Soil fertility has been gauged by different measures. Howard
repeatedly insisted that the only good yardstick was humus content.
Others are so impressed by the earthworm's essential functions that
they count worms per acre and say that this number measures soil
fertility. The two standards of evaluation are closely related.
When active, some species of earthworms daily eat a quantity of soil
equal to their own body weight. After passing through the worm's
gut, this soil has been chemically altered. Minerals, especially
phosphorus which tends to be locked up as insoluble calcium
phosphate and consequently unavailable to plants, become soluble in
the worm's gut, and thus available to nourish growing plants. And
nitrogen, unavailably held in organic matter, is altered to soluble
nitrate nitrogen. In fact, compared to the surrounding soil, worm
casts are five times as rich in nitrate nitrogen; twice as rich in
soluble calcium; contain two and one-half times as much available
magnesium; are seven times as rich in available phosphorus, and
offer plants eleven times as much potassium. Earthworms are equally
capable of making trace minerals available.
Highly fertile earthworm casts can amount to a large proportion of
the entire soil mass. When soil is damp and cool enough to encourage
earthworm activity, an average of 700 pounds of worm casts per acre
are produced each day. Over a year's time in the humid eastern
United States, 100,000 pounds of highly fertile casts per acre may
be generated. Imagine! That's like 50 tons of low-grade fertilizer
per acre per year containing more readily available NPK, Ca, Mg and
so forth, than farmers apply to grow cereal crops like wheat, corn,
or soybeans. A level of fertility that will grow wheat is not enough
nutrition to grow vegetables, but earthworms can make a major
contribution to the garden.
At age 28, Charles Darwin presented "On the Formation of Mould" to
the Geological Society of London. This lecture illustrated the
amazing churning effect of the earthworm on soil. Darwin observed
some chunks of lime that had been left on the surface of a meadow. A
few years later they were found several inches below the surface.
Darwin said this was the work of earthworms, depositing castings
that "sooner or later spread out and cover any object left on the
surface." In a later book, Darwin said,
"The plow is one of the most ancient and most valuable of man's
inventions; but long before he existed the land was in fact
regularly plowed and still continues to be thus plowed by
earthworms. It may be doubted whether there are many other animals
which have played so important a part in the history of the world,
as have these lowly organized creatures."
Earthworms also prevent runoff. They increase percolation of water
into fine-textured soils by making a complex system of
interconnected channels or tunnels throughout the topsoil. In one
study, soil lacking worms had an absorption rate of 0.2 inches of
rainfall per minute. Earthworms were added and allowed to work over
that soil sample for one month. Then, infiltration rates increased
to 0.9 inches of rainfall per minute. Much of what we know about
earthworms is due to Dr. Henry Hopp who worked for the United States
Department of Agriculture during the 1940s. Dr. Hopp's interesting
booklet, _What Every Gardener Should Know About Earthworms._ is
still in print. In one Hopp research project, some very run-down
clay soil was placed in six large flowerpots. Nothing was done to a
pair of control pots, fertilizer was blended in and grass sod grown
on two others, while mulch was spread over two more. Then worms were
added to one of each pair of pots. In short order all of the worms
added to the unimproved pot were dead. There was nothing in that
soil to feed them. The sod alone increased percolation but where the
sod or mulch fed a worm population, infiltration of water was far
better.
Amendment to clay soil Percolation rate in inches per minute
Without worms With worms
None 0.0 0.0
Grass and fertilizer 0.2 0.8
Mulch 0.0 1.5
Most people who honestly consider these facts conclude that the
earthworm's activities are a major factor in soil productivity.
Study after scientific study has shown that the quality and yield of
pastures is directly related to their earthworm count. So it seems
only reasonable to evaluate soil management practices by their
effect on earthworm counts.
Earthworm populations will vary enormously according to climate and
native soil fertility. Earthworms need moisture; few if any will be
found in deserts. Highly mineralized soils that produce a lot of
biomass will naturally have more worms than infertile soils lacking
humus. Dr. Hopp surveyed worm populations in various farm soils. The
table below shows what a gardener might expect to find in their own
garden by contrasting samples from rich and poor soils. The data
also suggest a guideline for how high worm populations might be
usefully increased by adding organic matter. The worms were counted
at their seasonal population peak by carefully examining a section
of soil exactly one foot square by seven inches deep. If you plan to
take a census in your own garden, keep in mind that earthworm counts
will be highest in spring.
Earthworms are inhibited by acid soils and/or soils deficient in
calcium. Far larger populations of worms live in soils that
weathered out of underlying limestone rocks. In one experiment,
earthworm counts in a pasture went up from 51,000 per acre in acid
soil to 441,000 per acre two years after lime and a non-acidifying
chemical fertilizer was spread. Rodale and Howard loudly and
repeatedly contended that chemical fertilizers decimate earthworm
populations. Swept up in what I view as a self-righteous crusade
against chemical agriculture, they included all fertilizers in this
category for tactical reasons.
Location Worms per sq. ft. Worms per acre
Marcellus, NY 38 1,600,000
Ithica, NY 4 190,000
Frederick, MD 50 2,200,000
Beltsville, MD 8 350,000
Zanesville, OH 37 1,600,000
Coshocton, OH 5 220,000
Mayaquez, P.R.* 6 260,000
*Because of the high rate of bacterial decomposition, few earthworms
are found in tropical soils unless they are continuously ammended
with substantial quantities of organic matter.
Howard especially denigrated sulfate of ammonia and single
superphosphate as earthworm poisons. Both of these chemical
fertilizers are made with sulfuric acid and have a powerful
acidifying reaction when they dissolve in soil. Rodale correctly
pointed out that golf course groundskeepers use repeated
applications of ammonium sulfate to eliminate earthworms from
putting greens. (Small mounds of worm casts made by nightcrawlers
ruin the greens' perfectly smooth surface so these worms are the
bane of greenskeepers.) However, ammonium sulfate does not eliminate
or reduce worms when the soil contains large amounts of chalk or
other forms of calcium that counteract acidity.
The truth of the matter is that worms eat decaying organic matter
and any soil amendment that increases plant growth without
acidifying soil will increase earthworm food supply and thus worm
population. Using lime as an antidote to acid-based fertilizers
prevents making the soil inhospitable to earthworms. And many
chemical fertilizers do not provoke acid reactions. The organic
movement loses this round-but not the battle. And certainly not the
war.
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.