By Haiying Tao, Washington State University Extension, Washington State University Farmers Network; Dawn Pettinelli, University of Connecticut Home & Garden Education Center
Happy are the farmers and gardeners who cultivate their fields or turnover their garden beds to find numerous earthworms working hard to improve their soil. Typically, high numbers of earthworms are associated with a healthy soil. They are even one of the field indicators for soil health assessments and for good reason. Earthworms are considered ‘ecosystem engineers’ meaning they have the ability to affect and change basic ecosystem processes and functions.
In agricultural fields and home gardens, earthworms usually bring welcome beneficial changes. As they tunnel through the soil, they create channels for air and water and plant roots to travel. This helps in water distribution, gas exchanges, and deeper roots penetration. A healthy soil breathes. As it heats up during the day, it expands allowing atmospheric gases like oxygen to enter and when the soil cools at night, it contracts allowing gases that built up in the soil, like carbon dioxide to escape.
As the earthworm feed on decaying organic matter, they also consume soil microbes and small particles of soil. After being ground up and digested, what is not used by the earthworm is excreted in the form of casts, small aggregates of nutrient rich droppings. Analyses of casts have found them to be higher in nitrogen, phosphorus and potassium than the surrounding soil. It is estimated that a single earthworm could potentially produce 10 pounds or more of casts annually. This aggregation is welcome in compacted soils as it improves the soil’s structure increasing pore spaces for air and water to travel.
The nutrients bound in organic matter are transformed by the earthworm into forms that plants can easily take up. This increases readily available nutrient levels in the soil plus nutrients are also contained in the casts. As the casts slowly break down, more nutrients are released over time. Depending on the earthworm species, casts may be deposited on top of the soil, at surface levels or deeper in the soil.
Although there are diverse species of earthworms, not many species were found in Washington State. Only two species of earthworms were found in an earth worm survey conducted on 36 fields in the southeastern Washington, western Idaho, and northeastern Oregon. The dominant species found was aporrectodea trapezoides, followed by lumbricus terrestris. This was not surprising because the A. trapezoids is an exotic-invasive species and successfully adapted in various environment worldwide. There was a possibility of existence of anecic species such as L. terrestris, however, this survey was not able to confirm the existence due to the limitations of the sampling technique, according to a publication by Dr. Johnson-Maynard’s team from University of Idaho.
Many factors of climate and soil conditions affect variation of species and population density of earthworms. The abundance can also vary significantly within a field due to spatial variation of soil properties, such as soil temperature, soil moisture, soil organic matter, soil texture, and soil pH. Management practices that alter these soil conditions, therefore, affect diversity and population of earthworms.
In hilly fields, soil temperature and moisture can differ significantly in different aspect of the slopes, and soil organic matter content is typically higher in the bottom of hills than steep slope positions. Soil temperature and moisture outside of earthworm’s comfort zone can cause reduced number of species and populations. Soil organic matter is important for earthworms because it provides food for them. Earthworm populations is generally low in well drained coarse textured soils because these soils dry out quickly during dry seasons. Earthworms also avoid heavy clay soils as these soils tend to be waterlogged during heavy rainfall seasons, creating anaerobic conditions. The optimum soil pH for earthworms is between 5.0 and 7.4 and they generally are not found in soils when soil pH is below 4.5.