Successful pond fertilization is dependent on drying, tilling and liming of the pond bottom. Drying accelerates the decomposition of organic matter that accumulates in the substratum. A drying period of 2-3 week after harvesting is usually adequate.
After drying, the pond bottom should be limed to reduce the acidity as well as to kill any pathogenic organisms. The quantity of lime required depends on the type of soil and pH (Table 2). If pond water alkalinity is less than 30-40 mg/L (CaCO3) then liming is necessary and if it is more than 60 mg/L then liming is not required. An application rate of 1000 kg/ha of burnt lime or 1500 kg/ha of hydrated lime is usually sufficient to disinfect the pond bottom (Boyd, 1995). Tilling (ploughing) the pond bottom during the drying period can enhance its oxygen content and should be limited to a depth of 5–10 cm (New and Valenti, 2000).
For successful prawn production it is necessary to maintain an adequate phytoplankton density. However, it is often unnecessary to fertilize because this is rapidly achieved by the feeding regime. Ponds in sandy-clay soil may require fertilization. Where necessary, 25 kg/ha/month of triple superphosphate (Na3PO4) will keep the water green. An adequate benthic fauna is very important for successful freshwater prawn farming and this is achieved by an appropriate fertilization schedule. Animal manures are used for this purpose (e.g. 1 000-3 000 kg/ha of cattle manure). Animal manures can be substituted by other organic materials, such as distillery by-products or other plant residues. Animal manures have a variable composition and relatively low nitrogen and phosphorus content, which means that large amounts must be applied (Byod and Tucker, 1998). Animal manure should normally be discouraged for use in prawn ponds and mineral fertilizers should be favoured (New and Valenti, 2000). The recommended N:P2O5 ratio in freshwater prawn ponds is 1:3 or 1:4. Diammonium phosphate (18 percent N and 48 percent P2O5) or mono-ammonium phosphate (11 percent N and 48 percent P2O5) are excellent pond fertilizers, but the desired ratio of nitrogen and phosphorus can also be obtained by mixing various other fertilizer materials (Byod and Tucker, 1998).
Polyculture with tilapia, common carp, silver carp, grass carp, bighead carp and catfish is an alternative way to improve pond productivity. Tilapia and prawns feed on benthic detritus in multi-species polyculture system, hence decreasing the amount of material to be decomposed by bacteria and thus reducing the biochemical oxygen demand (Hepher and Pruginin, 1981). In addition tilapia and freshwater prawn consume the faeces of grass and common carp. Common carp feed on partially digested algae from silver carp faecesa and silver carp feed on phytoplankton (New and Valenti, 2000).
