and lakes. This leads to high growth of water plants. Plants grow and produce food by
photosynthesis which requires high quantities of oxygen. The high oxygen demand of the
rapidly growing water plants removes oxygen available to other organisms in the rivers and
lakes. These organisms then suffocate and die due to lack of oxygen. The biodegradation of
the dead organisms results in a massive increase in bacteria, fungi and algae degrading the
dead organic matter, which also require oxygen. This further depletes the available oxygen,
and further contributes to the death of fish and other aquatic species.
Figure 2.2: Schematic diagram showing the processes that lead to eutrophication.Figure 2.3: Algae and dead fish in a lake that
has undergone eutrophication. Figure 2.4: Algal bloom in a river following
eutrophication.Natural fertilisers: an application of indigenous knowledge systems
The fertilisers discussed above are non-natural inorganic compounds such as nitrates, phos-
phates etc. However, as a means of reducing the negative impact of the inorganic fertilisers
discussed earlier, organic fertilisers that occur naturally can be used. Natural fertilisers con-
sist of organic compounds derived from manure, slurry, worm castings, peat, seaweed etc.
Natural fertilisers supply nutrients to the soil through natural processes such as composting.
This means that the nutrients are released back to the soil slowly, and excessive nutrients
do not wash off into rivers causing over-fertilisation and eutrophication. However, the use
of organic fertilisers is more labour-intensive and the nutrient composition tends to be more
Chapter 2. The chemistry of life 29