Agriculture Reference
In-Depth Information
decrease with a decrease because of chemical binding of between 35 and 55% in bioavailable
metals in 2 months (
Figure 20.3D)
. Similar results have been reported in other studies of both
composting and vermicomposting, and this implies a lower availability of heavy metals for plants
from composts or vermicomposts. During vermicomposting, heavy metals tend to form complex
aggregates with the humic acids and the most polymerized organic fractions.
H
UMIFICATION
DURING
V
ERMICOMPOSTING
Saviozzi et al. (1988) reported that organic wastes, to be compatible with their agricultural uses
and to avoid adverse effects on plant growth, must be transformed into a humuslike material and
become stabilized. In our case study, decreases in the carbon from fulvic acids and increases in
the percentages of the carbon from humic acids were observed throughout the vermicomposting
process (Figure 20.3E), so, clearly, earthworm activity accelerates the humification of organic
matter. Moreover, during vermicomposting, the amounts of humic materials increased from 40 to
60%, which was more than the values obtained in a composting process using the same waste
materials. Humification processes are accelerated and enhanced not only by the fragmentation and
size reduction of the organic matter, but also by the greatly increased microbial activity within the
intestines of the earthworms and by aeration and turnover of the organic matter through earthworm
movement and feeding.
S
TABILITY
OF
O
RGANIC
W
ASTES
AND
M
ATURITY
OF
THE
V
ERMICOMPOSTS
The stability and maturity of organic wastes, which imply a potential for the development of
beneficial effects to plants when they are used as growth media, can be determined by plant
germination experiments and growth bioassays (Chen and Inbar 1993). In our example,
the germi-
nation percentages of
Lepidium sativum
indicated that the initial organic wastes were toxic to the
plants, probably because of their high ammonium content, but this toxicity was removed gradually
during the vermicomposting process. Moreover, the results obtained for the germination index
(which combined germination percentages and coleoptile elongations) demonstrated a beneficial
effect of the earthworms on germination (Figure 20.3F).
V
ERMICOMPOSTING
AND
H
UMAN
P
ATHOGEN
D
ESTRUCTION
Preliminary research in our laboratory, and in the Soil Ecology Laboratory at The Ohio State
University, has shown that vermicomposting involves a great reduction in populations of human
pathogenic microorganisms, as in composting. It is generally accepted that the 72 hours of the
thermophilic stage of the composting process eliminate pathogenic organisms, but these studies
have shown that human pathogens also do not survive vermicomposting. After 60 days of vermi-
composting, the amounts of fecal coliform bacteria in biosolids dropped from 39,000 MPN (most
probable number)/g to 0 MPN/g. In that same time period,
Salmonella
sp
.
dropped from <3 MPN/g
to <1 MPN/g. Similar results have been reported by Eastman (1999), also for fecal coliforms and
Salmonella
sp
.
and for enteric virus and helminth ova, and other authors (see
Chapter 18
this
volume).
SOIL FOOD WEBS IN THE VERMICOMPOSTING SYSTEM
Earthworms participate in soil functions through the
drilosphere
, which is defined as the space of
interactions among earthworms, soil or waste physical structure, and the whole microbial and inver-
tebrate communities (Lavelle et al. 1998). As a result of organic matter digestion processes by
earthworms and the creation of soil structures (see
Chapter 11
this volume), the overall composition,
structure, and the relative importance of the drilosphere is clearly determined by environmental
conditions, soil characteristics, and the quality and amounts of the organic matter inputs.
