Agriculture Reference
In-Depth Information
One hundred years later, the first scientific observations on the effects of earthworms on soil
structure were conducted by Darwin (1881) and centered mostly on how earthworms contribute to
the geologic evolution of soils and landscapes. Like White, he recognized that earthworms promote
the growth of vegetation by creating an intimate mixture of organic and mineral matter that aids
in water retention and nutrient release and provides a medium suitable for root proliferation. He
also recognized that deep-burrowing earthworms affect water movement in the soil and Ñmaterially
aid in its drainage.Ò Darwin also postulated that earthworm activity can have negative aspects by
contributing to ÑdenudationÒ (soil erosion) by both wind and water. This was based on observations
that casting activity by earthworms can result in the deposition of weakly aggregated material at
the soil surface that can flow or be washed or blown downslope.
In the ensuing years since these pioneering naturalists published their findings, a number of
scientific studies have confirmed their observations, and there are now detailed data on the effects
of earthworms on soil aggregation and soil porosity. We are also beginning to understand the
chemical and physical processes by which earthworms affect soil structure and the consequences
of their activity, both positive and negative, and the interrelationships between soil management
and earthworm activity.
ECOLOGICAL CLASSIFICATION OF EARTHWORMS
Earthworms affect soil physical properties when they ingest and excrete soil to construct burrows
and as part of their feeding activities. Because different earthworm species have different ecological
strategies, their effects on soil aggregation and porosity can vary considerably. Most earthworms
are placed in one of three ecological groups: epigeic, anecic, or endogeic (Bouch 1977).
species of earthworms generally forage within accumulations of organic matter and
rarely burrow into or ingest much soil. Typical habitats include forest litter or manure piles; thus,
they have little direct effect on the structure of mineral soils. For example, Hamilton and Dindal
(1989) noted that the epigeic earthworm
Epigeic
Eisenia fetida
had no effect on aggregation in a sludge-
amended soil.
earthworm species normally live in permanent or semipermanent burrows that can extend
deep into the soil. They feed primarily on decaying surface organic residues, which they frequently
pull into their burrows or mix with excrement to form a midden. The midden promotes further
decay of the incorporated organic residues and covers the burrow entrance.
Anecic
earthworm species burrow extensively belowground and obtain their nutrition by
ingesting a mixture of soil and organic matter. They form extensively branched, subhorizontal
networks of burrows in search of food, but most of their activity is in the upper 10 to 15 cm, where
organic matter levels are generally highest. Portions of their burrows are often occluded with their
casts, and they occasionally cast on the soil surface. These classifications are not absolute because
the behavior of many species is intermediate to these groupings and can vary with environmental
conditions (Edwards and Bohlen 1996).
Endogeic
AGGREGATION
I
R
P
C
NGESTION
ATES
AND
ROPERTIES
OF
ASTS
Although earthworms feed on decaying organic matter and the microorganisms that colonize it,
the material ingested by endogeic and anecic species during feeding and burrowing is predom-
inantly mineral matter. This mineral and organic material is mixed thoroughly in their digestive
tracts and excreted as casts on the soil surface or belowground, depending on the species of
earthworm, location of the food source, and soil bulk density (Binet and Le Bayon 1999). The
amount of soil ingested is highly dependent on the size, composition, and activity of the
earthworm population and is hard to measure accurately because subsurface activity is difficult
