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Système d’information sur les ressources alimentaires et d’engrais en aquaculture
 

Indian white prawn - Fertilizers and fertilization

Pond preparation aims at optimizing the pond environmental conditions for the growout cycle, while fertilization establishes the desired water quality conditions and natural production in the pond (Figure 18). Feed management then drives the production cycle until harvest. Water quality management is ongoing throughout the cycle to optimize the animal’s environment for best growth. The pond sequence usually follows: draining/harvest, cleaning/drying, ploughing/tilling, liming, filling, (chlorination), fertilization, predator eradication, stocking and growing.

Harvest to pond liming
Pond preparation between crops is an important part of the shrimp production cycle. The basic objectives of pond preparation are to (Suresh, Aceituno and Oliva, 2006):

  • oxidize organic wastes and reduced inorganic compounds that accumulate in pond bottoms during the growout period;
  • eradicate predators, pathogens and vectors of pathogens that may be present from the previous culture cycle or enter before the pond is stocked for the next cycle;
  • improve soil pH and alkalinity of the water;
  • enhance the availability of natural food organisms before stocking; and
  • remove or redistribute sediment as necessary.

Organic matter accumulation in shrimp ponds results in the deterioration of the pond ecosystem (Avnimelech and Ritvo, 2003). After harvest, the ponds are allowed to dry out to eliminate or reduce Vibrio and Pseudomonas species bacterial pathogens (Nimrat et al., 2008) and pond sediments are then removed (FAO, 2007b) (Figure 19).

Ponds should be dried for at least 7 (CIBA, 2003a) to 14 days (Boyd, 1990). Diab and Shilo (1986) found that the microbial nitrification rates, which are low when the soil is flooded, increase in the drained soil. Air exposure oxidizes the black sludge within a few hours. This can be followed in the field through the colour change of the sediments from black to brown, with deeper sediments taking longer to oxidize. Where the pond cannot be dried, chemicals such as chlorine compounds and burnt or hydrated lime can be used on the wet areas. Wet areas can be treated with burned or hydrated lime at 0.25 kg/m2 or saturated chlorine solution at one litre/m2 to effect disinfection.

Treatment of wet patches with sodium, potassium or calcium nitrate at 0.1 to 0.2 g/m2 can also enhance oxidation. Sludge can also be flushed from the ponds, but must then be directed to a settling pond for treatment and not released to the environment (Avnimelech and Ritvo, 2003; Suresh, Aceituno and Oliva, 2006).

Lime is applied to regulate soil pH at 7.5–8.5 (Table 10).

Tilling and liming are often done at the same time. The purpose of tilling is to enhance the oxidation of organic matter by pulverizing the soil to increase the exposure of wastes to sunlight and air. Tilling the upper 5–10 cm of soil is most important, for waste from the previous crop accumulates in this layer (Suresh, Aceituno and Oliva, 2006). Liming should be performed when the pond bottom is still wet, because some moisture is required for microbial activity (Boyd, 1995). Liming can decrease phosphorus availability in pond water through the precipitation of low solubility calcium and magnesium phosphate minerals. It is therefore important to leave sufficient time between lime application and pond fertilization. Proper liming can however improve soil phosphorus availability and greatly enhance pond productivity (Wurts and Masser, 2004). Shrimp farmers also use lime during pond preparation to prevent disease, neutralize acidity in pond soil and water, and increase alkalinity and hardness of water (Boyd, Boonyaratpalin and Thunjai, 2002; FAO, 2007b).

Pond filling and fertilization are usually done together. 200 µm mesh filters are used to remove pathogens and their vectors (Figure 20). A series of progressively finer filters installed between the intake point, reservoir and ponds is employed at many farms. Sediment ponds in modern farms provide initial water treatment, maintain water levels and provide a reservoir of water. Calcium hypochlorite can be applied to chlorinate the water before use (CIBA, 1996). Chlorine is usually applied at concentrations ranging from 3 to 30 mg/litre to eliminate Vibrio spp., Monodon baculovirus(MBV) and vectors of other viruses, particularly wild crustaceans (Chanratchakool, 1995; Hedge, Anthony and Rao, 1996; Boyd and Massaut 1999). During sterilization, the pond is oxygenated using a paddle wheel for 2 to 3 h and then allowed to settle for 4 to 5 d. Chlorination is expensive, will also eliminate beneficial organisms (0.25 mg/litre adversely af­fects phytoplankton) and is not fully effective in killing all organisms (Husnah and Lin, 2002).

Tea seed cake and pesticides (e.g. rotenone at 20 to 25 mg/litre) are commonly used to eliminate fish in the ponds. Application rates for tea seed cake ranged from 150 (Minsalan and Chiu, 1986) to 300 kg/ha (Suresh, Aceituno and Oliva, 2006). The cakes are soaked in water overnight and the resulting liquid is applied uniformly over pond surfaces one week before stocking so there are no residual effects on the postlarvae.

Fertilizers used are either organic or inorganic (Table 11b). The aim of fertilization is to achieve a healthy bloom of zoo- and phytoplankton in ponds before stocking so that natural food organisms are available to the newly stocked postlarvae (Suresh, Aceituno and Oliva, 2006). Farmers tend to prefer diatom blooms (Boyd and Tucker, 1998). Target nitrogen levels through fertilization range between 0.3 and 2 ppm and for phosphorus, between 0.1 to 0.2 mg/litre (Treece, 2001; Boyd, 2003a; Fox, 2008b) (Table 11a). The fertilizers available will differ regionally (see FAO, 2007b). The nutrient ratio for diatoms is C:Si:N:P = 106:15:16:1 (Brzezinski, 1985). A ratio of N:P of around 20:1 is reported for good diatom blooms (The Redfield ratio) (FAO, 1987d; Boyd and Tucker, 1998; Fox, 2008b). A secchi disk reading of 40 to 25 cm should be maintained by fertilization, with no fertilizer applied below 30 cm (Treece, 2001). N:P of 15:1 promotes diatom blooms (Boyd and Tucker, 1998).

Organic materials
Various organic materials can be used as fertilizer (Table 11b). Research has found minimal risk from bacterial contamination (Salmonella and other human pathogenic bacteria) when using organic fertilizers (Dalsgaard and Olsen, 1995; Dalsgaard, 1998).

Organic fertilizers are widely used in shrimp ponds, with applications up to 1 000 kg/ha manure (or meals like soybean, sorghum and cottonseed at 200 kg/ha) to pond bottom during preparation. This is followed by applications of 200 to 300 kg/ha at one to two week intervals (Boyd, 1989; Fox, 2008b) or maintained thereafter with inorganic fertilizers. Organic fertilizers have low cost, are usually available and decompose to release nutrients slowly (high C/N ratio). They provide nutrients other than nitrogen and phosphorus, as well as substrate for the growth of microbial food organisms suited to the shrimp’s natural behaviour. Zooplankton feed directly on particles of manure, enabling a rapid increase in zooplankton abundance (Boyd, 1989). With their low inherent concentration of nutrients, they are best applied together with inorganic fertilizers. 2.67 kg urea contains as much nitrogen as 100 kg of poultry manure. Wyban et al. (1987) established a carrying capacity of over 1 700 kg/ha of shrimp in ponds fertilized with cattle manure at 1 800 kg/ha/week.

Molasses is also used as an organic fertilizer, as it provides a readily available source of carbon and stimulates the growth of beneficial bacteria. Typical application rates range from 50 to 100 kg/ha. Molasses can be used as a tool to prevent an increase in ammonium and nitrite (Samocha et al., 2007). Grain residues such as rice bran are also widely used. Initial application rates range from 250 to 750 kg/ha.

Organic fertilizers will also include the use of feedstuffs and feeds: soybean meal, chicken feed, palm by-products (Gautier, 2002), cottonseed meal, rice bran, wheat bran, alfalfa meal and other processed grains or hays (Treece, 2001; Wurts, 2004).

Inorganic fertilizers
Inorganic nitrogen and phosphorous fertilizers can be applied to shrimp ponds to stimulate algal growth to increase zooplankton. 1 kg of urea and 1 kg of 1riple super phosphate (TSP) together contain an amount of available nitrogen and phosphorus equivalent to about 100 kg of chicken manure (FAO, 2007b).