There are about 2,700 species of earthworms, ranging in size from about ½ inch long to about 3 to 4 feet long and 1 inch in diameter. Aside from the fearful prospect of running into a giant Gippsland earthworm, most people consider earthworms to offer more benefits than drawbacks and many gardeners go to great lengths to attract earthworms to their gardens. However, there are circumstances where fewer earthworms would be better.
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Why Kill Earthworms?
Earthworms are a critical part of the ecosystems that sustain plants, crops and trees. They aerate the soil by digging tunnels, allowing air to get to the roots of plants. And helping to prevent the soil from compacting and crusting. They increase the capacity of soil to hold water as well as making the soil drain better. They eat, digest and expel organic matter. The excrement, or castings, "has five times the nitrate, 11 times the potash, seven times the phosphorous and three times the magnesium of the soil around them," according to the Oxford Croquet website. To put it another way, the castings contain 10 times more nutrients than does commercial topsoil. These castings that provide such benefits to sustain flora are also the problem. Castings on lawns are unattractive. On sports fields and golf courses, they interfere with using the area as intended and therefore a lot of effort and money goes into cleaning them up. Getting rid of the earthworms, by killing them or by other means, would solve that problem.
Using Vinegar to Kill Earthworms
Earthworms have no lungs, so they breathe through their skin. In order for their breathing mechanism to work, their skin must remain moist. Vinegar causes moisture to be released from plants and animals that it touches, which is why it works so well to kill weeds growing up in brick patios or concrete or asphalt driveways. So, yes, you can spray earthworms with vinegar and the vinegar will kill them. The difficulty is that you cannot just spray the vinegar on the lawn or sports field from which you want to remove the worms because you will kill the grass as well.
According to the website All About Worms, earthworms will come to the surface if their environment is too moist, water would take the place of the oxygen in the soil the worms need to breathe. If the soil gets too wet, the earthworms may come to the surface at night and migrate to dryer conditions. If they come to the surface during the day, the sunlight will probably kill them. The challenge here is to flood out the worms without killing the grass that you are trying to protect. The other thing you can try is to reduce their food sources (leaves and dead grass, which provide the bacteria, algae and fungi the worms need). Another option, suggested by Oxford Croquet, is to manipulate the acidity of the soil to make it higher and thereby draw the worms deeper into the earth. Again, the challenge is to change the acidity of the soil without killing the grass.
Many pesticides and fertilizers used on plants are not good for earthworms, so that might provide a path by which you could keep the grass green and eliminate, or at least cut down, on the earthworms.