Fall, 1990 (v3n1)
by Matthew Werner,
UC Santa Cruz Agroecology Program, and
Robert L. Bugg, SAREP
The Hindu pantheon includes
a multitude of deities, but the principal trinity are Brahma the creator,
Vishnu the sustainer, and Shiva the destroyer. Creation, sustention, and
destruction, they interconnect and operate in balance through the cycles
of nature. By contrast, in western religions the destructive forces of
the universe are generally seen as evil. Emblematic of these destructive
forces is the worm, destroyer of all things mortal. Yet in our efforts
to create a more sustainable agriculture, we may need more balance in
our appraisal of the great destructive forces of nature, and of the worm
in particular.
For all its destructive reputation,
the worm, paradoxically, has a crucial role to play in building. As noted
by Charles Darwin in his 1882 classic, The Formation of Vegetable Mould
Through the Action of Earthworms with Observations on Their Habits, earthworms
process huge quantities of plant litter and help convert it into rich
topsoil, liberating nutrients for renewed plant growth. More recent studies
show that earthworms can help reduce soil compaction, improving permeability
and aeration. Earthworms do this through burrowing activities, ingestion
of soil along with plant debris, and subsequent excretion of casts. Upon
drying, these casts form water-stable soil aggregates. These aggregates
are clumps of soil particles bound together by organic compounds, and
their presence helps improve soil structure, retain nutrients that might
otherwise be leached, and reduce the threat of erosion.
Earthworms are only part
of the complex of organisms termed "decomposers" in agroecology.
Other destroyers include springtails (Collembola), nematodes, bacteria,
protozoa, and fungi. Earthworms themselves fall into several subgroups
based on their behavioral ecology: epigeic, endogeic, and anecic.
Epigeic earthworms are those
that live in the superficial soil layers and feed on undecomposed plant
litter. These worms are usually small and produce new generations rapidly.
Endogeic species are those
which forage below the soil surface in horizontal, branching burrows.
These species ingest large amounts of soil, showing a preference for soil
rich in organic matter. Endogeics may have a major impact on the decomposition
of dead plant roots, but are not important in the incorporation of surface
litter.
Anecic earthworms build permanent,
vertical burrows that extend deep into the soil. This type of worm comes
to the surface to feed on manure, leaf litter, and other organic matter.
Anecics, such as the nightcrawlers, Lumbricus terrestris and Aporrectodea
longa, have profound effects on decomposition of organic matter and
the formation of soil.
Deep tillage is generally
harmful to earthworms. It can kill them outright, disrupt their burrows,
lower soil moisture, and reduce the availability of surface litter. Shallow
tillage, ridge-tillage, and surface management of crop residues has often
led to increases in earthworm activity compared to areas where deep tillage
is practiced. Earthworms favor leaf litter that has a low ratio of carbon
to nitrogen, and tend to prefer residues of clovers and other legumes
over residues of grass. Residues left as surface mulch are very useful
in enhancing earthworm densities.
Certain pesticides are particularly
harmful to earthworm populations. These include aldicarb, carbaryl, carbofuran,
benomyl, and most soil fumigants. Most herbicides are probably not directly
damaging to earthworms at the normal rates of use. Some inorganic fertilizers,
especially ammonium sulfate, can be harmful to earthworm populations,
possibly due to an acidifying affect.
Much of the research on earthworm
agroecology has been conducted in areas with humid cool-temperate climates.
Relatively little research has occurred in regions with Mediterranean
climates. Native California earthworms particularly are still poorly documented
and understood. The most complete collection of native earthworms was
destroyed in the 1906 San Francisco earthquake and has never been restored.
There is still little research
on the roles of earthworms in Californian agroecosystems. Preliminary
observations suggest that earthworms in some California agricultural soils
tend to have small populations of endogeic species that are active for
short periods during the rainy season. More information is needed to take
full advantage of the potential benefits of including earthworms in sustainable
agriculture practices. What earthworm species/behavioral groups inhabit
California ecosystems? Can earthworm species be introduced that are suited
to particular agroecosystems? How does irrigation affect earthworm activity
patterns? Are earthworms affected by botanical insecticides? What management
practices will sustain crop productivity by promoting earthworm activity?
These questions are a starting point for observation and experimentation.
FURTHER EARTHWORM REFERENCES:
Dindal, Daniel. 1990. Soil Biology Guide. Wiley Publishing,
1349 pages.
Edwards, C.A. 1972. Biology
of Earthworms. Bookworm Publishing Co., Russelville, AR, 283 pages.
Edwards, C.A., and Lofti,
J.R. 1977. Biology of Earthworms. Chapman and Hall, London.
Gaddie, Ronald E., &
Donald Douglas. 1975. Earthworms for Ecology and Profit, in three
volumes: Vol. 1, Scientific Earthworm Farming; Vol. 2, Earthworms
and the Ecology; Vol. 3, Successful Earthworm Marketing. Bookworm
Publishing Co., Russelville, AR.
Lee, K. 1985. Earthworms:
Their Ecology and Relationships with Soil. Academic Press, NY, 432
pages.
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