Agriculture Reference
In-Depth Information
they provide suitable conditions for development of earthworm populations or if the earthworms
develop a burrow system that improves mass transfer properties in this soil and then respond to
soil environmental limitations. Observations of the development of earthworm burrow systems in
cultivated soils previously deprived of any earthworm populations (e.g., pastures at Manatuke
Experimental Station, Gisborne, New Zealand; personal observations) show interesting strategies
of soil colonization by earthworms, but it is not possible to find any earthworm burrows (mainly
caused by
occurring below the depth reached by the plant roots. Con-
versely, the burrow systems observed in a pasture in northern France (Kretzschmar 1982) contained
a high density of earthworm burrows at a depth where very few roots were present.
In these two cases, it is difficult to be sure whether the mechanism having the primary effects
on soil structure is the effect of existing root system on the development of earthworm burrows or
the opposite. The answer is almost certainly that roots and earthworms develop together, and in
forming burrows that influence root patterns, earthworms adapt their strategies to suit the pedolog-
ical environment.
When studying the possible effects of the presence of earthworm burrows opening at the soil
surface on the pattern of surface cracks under experimental conditions (Chadoeuf et al. 1994), the
best that could be proved was that there are spatial relationships between the distribution of
earthworm burrows and the distribution of cracks; none was assessed as the prime factor, and the
conclusion was that the density of earthworm burrows in the vicinity of cracks (distance less than
2.5 cm) was greater than the density that would be predicted if the burrows had been distributed
following a stationary Poisson pattern. It was not possible to decide whether the burrows usually
opened closer to the cracks or if the cracks developed closer to the earthworm burrows (experimental
conditions could be changed without solving this dilemma). This chicken-and-egg type of question
demonstrates the limits of searching for causal relationships between earthworm activities and soil
physical and biological properties, for which the former could cause variations in the latter.
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TRUCTURE
EPENDENT
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In temperate zones, soils with a Ñnatural good structure,Ò Ñhigh fertility,Ò and Ñno earthwormsÒ are
known in North America and New Zealand (as well as tropical soils). It is then questionable how
to define an improvement in soil physical properties in those types of soil after earthworms are
introduced because the introduction of earthworms is often made at the same time as the introduction
of new soil management practices. The improvements in pasture productivity in New Zealand after
the widespread and monitored introduction of northern European lumbricid species of earthworms
brought remarkable and stable increases in dry matter production (Springett 1985) in pastures that
were then managed (i.e., fertilized and grazed) in the same way as northern European pastures.
Earthworm populations developing in the pastures of South Australia have the advantage of living
in soils receiving lime applications for pH control (Baker et al. 1992).
Experimental evidence is available on the conditions under which earthworm activities in soils
create significant modifications in soil properties. When a soilÔs air-filled porosity is in the median
zone (i.e., 15% soil volume), the development of an earthworm burrow system does not bring
significant improvements in gaseous diffusion compared with soils with 25% air-filled porosity;
conversely, when air-filled porosity is down to about 10%, conditions at which gaseous diffusion
is severely limited (Glinski and Stepniewski 1985), a single earthworm burrow can dramatically
change the rates of gas diffusion and its effects (Kretzschmar and Monestiez 1992; Kretzschmar
and Ladd 1993).
If the presence of earthworm burrows could be simulated as a short circuit in the mass transfer
pathways in the soil matrix, the efficiency of this short circuit would depend entirely on the degree
of connectivity associated with the burrows; furthermore, this connectivity must be considered from
different views. In the soil profile, such a short circuit could join two zones of equal permeability
crossing a less-permeable zone; at the scale of the soil matrix in the earthworm burrow vicinity,
