Chemistry Reference
In-Depth Information
population,
whereas
inorganic
fertilisers
negatively
impacted
on
these
organisms.
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5.4 Beneficial Microorganisms: Soil Fertility Promoters, Plant
Growth Regulators and Biocontrols
Soil ecosystems are among the most complex of all terrestrial communities,
and the role of the soil biota in maintaining plant health is progressively being
understood. The composition of the soil biota is strongly influenced not only
by the nature of the underlying organic matter and mineral components, but
also by environmental variables such as temperature, pH and moisture.
Natural soils have been shown to harbour large populations of microorgan-
isms which exist in a state of dynamic equilibrium and controlled changing
balances. These microorganisms primarily compete with each other for
nutrition and space. A majority of the microbes are classified as fungi and
bacteria which play beneficial and often vital roles in natural environments and
agriculture. Numerous benefits are accrued from these microbes including (1)
direct symbiotic association with roots (mycorrhizae, legume nodulating
bacteria); (2) nutrient cycling which involves breakdown and release of
minerals from organic matter present in the soil, resulting in increases in
essential element availability to higher plants; and (3) biocontrol agents,
through predation of disease-causing microorganisms and/or suppressing
growth, or reproduction activity of harmful disease-causing microorganisms
through other interactions such as chemical inhibition. Details of selected
microbes with economic potential which have been well investigated in African
soils are discussed in the sections below.
d
n
1
r
2
n
g
|
4
5.4.1 Legume Nodulating Rhizobia (LNB)
Biological nitrogen fixation is the ability of living organisms to convert inert
dinitrogen gas in the atmosphere (N
2
) into nitrogen-containing organic
compounds through asymbiotic, associative or symbiotic processes.
Microbially mediated nitrogen fixation accounts for 175 million tonnes per year
in terrestrial and aquatic environments.
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This provides two thirds of the nitrogen
required in the biosphere, most of which comes from the contribution of the
association between modulating rhizobia bacteria with compatible host legumes.
The organisms that possess the nitrogenase enzyme have attracted consider-
able interest. These prokaryotes in the Eubacteria and Archaebacteria kingdoms
which can fix nitrogen are metabolically diverse and the different bacterial N-
fixing systems have been reviewed.
78
For almost 100 years the term Rhizobium
was used to represent those organisms capable of forming nodules with specific
homologous host legumes. Recently, phylogenetic analysis which uses 16S rRNA
has become the standard for classification of bacteria. This new classification,
which is dependent on the phenotypic traits, has confirmed a number of
taxonomic
divisions
which
include
Azorhizobium,
Bradyrhizobium,
