Food supply primarily determines earthworm population. To increase their numbers it is merely necessary to bring in additional organic matter or add plant nutrients that cause more vegetation to be grown there. In one study, simply returning the manure resulting from hay taken off a pasture increased earthworms by one-third. Adding lime and superphosphate to that manure made an additional improvement of another 33 percent. Every time compost is added to a garden, the soil's ability to support earthworms increases.
Some overly enthusiastic worm fanciers believe it is useful to import large numbers of earthworms. I do not agree. These same self-interested individuals tend to breed and sell worms. If the variety being offered is Eisenia foetida, the brandling, red wiggler, or manure worm used in vermicomposting, adding them to soil is a complete waste of money. This species does not survive well in ordinary soil and can breed in large numbers only in decomposing manure or other proteinaceous organic waste with a low C/N. All worm species breed prolifically. If there are any desirable worms present in soil, their population will soon match the available food supply and soil conditions. The way to increase worm populations is to increase organic matter, up mineral fertility, and eliminate acidity.
Earthworms and their beneficial activities are easily overlooked and left out of our contemplations on proper gardening technique. But understanding their breeding cycle allows gardeners to easily assist the worms efforts to multiply. In temperate climates, young earthworms hatch out in the fall when soil is cooling and moisture levels are high. As long as the soil is not too cold they feed actively and grow. By early spring these young worms are busily laying eggs. With summer's heat the soil warms and dries out. Even if the gardener irrigates, earthworms naturally become less active. They still lay a few eggs but many mature worms die. During high summer the few earthworms found will be small and young. Unhatched eggs are plentiful but not readily noticed by casual inspection so gardeners may mistakenly think they have few worms and may worry about how to increase their populations. With autumn the population cycle begins anew.
Soil management can greatly alter worm populations. But, how the field is handled during summer has only a slight effect. Spring and summer tillage does kill a few worms but does not damage eggs. By mulching, the soil can be kept cooler and more favorable to worm activities during summer while surface layers are kept moister. Irrigation helps similarly. Doing these things will allow a gardener the dubious satisfaction of seeing a few more worms during the main gardening season. However, soil is supposed to become inhospitably hot and dry during summer (worm's eye view) and there's not much point in struggling to maintain large earthworm populations during that part of the year. Unfortunately, summer is when gardeners pay the closest attention to the soil.
Worms maintain their year-round population by overwintering and then laying eggs that hatch late in the growing season. The most harm to worm multiplication happens by exposing bare soil during winter. Worm activity should be at a peak during cool weather. Though worms inadvertently pass a lot of soil through their bodies as they tunnel, soil is not their food. Garden worms and nightcrawlers intentionally rise to the surface to feed. They consume decaying vegetation lying on the surface. Without this food supply they die off. And in northern winters worms must be protected from suddenly experiencing freezing temperatures while they "harden off" and adapt themselves to surviving in almost frozen soil. Under sod or where protected by insulating mulch or a layer of organic debris, soil temperature drops gradually as winter comes on. But the first day or two of cold winter weather may freeze bare soil solid and kill off an entire field full of worms before they've had a chance to adapt.
Almost any kind of ground cover will enhance winter survival. A layer of compost, manure, straw, or a well-grown cover crop of ryegrass, even a thin mulch of grass clippings or weeds can serve as the food source worms need. Dr. Hopp says that soil tilth can be improved a great deal merely by assisting worms over a single winter.
Gardeners can effectively support the common earthworm without making great alterations in the way we handle our soil. From a worm's viewpoint, perhaps the best way to recycle autumn leaves is to till them in very shallowly over the garden so they serve as insulation yet are mixed with enough soil so that decomposition is accelerated. Perhaps a thorough garden clean-up is best postponed until spring, leaving a significant amount of decaying vegetation on top of the soil. (Of course, you'll want to remove and compost any diseased plant material or species that may harbor overwintering pests.) The best time to apply compost to tilled soil may also be during the autumn and the very best way is as a dressing atop a leaf mulch because the compost will also accelerate leaf decomposition. This is called "sheet composting" and will be discussed in detail shortly.
Certain pesticides approved for general use can severely damage earthworms. Carbaryl (Sevin), one of the most commonly used home garden chemical pesticides, is deadly to earthworms even at low levels. Malathion is moderately toxic to worms. Diazinon has not been shown to be at all harmful to earthworms when used at normal rates.
Just because a pesticide is derived from a natural source and is approved for use on crops labeled "organically grown" is no guarantee that it is not poisonous to mammals or highly toxic to earthworms. For example, rotenone, an insecticide derived from a tropical root called derris, is as poisonous to humans as organophosphate chemical pesticides. Even in very dilute amounts, rotenone is highly toxic to fish and other aquatic life. Great care must be taken to prevent it from getting into waterways. In the tropics, people traditionally harvest great quantities of fish by tossing a handful of powdered derris (a root containing rotenone) into the water, waiting a few minutes, and then scooping up stunned, dead, and dying fish by the ton. Rotenone is also deadly to earthworms. However, rotenone rarely kills worms because it is so rapidly biodegradable. Sprayed on plants to control beetles and other plant predators, its powerful effect lasts only a day or so before sun and moisture break it down to harmless substances. But once I dusted an entire raised bed of beetle-threatened bush bean seedlings with powdered rotenone late in the afternoon. The spotted beetles making hash of their leaves were immediately killed. Unexpectedly, it rained rather hard that evening and still-active rotenone was washed off the leaves and deeply into the soil. The next morning the surface of the bed was thickly littered with dead earthworms. I've learned to treat rotenone with great caution.
Microbes and Soil Fertility