Of the 3,000 varieties of earthworms that exist on this planet, only about a half dozen varieties are found naturally all over the United States in numbers that have impact on the soil here. They are not as abundant in the arid regions of the west and especially southwest. Even there, they can be found in irrigated areas and along the banks of rivers and streams. They range in size from the large Night crawler (nearly 1 foot in length) to the small Green Worm (2 to 2 1/2 inches in length). Besides having use as fishing bate, they are beneficial to soil where crops and landscape plantings are grown.

Earthworms help prevent erosion where the soil is primarily clay. Water does not penetrate into clay fast enough, which causes gullies and washouts of layers of soil. When earthworms are present, they create channels through the soil that allows water to penetrate deeper into the clay, enabling the soil to retain moisture to a deeper level and allowing oxygen to aerate the soil. Nutrients are added to the soil by the earthworms which improves the quality of the soil.

Aerated soil is not only caused by the channels of earthworms, but also by birds, especially Robins, which penetrate the soil by pecking holes in search of the worms for food. This helps the top layer of soil begin to receive the water that runs deeper into the channels caused by the burrowing earthworms.

Soil that is compact or that is mostly clay on flat land will allow water to stand for long periods depriving the soil of adequate oxygen for earthworms and most plants.

Earthworms may be added between layers of compost to aid in the process of breaking down the matter into rich garden soil. Earthworms may be obtained at bait houses or can be dug up from other locations for free. Bringing earthworms from other locations usually will not be necessary; there are likely to be a few worms in nearly any location. The existing population, scant though it may be, can be made to increase in numbers. If there are no worms at all, the soil may have some kind of problem such as contamination. Soil in such cases should be tested before trying to add worms to conditions that may not support life.

Earthworms require food just like all other living organisms. Besides minerals that are present in soil, earthworms require any kind of organic matter. Decaying grasses will supply nutrients in open fields while decaying leaves will provide nutrients in wooded areas. Nutrients must be added continuously to the soil for the earthworm population to remain at a high level. When the soil is deprived of nutrients, the earthworm population will decrease.

Besides decaying grass, brush, leaves and wood as occurs in nature, there are many nutrients that can be added to soil to help raise the earthworm population. These nutrients are free since they are simply the garbage taken from the average kitchen. Just about anything except grease, fat, glass, plastic, metal, petroleum products and polyester fabrics will supply nutrients to the soil for the benefit of the earthworms.

Such organic items as vegetable scraps from salads, coffee grinds, potato peals, banana peals, apple cores, melon rinds, and the usual items that need to be thrown out after a typical meal will be welcome nutrients to the earthworms.

Some of the items from the pruning of plants and bushes can be ground up for mulch, such as dead plant parts (not diseased), stems, leaves, and bark. Grass clippings, and uprooted weeds can be added.

Other sources of organic material may come from vacuum cleaner bags, lint traps from the clothes dryer (as long as there are not too many polyester fibers), sawdust or wood shavings, and shredded or torn strips of newsprint paper.

Manures can be added to the soil, or it is possible to grow "green manure" from such crops as fenugreek, clover and alfalfa. These crops produce nitrogen rich nodules attached to their roots which benefit the soil and the earthworms.

 Besides food, the earthworm population will benefit from occasional watering, especially during periods of drought. Oxygen is also important. A tightly compacted clay that allows water to stand for long periods will not allow enough oxygen below the surface to benefit the worms. Below the surface of such standing water, there may not even be enough moisture because the water is not seeping down into the ground. Aerating or turning over such soil and the addition of a little sand and a lot of leaves will make such ground more habitable for earthworms.

Generally, any material that can be used to make compost can also be added directly to the garden soil at regular intervals to keep the earthworm population thriving.