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EAP Publication - 6
by Jennifer A. Ramsay and Stuart Hill
While nature works slowly in the production of topsoil, often over centuries, man, through poor agricultural practices, may deplete this valuable resource within an individual's lifetime. In the absence of a rich population of soil animals, 500 to 1000 years may be required to create an inch of topsoil. However, under favourable conditions, earthworms, lowly creatures to many people, can speed up this process to only five years. As agriculture, and ultimately civilization, depend on the maintenance of a fertile topsoil (Hyams, 1952; Mitchell, 1946), it is in our best interest to encourage earthworms in their soil building activities.
Long before the invention of agricultural implements, earthworms ploughed the soil, mixing, tilling and building topsoil as they burrowed through the earth. Their importance has been clearly recognized for nearly 200 years, and even in the Fourth Century B.C., Aristotle, it is said, aptly referred to earthworms as "the intestines of the earth" though he may well have been referring to their appearance rather than to their function. But what do we know about these animals? The following will help us to understand earthworms and how we may be able to benefit from their activities.
Relatively few people realize that the "common" earthworms, of which the best known species, but not always the commonest, is called Lumbricus terrestris by biologists, are just as much newcomers to North America as we are. During the last glaciation period even native earthworms were unable to survive in those areas covered with ice and have only been reintroduced (often accidentally) to these soils by man, the familiar ones since European settlement. In fact, there are many areas in Canada where earthworms are absent and where the productivity of the soils could be substantially increased if they were introduced.
Earthworms are most numerous in grassland and mull soils, relatively rare in acidic (or mor) soils, and intermediate and variable in numbers in arable lands. However, other factors such as soil texture, moisture, temperature, and food supply also determine whether a field may have more earthworms than one on a neighbouring farm. Earthworm populations vary not only from one soil to another, but also throughout the year, being most numerous in the spring and early summer.
There are many species of earthworms and each generally has different preferences for soil conditions. Of the 200 species found in North America only 18 have been found in Canada; only six of these are native to this country. Some species are only found within the top surface layers while others, such as Lumbricus may be able to penetrate several feet to the subsoil horizon. Those that live within the surface layers generally migrate to lower depths during the summer as the soil becomes drier. Cultivation of the soil may enable earthworms to penetrate further into the soil.
The accidental introduction of earthworms to North America may have been the best outcome of contact between the early settlers and native peoples. While the Indians suffered from lack of exposure and immunity to European diseases, the fertility of many North American soils was greatly enhanced through the recycling of organic matter by earthworms.
The activity of earthworms is most easily appreciated by comparison with the situation in regions where they are absent: the decomposition of organic matter there is slow, such that layers of litter accumulate on the soil surface and fail to be incorporated into the soil. More specifically, the activity of earthworms is important to the agriculturist in four respects, in that they:
The burrowing of earthworms improves the physical structure of the soil, creating channels through which plant roots may more easily penetrate the soil. In addition to increasing soil porosity and aeration, this activity also improves soil drainage and water penetration while eliminating hardpan conditions. Earthworms may also enhance soil structure through the formation of aggregates. Secretions in earthworm intestines cement soil particles together into aggregates which aid in erosion control. Man, through agricultural practices, such as cultivation, may temporarily improve soil structure, but the earthworm has longer-term effects in maintaining soil tilth.
As earthworms burrow through the earth, they consume large quantities of soil and fresh or partially decomposed organic matter from the soil surface, depositing it as fecal matter, or casts, in the lower soil horizons. Similarly, soil from the subsoil horizon is moved by these animals to the upper levels where it is mixed with the surface soil, resulting in a more uniform distribution of plant nutrients. Charles Darwin (1881), the naturalist famous for his ideas on evolution, estimated that 10.6 tons of materials are brought to the soil surface of each acre by earthworms (or approximately St/hectare).
Through the ingestion of organic matter earthworms are important to the initial breakdown as well as to subsequent decomposition of organic matter. In fact, earthworms may consume more surface organic matter than all other soil animals together. This material is eventually excreted as casts, concentrating nutrients and rendering them more water-soluble and available to plants. Researchers have found that worm casts are generally richer in exchangeable calcium, potassium, and phosphorus than the surrounding soil, while earthworms themselves and their excretions are valuable sources of nitrogen. By bringing soil nutrients to the upper horizons from the lower subsoil, the earthworms counteract the effects of leaching whereby many nutrients are washed from the root zone and consequently rendered unavailable to plants.
As earthworms are a measure of soil fertility, so are they indicators of soil management practices. Consequently, the use of earthworms to our benefit depends not only upon a knowledge of their activities but also upon an awareness of how our own activities, in particular agricultural practices, may influence their distribution.
Earthworm population may be increased or decreased by the following agricultural practices:
As previously mentioned, earthworms are generally more numerous in grasslands than in arable land. Evidence indicates, however, that earthworm populations do not decline from mechanical damage during tillage operations, but rather from a reduction in the organic matter content of the soil. Repeated row cropping will reduce the number of earthworms, while the inclusion of grass or field crops in a rotation and intercropping will counter this effect.
Limestone generally increases earthworm populations and, in poor soils, nitrogen fertilizers may also benefit these indirectly. Most other mineral fertilizers have little effect on earthworm numbers, while organic matter such as manure, crop residues, or mulches favour earthworm multiplication by providing them with a source of food.
Many (though not all) of the insecticides, herbicides, and fungicides that are used to control agricultural pests are toxic to earthworms and may conflict with the natural biological control of pests. For example, earthworms play an important role in the control of apple scab, caused by the fungus Venturia inequalis, which overwinters on fallen leaves and twigs. Apple scab may be culturally controlled by burning these disease-carrying materials in the fall, or it may be chemically prevented through the use of copper sulphate, which is also toxic to earthworms. A less expensive, but equally effective means of controlling apple scab, however, is the introduction of earthworms, preferably Lumbricus terrestris, into orchard soils. These animals take the fallen leaves and twigs into their burrows where the vegetation eventually decomposes and ceases to be a source of disease. (One researcher found that earthworms may remove up to 90 per cent of leaf-fall in orchards (Raw 1962).)
Other pesticides that are lethal to earthworms include arsenic and copper compounds, chloropicrin, metham sodium, methyl bromide, D-D, chlordan, heptaclor, phorate and carbamate insecticides (Edwards and Lofty, 1972). Although other compounds may be less toxic to earthworms, these chemicals are concentrated in their bodies and may be lethal to birds and mammals when they are eaten.
Earthworms have been successfully introduced into areas where they are absent and have been found to increase the yield of crops. The long-term benefits of encouraging earthworms can be translated into dollars. Researchers have estimated that for every dollar invested in earthworms on New Zealand sheep farms, the farmer can expect a return of $3.34 and an increase in carrying capacity of 2.5 stock units/hectare or an increase in productivity of 25-30 per cent (Crump 1969).
When considering the use of earthworms to improve soil fertility it is important to remember that these animals thrive only under certain conditions. Most are unable to survive in sandy, dry, acid soils and all need organic matter for food. In addition, not all earthworm species are suitable for land reclamation. Species that are the easiest to cultivate, i.e., those grown on compost or manure piles, are usually not suitable for inoculation of arable lands.
Earthworms are familiar to the fisherman and poultry producer as bait or animal feed, but few North Americans realize that earthworms are regarded as a source of dietary protein, even a delicacy, by other cultures. However, popularized by events such as the Great Canadian Worm Recipe Contest, these animals may eventually worm their way into North American kitchens in the form of Mrs. Wiebe's Wiggly Cake (Waldon 1978). Earthworms have also been used for medicinal purposes since ancient times in the treatment of illnesses such as bladder stones, jaundice, rheumatism, fever and impotency. Their efficiency, however, requires proper scientific investigation!
Vermiculture, the art of breeding and raising earthworms, is a billion dollar enterprise, supplying eager fishermen, zoos, fish hatcheries, poultry producers, and biology classrooms. The production of earthworms requires large amounts of organic matter with which to feed them. Consequently, vermiculture could be easily integrated with industries such as canneries, breweries, slaughterhouses, and papermills where large quantities of organic waste are produced. Rabbit breeders have found that earthworms placed under hutches are very effective in controlling odours from animal droppings and provide extra income if the earthworms are sold. Similarly, many of our urban wastes could be recycled through earthworms, solving many of our current problems in respect of solid waste disposal and water pollution.
Whether we are backyard gardeners or fully fledged farmers, it must be remembered that earthworms are not the antidote to infertile soils and poor management. If soils are to be improved through the use of earthworms, we must provide them with sufficient food and moisture. Only then may we profit from their activities as ploughmen and builders of the soil.
Crump, D. R. 1969. Earthworms--a profitable investment. N.Z. J. Agric. 119 (2): 84-85.
Darwin, C. 1881 (1945 reprint). Darwin on humus and the earthworms: the formation of vegetable mould through the action of worms with observations on their habits, 153 pp. Faber & Faber, London.
Edwards, C. A. and J. R. Lofty. 1972. Biology of earthworms. 283 pp. Chapman and Hall Ltd., London.
Hyams, E. 1952. Soil and civilization. 312 pp. Thames and Hudson, London.
Mitchell, E. 1946. Soil and civilization. 141 pp. Angus and Robertson, London.
Raw, F. 1962. Studies of earthworm populations in orchards. l. Leaf burial in apple orchards. Ann. app. Biol. 50: 389-404.
Waldon, B. 1978. Vermiculture is good for you Harrowsmith 3 ((1): 47-50.
Barrett, T. J. 1949. Harnessing the earthworm. 166 pp. Wedgewood Press, London, Boston, Massachusetts.
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Hopp, H. 1973. What everyone who gardens should know about earthworms. 39 pp. Garden Way Publ., Charlotte, Vermont.
Kevan D. K. McE. 1968. Soil animals. 2nd edn. 244 pp. Witherby, London.
Reynolds, J. W. 1973. Earthworms (Annelida:Oligochaeta) ecology and systematics. pp. 95-120. In: Dindal, D. L., ed. Proc. 1st Soil Microcommunities Conference. U.S. Atomic Energy Commission.
Reynolds, J. W. 1977. The earthworms (Lumbricidae and Sparganophilidae) of Ontario. 141 pp. Royal Ontario Museum, Toronto.
Rodale, R. (ed). 1961. The challenge of earthworm research. 102 pp. Soil and Health Foundation, Emmaus, Pennsylvania.
Satchell, J. E. 1967. Lumbricidae. pp. 259-322. In: Burges, A. and F. Raw (eds.). Soil biology, Academic Press, New York.
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