Earthworms are the silent partners in our gardens. To a large degree, they are responsible for the well-being and health of our soil. But what do they do that's so important?

In the Yakima Valley, the yeoman's work of improving our soils is accom-plished just by two species of earthworms: the common field worms, Aporrectodea caliginosa, and nightcrawlers, Lumbricus terrestris.

Field worms plow through the soil ingesting soil, organic matter and minerals as they meander seemingly aimlessly through the soil. The result is a maze of tunnels, none of which is used more than once.

In early spring they break their dormancy deep down in the soil where they have spent the winter and tunnel nearer the surface. With the onset of hot summer weather, they retreat deeper to escape the heat. When the weather finally moderates in late summer, they once again return to the root zone of your garden and again become active. By the time winter has returned, they have tunneled well below the frost line and become dormant. So field worms, like all other earthworms, are most active during spring, early summer and fall.

Nightcrawlers operate somewhat differently. They make vertical tunnels that go down as much as 6 feet deep. Their tunnels are more or less permanent. While field worms feed under-ground, night-crawlers feed primarily at the soil's surface on organic matter that they pull into the entrance of their tunnels. Then they plug their surface entrance with their excretions, which are known as castings. Nightcrawlers are generally outnumbered 10-1 by field worms, but both kinds are important to gardeners.

How earthworms improve soil

Earthworms improve our soil in myriad ways. Here are some of the more important ways they accomplish this remarkable feat:

Earthworms eat their weight in soil, organic matter and minerals each day. In the process, their castings become five times richer in nitrogen, 11 times higher in potassium and are seven times higher in phosphorous as is the surrounding soil. The nutrients in castings are available almost immediately for use by the roots of the plants in our gardens.

As nightcrawlers delve deeply into the soil, they bring up minerals and nutrients that are often in short supply in the root zone. In the process, they also bring up nutrients that have leached out of reach.

Roots use burrows to help penetrate deeper into the soil to reach nutrients that would otherwise be out of their reach. Burrows also improve aeration and the ability of the soil to take in and store water. In short, the actions of earthworms help build soil structure.

Bacteria living in the intestines of worms also play an important part in making the nutrients in castings available to plants. Earthworms in turn spread these one-celled creatures throughout the soil in their castings where they continue to break them down along with other organic matter in the soil, which is them made available to plants.

Castings are excreted throughout the soil. Over time, castings help neutralize both acidic and alkaline soils. Most of our shrubs, trees, perennials and annuals grow best in soils that are in the neutral range that has a pH of 6.8 to 7.2.

In a year's time, earthworms rotate between 20 tons and 40 tons of soil per acre. This mixing of soil containing all the accumulated castings creates the ultimate soil for gardening.

Some bacteria living in the intestines of earthworms are able to detoxify hazardous chemicals in the soil. High concentrations of toxic matter in the soil, however, can be deadly to earthworms.

Encouraging earthworm population in your garden

Help your earthworm populations grow by adding compost, aged manure and shredded to the surface of the soil. Beneficial bacteria and other microorganisms, along with nightcrawlers, will slowly incorporate this organic matter into the soil where it will become available as nutrients to you plants.

Add organic mulch, such as shredded leaves and lawn clippings, during the growing season and again in late fall. Mulch moderates soil temperature both during the summer and fall. This keeps your earthworms, as well as beneficial microorganisms, working longer during the growing season.

Keep tilling to a minimum since it often mangles earthworms and brings microorganisms to the surface where they quickly dry out and die. If hoeing is needed to kill small weeds, cultivate just the top inch of soil to avoid damaging the soil's structure and destroying earthworms and beneficial microorganisms.

Earthworms are moisture sensitive. To keep them working, keep your soil evenly moist. Either oversaturation or letting soil dry out causes them to retreat deep into the soil, where they will no longer be your partners in gardening.

Considering the fact that earthworms are totally blind, deaf and spineless, wouldn't you agree that these silent partners in our gardens are more worthy of huge bonuses than many big bank CEOs are receiving?

* Freelance gardening columnist Jim McLain can be reached at 509-697-6112 or ongardening@fairpoint.net.