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.