100 FISH HATCHERY MANAGEMENT 



A number of phosphate fertilizers are available for use in ponds. Ordi- 

 nary superphosphate is available commercially more than any other form 

 and is satisfactory for pond use. More concentrated forms may save labor 

 in application, however. Sources of P^O-, in commercial phosphate fertiliz- 

 ers are listed in Table 13. When nitrogen also is desired, ammoniated phos- 

 phates are recommended as they are completely water-soluble and gen- 

 erally should give a more rapid response. An application rate of 8 pounds 

 P2O-, per surface acre is normal in pond fertilization. This amount supplies 

 about 1 part per million in a pond averaging about 3 feet deep. In the 

 United States, the usual practice is to supply the needed phosphorus 

 periodically throughout the growing season. In Europe, however, the sea- 

 sonal phosphorus requirements are supplied in one or two massive applica- 

 tions either before or shortly after pond is filled, or at the beginning and 

 middle of the fish production cycle. A 50-100"'i) increase over normal 

 applications is justified in ponds with unusually hard waters, large 

 amounts of iron and aluminum, or high rates of water exchange. 



Potassium generally is referred to as potash, a term synonomous with po- 

 tassium oxide (K2O). The most common sources are muriate of potash 

 (KCl) and potassium nitrate (KNO3). Potassium sulfate (K2SO4) also is a 

 source of potassium. 



Potassium is less important than nitrogen or phosphorus for plankton 

 growth, but it functions in plants as a catalyst. 



Increased phytoplankton growth occurs with increases in potassium from 

 to 2 parts per million; above 2 parts per million there is no additional 

 phytoplankton growth. Many waters have an ample supply of potassium for 

 plant growth, but where soils or the water supply are deficient or where 

 heavy fertilization with nitrogen and phosphorus is employed, addition of 

 potassium is desirable. It can be applied at the beginning of the production 

 cycle, or periodically during the cycle. It is quite soluble and unless ad- 

 sorbed by bottom deposits or taken up by plants, it can be lost by seepage 

 or leaching. 



Calcium is essential for both plant and animal growth. It seldom is defi- 

 cient to the point that it exerts a direct effect on growth. Many of its ef- 

 fects are indirect, however, and these secondary influences contribute sig- 

 nificantly to the productivity of a body of water. Waters with hardness of 

 more than 50 parts per million CaC03 are most productive, and those of 

 less than 10 parts per million rarely produce large crops. Calcium ac- 

 celerates decomposition of organic matter, establishes a strong pH buffer 

 system, precipitates iron, and serves as a disinfectant or sterilant. In some 

 cases, fish production can be increased 25-100% by adding lime at the rate 

 of 2 to 3 tons per acre. 



Calcium is available in three principal forms. It is 71"o of calcium oxide 

 (CaO) or quicklime, 54% of calcium hydroxide (Ca(OH)2) or hydrated 

 lime, and up to 40"ii of calcium carbonate (CaCO^) or ground limestone. 



