What Causes Tiny Mounds of Soil in My Lawn?

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earthworm castings

When night temperatures are warm, even during fall and winter when warm spells occur, I often get questions about tiny mounds of granular soil covering small or large areas of the lawn. The Pitt County Extension Master Gardeners and I have already had numerous calls and questions about this unusual disturbance of the soil this month. If you have seen this, then you may have a thriving population of earthworms.

Often earthworms leave small mounds or clumps of granular soil, which are called castings, scattered about in the lawn or garden. The castings may be seen as a nuisance when they accumulate. 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. Sometimes earthworms may enter drainage holes of containers sitting on the soil or sunken into the ground. The castings may clog the drainage as they accumulate in the container.

As earthworms tunnel through the soil, they ingest the soil and digest any organic matter in it. Organic matter is dragged into their burrows and is broken down. Although earthworms are most numerous in the top 6 inches, 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. In 100 square feet of garden soil, earthworms may bring from 4‑8 pounds of dirt to the soil surface each year.

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. For example, a 200 square feet garden with a low worm population of only 5 worms per cubic foot will be provided with over 35 pounds (about 1/3 pound per worm) of top grade fertilizer each year.

Not only do they produce this fertilizer, they spread it in the top 12 inches of soil. They may also incorporate it as far down as 6 ft. A soil that is well managed and rich in humus may easily support 25 worms per cubic foot, which translates into at least 175 pounds of fertilizer per year for the same 200 square feet of garden.

This means that your garden or lawn can be supplied with fertilizer of superior quality and soil building properties than a dry or granular fast acting fertilizer of 10‑20 pounds. Worms make other contributions, such as adding calcium carbonate, a compound that helps moderate soil pH. Over time, earthworms may help change acid or alkaline soils toward a more neutral pH.

Earthworm tunnels help to aerate and loosen the soil. This allows more oxygen in, 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, improve plant root growth and minimize thatch build up in lawns. Since earthworms are beneficial, control measures are not required. Once warm season grasses begin to grow actively they will cover the soil and castings earthworms have brought to the surface. Earthworm populations will be higher in very moist areas. Core aerating and topdressing lawns with thin layers of sand over time may improve surface drainage and help reduce earthworm numbers to a tolerable level. In heavy, wet soils, installation of drainage systems may remove excess moisture but it may not be enough to reduce earthworm populations. Except in extreme cases it is best to enjoy the fact that nature is at work in your yard and you have a work force out there improving the soil day and night.

If are looking for more information on “earthworms as pests in home lawns”, we have an on-line publication titled just that way. You can find it at https://www.ces.ncsu.edu/depts/ent/notes/O&T/lawn/note125/note125.html or simply search using the keywords “earthworms ncsu”. If you have gardening questions call the N.C. Cooperative Extension Master Gardener Volunteers at 902-1705 or email pittcomgv@hotmail.com. Voicemail is available at anytime. Volunteers have started their winter schedule and will be in the office to return and take calls and walk in visitors on Monday and Thursday from 10 a.m. to 12 p.m. More gardening information is always available at http://pitt.ces.ncsu.edu.

Written By

Danny Lauderdale, N.C. Cooperative ExtensionDanny LauderdaleArea Specialized Agent, Ornamental Nursery and Greenhouse, Eastern Region Serves 44 CountiesBased out of WilsonCall Danny Email Danny N.C. Cooperative Extension, Wilson County Center
Posted on Nov 18, 2015
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