To ensure pond phytoplankton productivity, various nutrients such as nitrogen, phosphorus, potassium, carbon and calcium need to be added regularly. Fertilizers can be inorganic fertilizers or mineral fertilizers, or else organic fertilizers or manures of plant and animal origin.
Organic manures have been used for a long time. They can be obtained from a number of places: dung of cows, sheep, goat and pig, and poultry and duck droppings. They can also be found in farmyard manure, compost, green manures, sewage, etc. Of these, cow dung is the most widely used in undrainable pond culture systems. As most of the organic manures are waste products of local agriculture they are generally cheap and easily obtainable. They provide all the basic nutrients required for biological production. Several of these manures are quickly used by the pond organisms, especially by the zooplankton and even by some species of carp. By improving the quality of the pond’s bottom mud they encourage the growth of bacteria, which leads to more zooplankton.
Though the major elements are present in
manures, their levels are generally low, which means that large amounts may be required. Unless proper care is taken in using manure, the level of dissolved oxygen in the pond water is likely to drop.
Commercially available inorganic compounds containing major nutrients (nitrogen, phosphorus and potassium) are known as inorganic or chemical fertilizers. They contain a high and fixed percentage of the three basic
kinds of nutrients: nitrogenous, phosphatic and potassic, or mixed. As they can easily dissolve in water, these nutrients are quickly available to the pond.
Nitrogenous fertilizers usually contain nitrogen as the main element and are commercially available as ammonium sulphate, ammonium nitrate, or urea. Most of these fertilizers make the pond soil acidic and it is therefore important to select a fertilizer based on the soil of the pond. Nitrogenous fertilizers are essential for newly constructed ponds poor in nitrogen and without sufficient organic matter on its bottom. Older ponds tend to have a good layer of organic mud which is able to produce nitrogen by itself. To further complicate matters, the strength of nitrogenous fertilizers depends on how much phosphorous is available. It is best to maintain the phosphorous : nitrogen ratio at 1 : 4.
Phosphatic fertilizers are the best fertilizers for fish culture. This is important because almost all fish ponds do not have enough phosphorus.
The most common phosphatic fertilizers are the orthophosphates. Superphosphates are the most soluble in water, and single superphosphate is the most widely used and is easily available. The more concentrated triple superphosphate is also used. Phosphatic fertilizers are released slowly over a number of years depending on the nature of the pond bottom.