Agriculture Reference
In-Depth Information
evidence of hormonal effects produced by earthworm activity, which was supported by the high
levels of cytokinins and auxins they found in the vermicomposts.
Krishnamoorthy and Vajranabhiah (1986) showed, in laboratory experiments involving large
earthworm populations, that seven species of earthworms could promote the production of cytoki-
nins and auxins in organic wastes very dramatically. They also demonstrated significant positive
correlations (
r
= 0.97) between earthworm populations and the amounts of cytokinins and auxins
occurring in 10 different field soils, and they concluded that earthworm activity was linked strongly
with PGR production. They reported that auxins and cytokinins produced through earthworm
activity could persist in soils for up to 10 weeks but degrade in a few days if exposed to sunlight.
The biological activities of humic substances have been investigated extensively (McCarthy et
al. 1990; Hayes and Wilson 1997). Studies of the effects of humic substances on plant growth
under conditions of adequate mineral nutrition have resulted consistently in positive plant growth
effects (Y. Chen and Aviad 1990; Hayes and Wilson 1997). For instance, humic substances increased
the dry matter yields of corn and oat seedlings (Albuzio et al. 1994); numbers and lengths of
tobacco roots (Mylonas and McCants 1980); dry weights of shoots, roots, and nodules of soybean,
peanut, and clover plants (Tan and Tantiwiramanond 1983); and vegetative growth of chicory plants
(Valdrighi et al. 1996) and induced shoot and root formation in tropical crops grown in tissue
culture (Goenadi and Sudharama 1995).
Vermicomposts originating from animal manure, food wastes, sewage sludges, or paper-mill
sludges have been reported to contain high levels of humic substances (Canellas et al. 2000; Atiyeh
et al. 2002; Arancon et al. 2004f). The biological activities and effects of humic substances derived
from earthworm feces on plants have been investigated (DellÔAgnola and Nardi 1987; Nardi et al.
1988; Muscolo et al. 1993). For instance, DellÔAgnola and Nardi (1987) reported hormonelike
effects of depolycondensed humic fractions obtained from the feces of the earthworms
Allolobo-
phora rosea
and
Allolobophora caliginosa
. Treating carrot cells with humic substances obtained
from the feces of the earthworm
A. rosea
increased cell growth and induced morphological changes
similar to those induced by auxins (Muscolo et al. 1996). Work in the Soil Ecology Laboratory at
OSU indicates that it seems that vermicomposts, which consist of an amalgamate of humified
earthworm feces and organic matter, can stimulate plant growth beyond that produced by mineral
nutrients because of the direct or indirect effects of the humic substances in the vermicomposts
acting as PGRs; this has been confirmed (Atiyeh et al. 2000a,b,c,d,e; Arancon et al. 2004f).
In this work at OSU, typical growth responses after treating plants such as tomatoes with humic
substances were for increased growth correlated with increasing concentrations of humic substances
but usually with a decrease in growth at higher concentrations of the humic materials (
Figure 18.5
)
.
This stimulatory effect of humic substances on plant growth at low concentrations has been
explained by various theories, the most convincing of which hypothesizes a ÑdirectÒ hormonal
action on the plants together with an indirect action on the metabolism of soil microorganisms, the
dynamics of soil nutrients, and soil physical conditions (Cacco and DellÔAngola 1984; Nardi et al.
1988; Casenave de Sanfilippo et al. 1990; Y. Chen and Aviad 1990; Muscolo et al. 1993, 1996,
1999; Albuzio et al. 1994).
Laboratory and greenhouse research at OSU has provided new evidence that the effects of
earthworm activity on organic matter to produce vermicomposts can lead to the production of
water-extractable and base-extractable plant PGI substances in vermicomposts in quantities that
could significantly influence plant germination, growth, flowering, and yields of greenhouse crops.
For instance, in bioassays, the leaf development of radish seedlings grown in a full Hoagland
nutrient solution was compared with that in complete nutrient solutions amended with 2 or 5%
aqueous extracts of vermicomposts. The extracts increased the leaf area significantly, suggesting a
nonÏnutrient-mediated plant growth response. In aqueous extracts of vermicomposted cattle waste,
in fractions separated by high-performance liquid chromatography (HPLC) and then analyzed by
gas chromatography-mass spectrometry (GC-MS), indole acetic acid (IAA) was reported conclu-
sively, and smaller amounts of gibberellins and cytokinins were present.
