Agriculture Reference
In-Depth Information
Earthworm casts on the soil surface. Photograph by the author. See color insert.
Figure 2.14
exceptions occur in the seasonal-rainfall tropics and subtropics (where termites are
generally predominant) and in soils under coniferous forests and heath where mor
humus forms.
Earthworms feed exclusively on dead organic matter. A few species, such as
Lumbricus rubellus
and
L. castaneous
, live mainly in the surface soil and litter layer,
provided that temperature and moisture conditions are favorable. The majority of
earthworms burrow more deeply into the soil, and in the course of feeding ingest
large quantities of clay and silt-size particles. Consequently, organic and mineral
matter is more uniformly mixed when deposited in the worm feces, which may
appear as
casts
on the soil surface (fig. 2.14). In a study of several vineyards on
contrasting soil types in the Barossa Valley of South Australia, the most abundant
species found was
Aporrectodea rosea
, followed by
A. caliginosa
. The latter species
is sensitive to dry soil conditions and was not found in the two sandiest soils
(Buckerfield and Webster 1996). Other useful species are
A. trapezoides
and the
two deep-burrowing species
A. longa
and
Lumbricus terrestris
.
As a result of earthworm activity, organic matter from the surface becomes
incorporated throughout the upper part of the soil profile. Earthworms are most
abundant under permanent grass swards because they are undisturbed and their
food supply is plentiful. Their numbers may then exceed 250 per m
2
, or 2.5 mil-
lion per ha. The size of the population is regulated not only by the amount of
suitable organic matter available, but also by soil temperature, moisture, and pH.
For example, few species of earthworms are found in soils of pH less than 4.5:
most species prefer neutral or calcareous soils. In hot, dry weather they burrow
deeply into the soil and estivate.
Table 2.4 shows the range of earthworm biomass found under different man-
agement practices in a vineyard on an acid soil. Straw mulch provides extra food
