Agriculture Reference
In-Depth Information
T
HE
E
ARTHWORM
T
HRESHOLD
C
ONCEPT
Probably the most important effects of earthworms on crop yields will occur when earthworms are
able to significantly modify factors that also happen to be those most limiting to plant growth. Almost
all the soil physical, biological, and chemical needs of plants can become limiting when they fall below
a certain lower threshold or become excessively above it. The range between these two thresholds is
the zone ideal for plant growth. Hence, possible effects of earthworms on plants can be regulated by
their ability to increase some of these factors that are limiting plant growth above the lower thresholds
and decrease factors adverse to growth below the upper thresholds. The combination of effects of the
drilosphere and the threshold level at which each of the biological, chemical, or physical factors are
limiting plant growth will ultimately determine the effects of earthworm activities on the plants.
For example, if a population of a parasitic nematode or an infestation by a particular fungal or
bacterial pathogen has reached disease proportions and become a primary limiting factor in plant
production, and if earthworms are able to reduce the population, it seems likely that this would be
a dominant mechanism influencing plant growth, although other factors, such as the influence of
earthworms on soil structure and fertility and biological interactions with soil microorganisms and
invertebrates, will also be important and operate simultaneously. Similarly, if availability of N or
P are limiting factors, earthworm-induced increases in the availability of the nutrients or changes
in the mycorrhizal colonization of roots may become important controlling mechanisms. Finally,
if soils are compacted or prone to compaction and associated hydrological limitations are compli-
cating plant growth, earthworm bioturbation and associated soil structural changes may be the most
important mechanisms that enhance plant productivity.
F
UTURE
N
EEDS
IN
E
ARTHWORM
R
ESEARCH
An ultimate goal in the process of assessing the potential effects of earthworms on plant productivity
would be the development of effective simulation models in which the soil environmental constraints
are matched with the earthworm and plant species (or communities) present at a site to predict any
potential direct and indirect influences of earthworm activities on soil physical, chemical, and biological
limitations and plant productivity. Unfortunately, there are still many gaps in knowledge of these
processes that need to be filled to be able to develop such models and predict accurately whether a
particular earthworm species, population, or community will enhance or suppress plant productivity.
Thus, future research should strive to use more holistic approaches using detailed experiments
that address potential mechanisms individually as well as in combination, but do not attempt to
Ñkill all birds with one stone.Ò On the positive side, the study of increased production of PGR
substances; promotion of beneficial rhizobacteria; reductions in plant pests, pathogens, or parasites;
the interactions of earthworms with plant seeds; and the potential attraction of earthworms to roots
and of roots to earthworm structures such as casts deserve more attention.
On the negative side, the assessment of plant growth suppression by direct and indirect effects
of earthworms also requires more research to verify any potential incompatibilities between existing
earthworm communities and established plant species and populations, soils, cropping systems,
and crops. There may be situations for which it is better to plant gramineous plant species rather
than legumes or vegetables (Puttarudriah and Sastry, 1961) or to manage the soil in a way to
decrease potentially negative effects of earthworm activity on plant production. Finally, many
earthworm species with potential benefits for plant growth have never been studied experimentally
and deserve more attention. Further studies should focus especially on the potential benefits of
earthworms to plant productivity in crop fields, pastures, and managed forests, where site-specific
management practices and technological innovations (e.g., precision agriculture) could be used to
manage the earthworm community for the benefit of plant growth.
