Agriculture Reference
In-Depth Information
are many possible combinations of soil types and associated plant and earthworm species. Because
several PGI mechanisms may be operating simultaneously, identifying the most important modes
of action on the plant in a given situation is a difficult task. However, there are guiding principles
that can be followed to help predict potential benefits of earthworm activities to plant growth and
identify some of the mechanisms involved.
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First, existing limitations to plant growth in a given site, situation, soil type, and abiotic and biotic
environmental conditions should be assessed and classified hiearchically. Then, the potential of
earthworms to ameliorate any of the limiting factors must be evaluated. Such potentials can be
assessed using pot and laboratory methodologies, not necessarily including plants. To be able to
deal adequately with the multiple limiting factors, commonly present under field conditions, and
identify the interactions between the limiting factors and effects of earthworms on the soil and
plants, a series of Ñall-minus-oneÒ trials could be designed in a greenhouse. In such experiments,
all the environmental, nutrient, and physical requirements would be provided to the plant except
one. Each requirement can then be altered to test how earthworms affect it and to determine any
associated plant responses.
Such experiments could begin with proper identification of all the soil-associated chemical
limitations to plant growth, which can be assessed by soil analyses, plant analyses, and limiting-
nutrient pot experiments. Fertilization with all but one nutrient and inclusion or exclusion of
earthworms could permit the assessment of the role of earthworms in facilitating nutrient availability
and uptake by the plants. Soil sterilization and inoculation with a range of different microorganisms
is also possible to help assess the role of their interactions with earthworms on plant productivity,
but this could be time consuming and laborious. In shorter-term experiments, the physical effects
of earthworms on soil studies are probably less important than the chemical and biological effects,
although as the length of the experimental periods increase, physical effects (especially limitations
when only one earthworm species is included) may become more important. To separate the
chemical, physical, and biological mechanisms, simultaneous experiments using different method-
ologies and experimental designs, such as pots with and without earthworms and pots with soil
previously processed by earthworms and various other treatment combinations (e.g., ° residues, °
fertilizers, ° sterilized or unsterilized), or with one or more earthworm species in combination
could be developed.
The approaches described are far from ideal because some interactions may occur when only a
single factor is manipulated, and such experiments do not take into account the complementary or
adverse roles of other organisms present in soils. Furthermore, such an experimental design will require
a very large number of replicates and treatments, making it very complex and somewhat impractical.
Therefore, experiments should be mainly in the field, while recognizing both their drawbacks
and benefits (Brown et al. 1999). In field experiments, a great many more variables, many of them
uncontrollable, can occur, although the results obtained should be much more realistic and useful
to practicing farmers, foresters, or other environmental managers. In such instances, proper assess-
ment of the soil physical limitations to plant growth is essential because, under such conditions,
physical effects of earthworms on soils may be of much greater importance (Alegre et al. 1996;
Barros 1999). Obviously, the characterization of changes in earthworm communities by periodic
population sampling, the quantification of earthworm casting and burrowing activities (e.g., by
physical description of soil cores or x-ray commuted tomography), and their chemical and biological
analyses are important to assess the extent of earthworm changes to soil structure, biology, and
fertility. In such studies, controls (without introduced or native earthworms) must be used and
maintained, and if earthworms are to be introduced to them, realistic biomass, populations, and
species assemblages should be used.
