The importance of earthworms in our garden soils cannot be unstated. They are a quick and visual guide to how healthy your garden soils are, lots of earthworms in a square foot of soil is the goal to having healthy plants. Few or no earthworms in your gardens indicates a big problem and a continuing fight to maintain your garden plants until you resolve the health aspects of the soil and worm numbers increase.
Now that the soils are moist you can easily find out what your worm populations are like by carefully lifting some soil in each of your garden plots. During dry times you are unlikely to find worms near the surface of the soil.
The recent publication of OrganicNZ magazine has an interesting article on earthworms in New Zealand which lead me to do a little research at (a free government NZ encyclopedia ) The following is information from that site:
The term “earthworm” cannot be satisfactorily defined in the scientific sense but it serves to describe a large number of species of the order Oligochaeta which inhabit soils and accumulations of decaying plant materials, and are occasionally found in shore and aquatic habitats. The earthworms of New Zealand consist of two groups of species. A large group of 173 native and five introduced species (27 genera) belong to the family Megascolecidae and a smaller group of 14 introduced species (seven genera) belong to the family Lumbricidae.
From the evidence of known distribution it is most likely that the native earthworms came originally from the Indo-Malayan or Australian region and entered New Zealand across a land-bridge connection from the north. They probably arrived in at least two waves, the first (subfamily Acanthodrilinae) in Cretaceous times and the second (subfamily Megascolecinae) in Tertiary times.
The largest, Spenceriella gigantea from North Auckland, attains a length of 4 ft 6 in. and a diameter approaching an inch, while a number of the smaller species are up to 1 in. long and less than 1/10 in. in diameter. Most of the native species are red or brown.
Earthworms are hermaphroditic, both male and female organs being present, but they are not self-fertilising and, when mating takes place, sperm cells are exchanged.
Native earthworms feed almost entirely on dead and decaying remains of plants and, because of their limited capacity to move about, they are obliged to live very close to their sources of food. The presence of free water is essential for they have virtually no mechanism for conserving moisture. Respiration takes place by diffusion of gases through the moist body wall; hence both moisture and dissolved oxygen are essential. Earthworms are injured and may die by exposure to daylight, except when the intensity is very low, the more pigmented species being more resistant to light damage than the less pigmented. They are killed by temperatures in excess of 85°F100F, but in most New Zealand habitats they escape the effects of extreme high or low temperatures by retreating to lower layers.
The pH tolerance varies from species to species but no native earthworms have been found in soils lower than pH4. Most earthworms are able to tolerate submersion in water and there are a few species that prefer an aquatic life. During heavy rains, however, earthworms are commonly driven to the surface, but this is most probably due to the shortage of oxygen in the water in their burrows.
The leaf-mould species are small, active, and heavily pigmented. The smallest is 15mm long and the largest 180 mm but most are between 20 mm and 50 mm. They do not make permanent burrows but move around freely in the loose material just as arthropods and other animals do; hence they are more prone to capture by predatory birds and are more frequently exposed to ultraviolet light than those species inhabiting topsoil or subsoil.
Both the topsoil and the subsoil dwellers have two distinct methods of making burrows in which to live. In the first method soil is swallowed and subsequently cast either at the soil surface or in natural cracks and cavities in the soil and in deserted burrows. In the second method the anterior end of the body is extended and inserted in spaces between the soil particles and then, by contracting the longitudinal muscles, the body is expanded laterally, compressing the soil to form a burrow. Usually burrowing consists of a combination of these two methods, the former predominating in more-compact soils and the latter in less-compact soils. As a burrow is formed it is lined with slime and thus has smooth walls firmly compacted by the lateral pressure applied during its construction.
Subsoil earthworms are usually large, sluggish, and unpigmented. The smallest is 32·5 mm and the largest 1,400 mm, but most are between 100 mm and 400 mm in length. The majority are circular in cross-section and have weakly developed body-wall muscles. They occasionally come to the surface or near to the surface for food, but otherwise are found only in the subsoil. They make very extensive burrows extending both laterally and vertically in the subsoil and occasionally going up into the topsoil. (Burrows of Spenceriella gigantea have been found about 20 mm in diameter and still continuing downwards at a depth of 11 ft 6 in.) They appear to make these burrows to obtain food by ingestion of soil and not primarily for shelter, like the burrows of the topsoil species. As they move forward they may deposit castings in the section of burrow left behind and it is not uncommon to find burrows partly filled with subsoil castings.
The most common introduced earthworms belong to the family Lumbricidae and, since such a large part of New Zealand has been cleared of the original vegetation and sown down to pasture, the lumbricid earthworms which feed on dead root and leaf material from pasture have become the dominant earthworm fauna both in pasture and in cultivated lands.
After the clearing of the land, native earthworms decline rapidly. The leaf-mould fauna is eliminated since there is no supply of leaf-mould; the topsoil fauna is usually eliminated but occasionally persists in a much reduced form; the subsoil fauna may be relatively unaffected but, if the soil is continually cultivated, this, too, fails to survive.
Earthworms affect soil fertility in various ways. Their burrows provide drainage channels through the soil, improve its aeration, and assist deep root penetration. The lumbricid species of New Zealand pasture lands are all topsoil dwellers but in summer, if the surface soil becomes too dry, they retreat into the subsoil and go into diapause a state of suspended animation. The vertical channels so made remain for a short time as subsoil drainage channels but, as their thin walls are not usually firmly compacted, they soon collapse.
The quantity of soil deposited at the surface in the form of worm casts was found (Evans 1948), on eight fields with different management histories, to range between 1 and 25 tons per acre per year. Calculations, based on total populations and taking account of species that cast beneath the surface, showed that from 4 to 36 tons of soil per acre per year passed through the alimentary systems of earthworms and were cast at or near the surface.
These topsoil earthworms play an important part in our grassland farming and gardens. They stimulate pasture growth by removing dead root material, loosening up the sod, and providing an enriched layer of cast soil in which perennial grasses, clovers and your plants are able to re-root year by year.
Too many gardeners destroy their worm populations by using herbicides, manmade fertilisers, chemical sprays and chlorinated water. If the worms are scarce then so is the rest of the soil life and your garden plants suffer as a result. You need to reverse this process if you want great gardens.