Fertilizer is the principal method of promoting natural productivity in ponds where the concentration of inorganic nutrients is low. Fertilizers are of two types: organic and inorganic. Although organic fertilizers provide additional material to boost benthic productivity, if used excessively their decomposition can create anaerobic conditions on the pond bed. Despite their disadvantages they are often readily available and cheap and can be ideal for small-scale and extensive aquaculture operations. It should also be remembered that there is a danger of animal wastes being contaminated with pesticides and other chemical products such as antibiotics (Lee & Wickins, 1992).

Fertilizers, particularly manures, are often applied to the pond bed in advance of filling and stocking. Once an initial phytoplankton population has been established, usually after 5-15 days, its maintenance usually requires further fertilizer applications unless a nutrient rich water supply is employed. Fertilizers may be slowly leached into the water from floating perforated plastic drums, sunken wicker silos or from porous sacks held in the inflowing water current or tied to stakes within the pond (Lee & Wickins, 1992).

Alternatively they may be placed on a wooden platform 30 cm below the water surface. It is advisable not to broadcast solid fertilizers over the pond since the nutrients will be deposited on the pond bed rather than used to fuel primary productivity in the water column. This may cause a carpet of benthic algae to develop in ponds where light can reach the bottom, an effect which is undesirable and sometimes necessitates the application of algicides (Lee & Wickins, 1992).

The control of phytoplankton productivity can be difficult, especially in large ponds. It usually requires manipulation of the rates of water exchange and feeding as well as fertilization. The objechve is usually to keep turbidity levels (as measured with a Secchi disc) within set limits. Supplementary feeds partly act as organic fertilizers, so fertilization rates usually need reducing as the standing crop and feeding rates build up in the later stages of a culture cycle (Lee & Wickins, 1992).

The most important components of fertilizers for phytoplankton are nitrogen and phosphorus. To a certain extent fertilization regimes can be designed to favour particular types of phytoplankton. Diatoms, which are usually prevalent in moderate or high salinity water, are usually preferred in shrimp ponds and can be encouraged with fertilizer high in nitrogen. In contrast generally undesirable blue-green algae, which often bloom in lower salinity water, are able to fix dissolved atmospheric nitrogen and are thus likely to be favoured by fertilizers high in phosphates. Although diatoms require silicates, the value of using silicate fertilizer is unknown and likely to be site specific (Lee & Wickins, 1992).