Geoscience Reference
In-Depth Information
Fig. 18.37 Soil moisture
retention curves in the surface
layer (0-10 cm) of control
and treated plots (Giusquiani
et al.
1995
)
Fig. 18.38 SEM images
showing general features of
water-stable aggregates
isolated from a a silt loam
soil and b a silt loam sludge
(5 %) mixture after 16 days
of incubation at 25 C
(Metzger and Yaron
1987
)
and field studies show that sludge application to soil induces an increase in the
number and size of water-stable aggregates (WSA).
Figure
18.38
illustrates the typical morphology of WSA (200-2,000 lmin
diameter) in a loamy soil following sludge application. Compared to control
aggregates (Fig.
18.38
a), WSA obtained in sludge-amended soil (Fig.
18.38
b) are
characterized by a large size and a loose and porous internal structure. From the
aggregation patterns observed in Fig.
18.38
, for three soils amended with sludge,
three distinct phases can be identified: a short 10-day period characterized by a
rapid rate of WSA formation, a period of similar duration displaying a moderate
decrease in WSA content, and finally a longer period characterized by a slow
secondary increase in WSA content over a scale of months (Metzger et al.
1986
).
Experiments performed on various soils and under different climatic conditions
confirmed that aggregate sizes in a sludge-amended soil decrease with time, over a
scale of months (Metzger and Yaron
1987
, and references therein). Moreover, long-
lasting effects of sludge on the structural stability of modified aggregates are
reported by Wei et al. (
1985
), who found aggregates in a modified silt clay loam
soil, 6 years after 112 t/ha of sludge was added to a field soil in a single application.
Metzger et al. (
1986
) argue that the two main binding mechanisms responsible
for aggregate formation are the cementation of primary particles by fungal
