98 FISH HATCHERY MANAGEMENT 



INORGANIC FERTILIZERS 



Inorganic fertilizers are relatively inexpensive sources of nitrogen, phos- 

 phorus, and potassium, which stimulate algal growth, and calcium, which 

 helps to control water hardness and pH. 



In nitrogen- free water, 0.3 to 1.3 parts per million of nitrogen must be ad- 

 ded to stimulate phytoplankton growth, and to sustain this growth about 

 one part per million must be applied at weekly intervals. In a normal 

 hatchery pond this comes to about eight pounds of nitrogen per surface 

 acre. Because nitrogen can enter the pond system from the atmosphere, 

 watershed, and decomposing organic matter, it is not always necessary to 

 add more. 



For the operation of warmwater hatchery ponds, it is recommended that 

 nitrogen be included in the fertilizer applications during the late spring 

 and summer months for all ponds except those which have been weed- free 

 for at least three years. If development of phytoplankton is delayed longer 

 than four weeks, nitrogen should be added. 



Forms of nitrogen available for pond fertilization are listed on Table 12. 



Phosphorus is an active chemical and cannot exist alone except under 

 very specialized conditions. It is generally considered to be the most essen- 

 tial single element in pond fertilization and the first nutrient to become a 

 limiting factor for plant growth. Plankton require from 0.018 to 0.09 part 

 per million as a minimum for growth. Several workers have recommended 

 applications of about 1.0 part per million phosphorus pentoxide (P^OrJ 

 periodically during the production season. 



Table 12. nitrogen fertilizers for pond enrichment. 



"Also contains 73",i P^r,. 

 'Also contains 48", 1 P^O-,. 

 'Also contains 48-52"(i P^Or,. 



