Fertilizers are natural or synthetic substances that are used in ponds to increase the production of the natural food organisms to be eaten by the fish. These organisms include phytoplankton, zooplankton and insects they are all part of a complex food web converging toward fish production. By increasing the availability of major nutrients, fertilizers promote the development of plank tonic algae, which provide food for many fish. Fertilization also leads to the development of animals which feed on algae, including some fish such as the Chinese silver carp and the Nile tilapia. (See Annex I)
When a fertilizer is added to a fish pond, the chemicals it contains dissolve in the water, where a portion is usually rapidly taken up by the phytoplankton present, either to be stored, sometimes in quite large proportions, or to be assimilated and used for growth, reproduction, etc.;
Another portion is attracted by and becomes attached to the organic and mineral particles present, both in the pond water and in the upper layers of the bottom mud or soil.
This second portion may also assist the development of bacteria, responsible for the decomposition of organic matter. The decomposition of organic matter may in turn release more nutrients back into the mud or water. The chemicals attached to soil particles may also later be released back into the water slowly, over a long period of time. They may also migrate deeper into mud and soil, where they will no longer affect the water body, unless the pond bottom is dried or ploughed.
Most of these phenomena are linked with and controlled by water quality and in particular temperature, pH, alkalinity and dissolved oxygen level. (Brunson et al, 1999)