Agriculture Reference
In-Depth Information
To gain further insights into the physical processes occurring in the earthworm gut, a number
of researchers have compared the stability of artificial casts with those made by earthworms. In
some cases, the artificial casts were less stable than natural casts, which may be related to applying
forces to the soil greater than those normally encountered within the earthworm gut (Zhang and
Schrader 1993; Hindell et al. 1997a; Schrader and Zhang 1997). This may be a particular concern
when artificial casts are made by forcing soil material through a syringe (Hindell et al. 1997a).
Zhang and Schrader (1993) also suggested that earthworm casts were more readily stabilized than
artificial casts because the organic and mineral fractions were mixed more intimately within the
earthworms than when the soil was artificially remolded. On the other hand, Marinissen and Dexter
(1990) reported that fresh casts produced by
were up to two times more dispersible
than artificial casts made by extruding the same soil through a syringe. In their work, they suggested
that the results were probably attributable to less intensive remolding in the artificial casts than in
the earthworm casts. Despite the difficulties in replicating the physical forces encountered by soil
material during passage through earthworms, Hindell et al. (1997a) pointed out that artificial casts
can be a useful model against which changes in soil structure that result from earthworm activity
can be tested.
A. caliginosa
M
S
A
C
EASUREMENT
OF
THE
TABILIZATION
OF
GGREGATES
IN
ASTS
Prior to the mid-1980s, most studies suggested that freshly excreted earthworm casts were imme-
diately more stable than uningested soil (Hopp and Hopkins 1946; Dutt 1948; Swaby 1950; Teotia
et al. 1950; Parle 1963; Lal and DeVleeschauwer 1982; Lal and Akinremi 1983). These results,
however, were mainly attributable to the fact that the samples were dried before analysis. Most
recent studies indicated that fresh, moist casts are less water-stable than uningested soil because
of the intense remolding that occurs during passage through earthworms (Shipitalo and Protz 1988;
Marinissen and Dexter 1990; Barois et al. 1993; Schrader and Zhang 1997; Decans et al. 2001).
As casts age, they are stabilized by a combination of physical, chemical, and biological
processes, which explains why some casts can persist at the soil surface for more than a year when
protected from raindrop impact and trampling by animals (Decans 2000). Although several studies
have concluded that aging or drying fresh casts reduced their dispersibility (Shipitalo and Protz
1988; Marinissen and Dexter 1990), this conclusion has not been universal (Haynes and Fraser
1998). To understand these seemingly contradictory findings, the methodology used to measure the
stability of earthworm casts (e.g., wet or dry sieving, clay dispersion, turbidimetric analysis), as
well as the effects of other treatments to which the casts have been subjected (wetting and drying
cycles, simulated rainfalls, sterilization and chemical treatments), must be taken into account.
Water-stable aggregation is an index for aggregate stability under wet conditions, whereas tensile
strength (determined by a crush test) is an index for aggregate stability under dry conditions. The tensile
strength of aggregates is influenced by their water and clay contents (Gill 1959) and decreases with
increased porosity (Dexter et al. 1984). In studies in which tensile strength was measured (Schrader
and Zhang 1997; Garvin et al. 2001), earthworm casts were significantly stronger than natural aggre-
gates. The tensile strength of casts, however, appears to be species dependent because Flegel et al.
(1998) observed a lower tensile strength for the casts of
Dendrobaena octaedra
compared with those
of
Similarly, Schrader and Zhang (1997) noted that water-stable aggregation was signifi-
cantly higher in casts of
L. terrestris.
L. terrestris
than in casts of
A. caliginosa
.
S
A
C
: P
, C
,
TABILIZATION
OF
GGREGATES
IN
ASTS
HYSICAL
HEMICAL
AND
B
P
IOLOGICAL
ROCESSES
One of the physical processes thought to contribute to the stabilization of casts with age is
thixotropic hardening (Shipitalo and Protz 1989). Thixotropic or age hardening is described by
Utomo and Dexter (1981) as a rearrangement of particles and water films and a restoration of
 
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