersibility increase was due to interparticle bond disruption caused by ingestion and peristalsis. Restoration and improvement in stability was probably due to thixotropic hardening and bonding of clay with incorporated organic debris. Because fresh worm casts are highly dispersible, surface casting activity in areas exposed to raindrop impact may contribute to soil erosion and crusting. Casting activity should enhance soil aggregate stability if casts are aged or dried before being subject to dispersion. Contribution of the Dep. of Land Resource Science, Univ. of Guelph.