Agriculture Reference
In-Depth Information
activity dramatically. Earthworms act as mechanical blenders, and by comminuting the organic
matter, they modify its biological, physical, and chemical status, gradually reducing its C:N ratio,
increasing the surface area exposed to microorganisms, and making it much more favorable for
microbial activity and further decomposition. During passage of organic matter through the earth-
worm gut, the fragments and bacteria-rich excrements are moved, thereby homogenizing the organic
material.
Vermicompost, which is the end product, is a stabilized, finely divided peatlike material with
a low C:N ratio and high porosity and water-holding capacity that contains most nutrients in forms
that are readily taken up by the plants. These earthworm casts are rich in organic matter and have
high rates of mineralization, which reflect greatly enhanced plant availability of nutrients, partic-
ularly ammonium radicals and nitrates.
EARTHWORMS
Earthworms can be defined as segmented and bilaterally symmetrical invertebrates with an external
gland (clitellum) producing an egg case (cocoon), a sensory lobe in front of the mouth (prostomium),
with the anus at the posterior end of the animal body, and with no limbs but possessing a small
number of bristles (chaetae) on each segment. They are hermaphrodites, and reproduction normally
occurs through copulation and cross fertilization, after which each of the mated individuals can
produce cocoons (oothecae) containing between 1 and 20 fertilized ova (although parthenogenesis
is also possible). The resistant cocoons, which can survive many years, are tiny and roughly lemon
shaped with specific characteristics. After an incubation period that varies according to species and
climatic conditions, the cocoons hatch. The young earthworms, which are white and only a few
millimeters in length after emerging from the cocoons, gain their specific adult pigmentation within
a day. Assuming favorable conditions, many species can reach sexual maturity within weeks after
emergence, although some species that live mainly in soil take longer. Mature individuals can be
distinguished easily by the presence of the clitellum, which is a pale or dark-colored swollen band
located behind the genital pores. After fertilization, the clitellum secretes the fibrous cocoon, and
the clitellar gland cells produce a nutritive albuminous fluid that fills the cocoon. The earthworms
can continue to grow in size after completing their sexual development but never add further
segments.
The number of earthworm species is enormous; according to Reynolds (1994), there are as
many as 7254 species in the Oligochaeta, of which about half (3627) are terrestrial earthworms,
with an average annual description of about 68 new species. For most earthworm species, the
original genus and species description is the only information available, and for many species, little
or nothing is known of their life cycles, distribution, ecology, and the like.
Through feeding, burrowing, and casting, earthworms modify the physical, chemical, and
biological properties of the organic matter. Physical properties in soils and wastes processed by
earthworms include improved aggregation, stability, and porosity; soil biological and chemical
properties that may be modified include nutrient cycling (mainly N and P), organic matter decom-
position rates, and chemical forms of nutrients in soil and their availability to plants. They also
change the soil pH, organic matter dynamics in terms of quality and quantity, microbial and
invertebrate activity (including production of enzymes and plant growth regulators), and the abun-
dance, biomass, species composition, and diversity of the microflora and fauna (Lavelle et al. 1998).
EARTHWORM LIFE HISTORIES
Earthworms, as all organisms, have to distribute the energy obtained in feeding to two main
compartments: the reproductive compartment and the somatic compartment. This assignment of
resources to either growth or reproduction can be modified according to evolutionary answers to
