The Soil

(Image from UMN 2005)

The first thing that Earthworms do when they invade a forest is to eat the O horizon, the layer that makes up the forest floor. This layer is composed of fresh and partially decomposed litter that has accumulated over many years. The litter contains twigs, leaves, seeds, bark and wood from small fragments of large logs. It is full of organisms and is very important to the overall functioning of the ecosystem. Within a matter of a few years, earthworms can consume the whole layer of litter and all the organisms that live in it. In many areas, this layer is completely eliminated and only bare soil remains. (UMN 2005) In a study that looked at the effects of invasive earthworms on leading edges in four northern hardwood stands, researchers found that “increasing total earthworm biomass was associated with rapid disappearance of the O horizon. Concurrently, the thickness, bulk density and total soil organic matter content of the A horizon increased, and it's percent organic matter and fine root density decreased.” (Hale 2005)

The A horizon was very thin before the earthworms arrived, about 1 cm, but now it gets very thick, between 10 and 15 cm. The original A horizon was very loose and moist because of the amount of organic material present. The post-earthworm A horizon lacks this organic material and therefore is compacted in comparison. (UMN 2005)

The E horizon developed beneath the A horizon. It’s thickness is about 10 to 20 centimeters and is composed of soil deposited both during the retreat of the glaciers and before it was covered by forests. The soil contains various amounts of clay, sand, silt and rocks. The top of the layers is dark black or gray and gets lighter in color it gets deeper. The dark color comes from organic molecules carried down from the A horizon, a process is called leaching. (UMN 2005)

The B horizon lies beneath the E horizon and is composed of the same material. This layer can be either very thick or thin depending on the site and is usually yellow, brown or red because how it was when it was originally deposited. However dissolved molecules of iron or salts that leach down with rainwater can also affect it. (UMN 2005)

The C horizon is under the B horizon and is made of the same material as E and B horizons. It has not been changed by leaching and is almost identical to what it was after glacial retreat. It is often referred to as “parent material”. (UMN 2005)

Research on earthworm invasions has focused on changes in soil structure and carbon and nitrogen cycling that occur following invasion. Read more.

View effects on forest soil