I have been getting several questions about tiny mounds of granular soil covering small or large areas of the lawn. The mounds will be clumped pellets of soil that look similar to small ant mounds. If you have seen this, then you may have a thriving population of earthworms.
Often earthworms leave small mounds or clumps of granular soil (castings) scattered about in the lawn or garden. The castings may be seen as a nuisance when they are in a high abundance. This situation is often noticed in fall, winter or spring when warm season grasses are dormant or grow slowly. Without constant growth of grass and mowing to knock them down, the castings brought to the surface are more noticeable.
As earthworms tunnel through the soil, they ingest the soil and digest any organic matter in it. Organic matter is taken into their burrows and is broken down. Although earthworms are most numerous in the top several inches of soil, they also work in the subsoil, bringing mineral rich soil from below to the surface. This adds to the supply of nutrients available to the plants.
Besides incorporating organic matter into your soil, earthworms are good manufacturers of fertilizer. Castings have a nutrient level and organic matter level higher than that of the surrounding soil. Each day they produce nitrogen, phosphorous, potassium and many micronutrients in a form that all plants can use.
Earthworm tunnels help to aerate the soil. This allows more oxygen in the soil, which not only helps the plant directly, but also improves conditions for certain beneficial soil bacteria. Finally, the tunneling of earthworms provides access to deeper soil levels for the numerous smaller organisms that contribute to the health of the soil.
Earthworm activity in your soil is beneficial and should be encouraged. They help incorporate organic matter, improve the soil structure, improve water movement through the soil and improve plant root growth. Since earthworms are beneficial, control measures are not required. Once warm season grasses begin to grow actively they will cover the soil and castings. Earthworm populations will be higher in moist areas. Core aerating and topdressing lawns with thin layers of sand over time may improve surface drainage and help reduce earthworms.
Earthworms may be the culprit of bringing some unwanted guests into your landscape. Earthworms are included in the diet of armadillos and moles. Be sure to check your lawn for grubs, mole crickets and earthworms to determine what is being eaten if you have an invasion of either of these pests in your yard. Except in extreme cases, it is best to let the earthworms do their work in the soil and allow your lawn to reap the benefits of such work.
Bill Tyson is the coordinator for Effingham County Cooperative Extension of the University of Georgia. Email him at wtyson@uga.edu or call the Extension office at 754-8040.