Chapter 10. The Misunderstood Earthworm

Contributions of various kinds are made to garden soils by "organisms" other than fungi and bacteria. I refer to creatures that live at least part of their lives in soil and are large enough to be seen with the naked eye. Most of them are outright garden or lawn enemies and are discussed (with recommended control measures) in Chapter Fifteen. The earthworm, however, defies such simple classi­fication. Is it a "good guy" or a "bad guy"? I shall try to bring an answer—or at least a better understanding—out of the fog of con­troversies, misinformation and half-truths that surround the earth­worm today as they have for many years.

Especially during the past two decades, flamboyant and extrav­agant advertising has created a picture which is completely false and misleading. The earthworm has been credited with being the source of all true fertility (ignoring completely the much more vital role of other soil organisms). It has been hailed as the savior of mankind. Articles have actually appeared in print which blame increases in human cancer on the destruction of earthworms by chemicals and modern tillage methods 1.

Darwin Data

The authority most frequently quoted to back up these claims is Charles Darwin, who in 1885 wrote a book, The Formation of Veg­etable Mold, in which he reported his observations of the role of earthworms in soil formation and modification. This was a sound piece of observation, and obviously the work of a man familiar with the scientific approach to problems. Although today we can add a few facts to his original observations, we cannot refute his conclusions. The difficulty lies not in what he said about earthworms but in how his account has been "doctored" or misinterpreted.

Darwin reported (and his findings have been confirmed by later observers) that earthworms in an acre of ground move as much as 15 tons of soil a year. This adds up to a layer about 1/lOth of an inch deep over the acre. This does not mean they move only the upper 1/lOth inch, since they do go quite deep at times. However, the total mass moved from place to place in a year's time will equal that amount of soil. And this gives the worm credit for always turn­ing over fresh soil, when it is obvious that considerable backtracking is inevitable so that the same soil may be moved several times a year. In any case, earthworms would take 70 to 100 years to turn over the same amount of soil that a man can turn over in one hour with a rotary tiller. Thus the soil-turning efforts of earthworms seem rather far from the magic process described for us in such glowing terms by earthworm-farm advertisers.

Right Species

The proper species must be present in the garden and, of course, in the cultures introduced from commercial sources. There are a number of earthworm species that occur in European and American gardens, but only two are important. One of these is Lumbricus terrestris, a dark red species found in soils that have a high organic content; the other is AUolobophora calignosa, a grayish-pink species which does not require quite as much organic matter to survive.

In both species, the body is made up of interlocking segments, interrupted about one third the way down by a smooth area known as the girdle.

Since these two species are the ones which propagate most readily in ordinary garden soils, one might reasonably expect to find one or the other in cultures supplied by commercial earthworm farmers. This is not the case. I have never found either species in the cans or packages supplied by earthworm farms.

The worm cultures used for worm propagating at such farms are usually mixed with large amounts of protein compounds and organic matter. As protein breaks down, it passes through a stage where ammonia is released. Ammonia is harmful to most earthworms and may even kill the two species already mentioned. For this reason, the earthworm farmer propagates only the manure worm, Eisenia foetida, which is capable of surviving and propagating in the pres­ence of ammonia. Unfortunately, this one is incapable of surviving in clay or loam soils unless these have been freshly manured. As a re­sult, about the only place they will survive in the average garden is in the compost heap. Directions accompanying shipments of worms usually recommend placing them there.

All earthworms (with the exception of tropical species outside the scope of this book) must have soils that are high in both organic matter and moisture, but the manure worm has the highest require­ments of all in this respect.

As for the hybrid worms offered by advertisers in organic garden­ing publications, these may exist but I have not seen any hybrids which were recognized by taxonomists familiar with these creatures. Species of earthworms tend to remain in special habitats so that opportunities for crossing are limited between species within the same genus.

Most earthworms are hermaphrodites, so that, in mating, both individuals fertilize each other. Since they live in habitats of a single species as a rule, this cuts chances for hybridity in half, even if it were possible. In other species, parthenogenesis eliminates crossing altogether.

CREATING VS. CONSUMING

A great deal of fuss is made of the role of earthworms in "building rich soil" through their castings. As is well known to scientists, but apparently not to organocultists, earthworms are incapable of sur­viving unless the soil is rich. Unless fertility, texture, structure and organic content of a soil are to the liking of worms, they will not live and breed there, or remain there if they are introduced artificially. This may come as a shock to those who have spent good money for worms in an effort to build up poor soils.

Equally true is that earthworms are far from being the powerful allies promised in extravagant advertising. Earthworms have no mechanism for creating plant foods, for capturing solar energy or for fixing nitrogen from the atmosphere.

To some small degree, earthworms may make available some deeply-buried fertility that would not be available to shallow-rooted plants like petunias and alyssum. Most plants, however, even lawn grasses, are capable of sending their roots as deep into the soils as earthworms normally burrow.

In the process of living, the earthworm uses up or degrades energy instead of creating it. For this reason, the worm makes the soil poorer rather than richer by the amount of energy it has burned up and passed off as carbon dioxide. Even when the worm dies and its body returns to the soil to increase organic content, that contribution is reduced by the amount of energy the worm used during its life.

Worms redistribute richness rather than create new food elements. In relatively poor soils, but rich enough to keep earthworms alive, the grass may seem greener in spots where castings are heaviest. This comes from the very small amount of nitrogen excreted in the castings. Similar effects can be observed when earthworm "manure" is used on poor soils.

The Price You Pay

The "richness" of earthworm castings is a myth so ridiculous that it is difficult to understand how it ever got started. It is clear that before making such claims, the sellers never took the trouble to chemically analyze the castings for nitrogen. I have done so: I purchased a $1 package weighing eight ounces and had it checked by an independent testing laboratory. The nitrogen content was determined as 5/l,OO0ths of an ounce of actual nitrogen. At that rate, nitrogen would cost $200 an ounce. With nitrogen at about 70f* a pound from commercial sources, this seems like a terrible price to pay just to keep a myth alive.

One Western state insists that even natural manures must carry fertilizer analysis tags (these are usually only required on mixtures of chemical fertilizers). One "manufacturer" of a worm-castings product tried to show the nitrogen reading in parts-per-million to make it sound more impressive. He was, however, compelled to mark his package as containing .005 per cent nitrogen, less than the nitro­gen content of some drainage waters.

Feeding Habits

This low percentage of nitrogen in worm castings is not surprising when the feeding habits of the two common worm species are stud­ied. They eat by dragging long strips of leaves or grass down into their burrows. In forests, their diet consists of fallen leaves, even during the summer months. Instead of seeking out other plants on which to feed, they prefer to work on and grind down the leaf fall of the previous autumn.

They are amazingly selective in their food, with a strong prefer­ence for leaves of certain kinds of plants. In brushing over the leaf cover in a mixed grove of maples and oaks, I was surprised to find that the only remaining leaves in spring were from oaks, while maple leaves had been consumed except for the tough leaf stems. In an­other grove, where oak, ash, dogwood and hickory were growing, leaves of oaks again were left behind. Leaves from hickories were eaten first; only then were the others touched.

In another forest, basswood, sugar maples, red maples, aspen and white pines were growing together in a mixed planting. The pine needles were untouched for two years, while basswood and aspen leaves were eaten completely. Maple leaves were attacked only after basswood and aspen leaves were stripped down to the last petiole. Herein lies a possible explanation for the slow breakdown of oak leaves in forest litter, as well as for the durability of pine needles when used as a mulch. Also, in both cases, bacteria and fungi have a hard time breaking through the outer cells of the foliage, so decay is delayed.

The important fact here, however, is that the preferred leafy diet of earthworms is relatively low in nutrients. If you analyze the mineral content of fallen leaves you will immediately realize that they are indeed very poor fare (as explained in the reference to leaves for composting in Chapter Eight). In order to stay alive, an earthworm must exist at a slow pace, yet consume enormous amounts of low-nutrient foods. It is interesting to note that when worms live in lawns, they feed only on grasses, never on nitrogen-rich clovers.

The Aeration Notion

Another claim made for earthworms is that they "aerate" the soil. Compared with the aeration effected by a single plowing, this is in­significant. Even in a lawn, earthworm aeration is more imagined than real. Worm burrows are of small diameter and quite short in length (at least in summer) and practically always plugged with a wad of grass. Since the burrows are open only at one end and are lined with a thin coating of lime, the amount of air that can move in and out is infinitesimal. This "aeration" is not the same as the move­ment of gaseous vapors between particles of a well-aerated soil, where gas dispersion takes place in all directions.

The slight aeration that earthworms do provide is more than offset by the harm done in spreading infection. Here we are not concerned with the role of the earthworm as a carrier of diseases to animals, but with its part in spreading organisms of plant maladies. I have seen a valuable planting of delphinium, for example, destroyed by mustard seed fungus when earthworms pulled infected plant material into their burrows between the plants. In an adjoining planting of del­phiniums the soil had been treated with arsenate of lead for Japanese beetle control (which, of course, destroyed earthworms in the proc­ess). Here not a plant was infected.

Earthworms rely on lime for proper functioning of their digestive apparatus, and this limits them to soils that are neutral or alkaline. They are never found in strongly acid soils.

A Lawn Evil

Another mark against the earthworm is its destruction of smooth lawn surfaces. Its ability to ruin putting greens is well known. The day-by-day deposit of castings is bad enough but can usually be partially overcome by regular mowing. The real trouble occurs in winter, particularly if snow falls early and remains on the surface most of the winter. Protected under a blanket of snow, soil remains unfrozen and worms are able to work night and day without inter­ruption. The height of their mounds under snow may reach one to two inches but it is difficult to judge because, as snow melts, the mounds usually are broken and spread over an area of several inches.

Earth thus brought to the surface leaves the lawn so uneven that the owner is compelled to roll it and thereby compact the soil, which more than offsets any slight improvement in aeration effected by the worms. Where owners complain of rough, uneven lawns and "heav­ing" of grasses, I often recommend treatment with chlordane, cal­cium arsenate or dieldrin to rid the soil of earthworms (and grubs) so that the need for rolling will not recur.

Chapter Digest

Do earthworms have any value in soils? Yes and No. In gardens, but not in lawns, we might make out a case for them as processors of organic matter as well as redistributors of plant foods. This may be partial repay­ment for the energy and nutrients they use up in the course of their life. Another demerit is the harm they do in spreading plant diseases and in depositing unsightly casts on lawn surfaces. Perhaps it all boils down to this: Except in special cases, earthworms do not do enough harm to war­rant the cost and labor of destroying them, or enough good to justify special efforts and expenditures to increase their numbers. A good soil will naturally and automatically have an adequate supply of earthworms; a poor soil will not attract them or support them if they are introduced.

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