Agriculture Reference
In-Depth Information
and mixing of the organic and inorganic components of soil and decrease the size of not only
organic particles, but also mineral particles (Shrickhande and Pathak 1951; Joshi and Kelkar 1952).
During passage through the earthworm gut, the different kinds of mineral particles become
mixed intimately with organic matter and form aggregates, which improve both the drainage and
moisture-loading capacity of the soil. These aggregates are usually very water stable and improve
many of the desirable characteristics of soils. There have been various suggestions as to the possible
ways in which earthworms form aggregates, such as by production of gums (Swaby 1950) or
calcium humate (Meyer 1943), by plant residues (Ponomareva 1953), or by means of polysaccharide
molecules (Parle 1963). Various authors have estimated that up to 50% of the aggregates in the
surface layers of soil are formed by earthworms (Kubiena 1953). Earthworms also contribute in
many ways to soil formation, structure, and physical characteristics (see
Chapters 10
and
11
, this
volume).
T
S
URNOVER
OF
OIL
As Darwin first noted, earthworms move large amounts of soil from the deeper strata to the surface.
The amounts moved in this way range from 2 to 250 tons per hectare per annum, equivalent to
bringing a layer of soil between 1 mm and 5 cm thick to the surface every year, creating a stone-
free layer on the soil surface. In temperate climates, all the upper 15 cm of soil may be turned over
every 10 to 20 years (Edwards and Bohlen 1996). However, much larger turnovers have been
reported from tropical agroecosystems (Lavelle et al. 1999).
S
A
D
OIL
ERATION
AND
RAINAGE
Earthworms also affect soil structure in other ways. Some species make permanent burrows, whereas
others move randomly through the soil, leaving cracks and crevices of different sizes. Both sorts
of burrows are important in maintaining soil aeration, drainage, and porosity. Moreover, earthworm
burrows are usually lined with a protein-based mucus that helps stabilize these channels, and many
of the species with permanent burrows cast their feces around the lining of the burrows, with the
cast material usually containing more plant nutrients in a readily available form than the surrounding
soil.
There is good evidence that earthworm activity increases both the porosity and the air-to-soil
volume (Wollny 1890; Hopp 1974; Edwards and Lofty 1977). Burrows are also important in
improving soil drainage, particularly because those of some species, such as
Lumbricus terrestris
L., penetrate deep into soil in permanent burrows (Edwards and Lofty 1978, 1982a,b) and can even
pass through layers of clay. The burrows and pores also increase the infiltration rate greatly (Slater
and Hopp 1947; Teotia et al. 1950; Carter et al. 1982), and there are numerous reports of water
penetrating the surface soil between two and ten times faster when earthworms were present than
when they were not (Stockdill 1966; Wilkinson 1975; Tisdall 1978). These effects on infiltration
can be of two kinds. The first is the presence of large surface-opening holes that are not usually
taken into account by soil scientists when conventional models of infiltration are developed
(Edwards and Lofty 1982a). Second, the crevices also created by earthworms, but which are smaller,
not only increase infiltration, but also aid in water retention (see Chapters 10 and 11, this volume).
Finally, earthworm activity makes a significant contribution to soil aeration (Stockli 1928;
Kretzschmar 1978) by creating channels, particularly in heavy soils, that allow air to penetrate into
deeper layers of soil, minimizing the incidence of anaerobic layers.
O
M
B
I
S
RGANIC
ATTER
REAKDOWN
AND
NCORPORATION
INTO
OIL
Although all species of earthworms contribute to the breakdown of plant-derived organic matter,
they differ greatly in the ways in which they break down organic matter and incorporate it into the
soil. Their activities can be of three kinds, each associated with a different group of species. Some
