Chemistry Reference
In-Depth Information
in the habitats that are subject to regular disturbance compared to the stable
ecosystems like shrub land and indigenous forest. It was also possible that
fungal tissues were fragmented and scattered in the course of farm operations,
thus increasing their frequency of detection. As such, agricultural practices can
exert positive or negative impacts on other microorganisms in the soil.
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According to Wang et al.,
128
some agricultural inputs stimulate build-up of
nematode-trapping fungi, hence the observed diversity, evenness and richness
with increased land-use intensity compared to land uses such as forest or shrub
land which are materially unchanged by human activity. Intensive cultivation
is characterised by increased movement of soil, which may result in increased
spread of the microorganisms in the field. Soil disturbance, coupled with
frequent changes in crop cover, subjects the soil biota to stress, making it
difficult for a particular species to establish itself in the soil to out-compete the
others. In contrast, soils under forest and shrub are less disturbed, meaning
that certain species of nematode-destroying fungi are able to establish and
suppress other species that are poorly suited to compete effectively.
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5.5 Farming Systems and Soil-borne Pests and Diseases
In conventional agriculture, addition of lime, inorganic fertilisers and
pesticides can change the physical and chemical nature of the soil environment,
thereby altering the number of organisms and the ratio of different groups of
organisms, resulting in adverse effects characterised in part by an increase in
soil-borne pests and diseases. Soil-borne pathogens (such as plant parasitic
nematodes, fungi, bacteria, phytoplasma, protozoa and viruses) are among the
most underestimated of the factors which affect plant productivity in tropical
regions. The reasons for the greater severity of soil-borne diseases and pests in
the tropics are the generally favourable climatic conditions, the greater
pathogenicity of pest species and the more severe disease complexes.
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In
addition, cropping systems in tropical regions are generally more diverse and
less reliant on chemical inputs compared to those in temperate regions. There
is also a greater diversity of nematodes and other pests in tropical regions.
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Table 8 lists some of the most common soil-borne pathogens in the tropics and
the crops and trees that may be affected in different systems.
In general, plants infected by soil-borne pathogens suffer from root rot,
collar rot, root blackening, wilting, stunting or seedling damping-off diseases.
To some extent, losses associated with soil-borne pathogens may be reduced by
a 4-5 year crop rotation programme, but this is not feasible due to economic
reasons. One way in which the soil-borne pathogens can be indirectly
suppressed is through the incorporation of organic amendment matter to
mineral soils. In addition to improving tilth, aeration and drainage of soils
where organic matter is incorporated, additional benefits occur such as
proliferation of populations of beneficial soil microorganisms. This was
demonstrated by Langat et al.
130
where amending soils with organic substrates
including baggase, molasses, tea and flower composts contributed to a change
