Fish Population Adjustment 165 



phorus available. German fish ciilturists usually mix potassium and 

 pliosphate fertilizers and apply them together.'*^ 



Phosphorus. Phosphorus is the most important fertilizing element in 

 lakes and ponds, but may be easily lost through combination with an 

 excess of calcium to form tricalcium phosphate (Ca3(P04)2).^'^ As carbon 

 dioxide increases, the precipitated salt may be converted to the more 

 soluble di- and monocalcium phosphates. ^^ As mentioned in connection 

 with manganese above, iron may unite with phosphate to form an in- 

 soluble precipitate. Phosphorus also may combine physically with micells 

 of ferric hydroxide or be absorbed directly on organic soil colloids on 

 the pond bottom. For these reasons, phosphorus added to a lake or pond 

 quickly goes out of solution, but still may be available on the pond 

 bottom. It therefore follows that phosphorus applied at one time in some 

 quantity may become available in small but useful amounts over a long 

 period of time. European workers recommend about 17 kilograms per 

 hectare (15.2 lb/acre) of phosphorus (applied as superphosphate) as an 

 optimum dose.^^ Experiments in this country do not seem to substantiate 

 this amount as optimum. 



Nitrogen. Nitrogen is more often used in this country than in Europe 

 as a fertilizing material. Some algae are able to fix nitrogen from the 

 atmosphere if phosphorus is available,-^' ^^' ^*^ particularly the bluegreen 

 algae, Anahaena and Nostoc.^^ However, nitrogen in fertilizers gives a 

 quick source of this element to the algae. 



Other Functions of Fertilizers. There are some uses of fertilizer other 

 than those of increasing phytoplankton. Swingle ^^^ mentions Irwin's work 

 on the use of inorganic fertilizer to cause clay particles to settle out of 

 muddy ponds ( see Chapter 3 ) . Ball ^ believed that the addition of 

 fertilizer to the entire shoal area of North Twin Lake ( Cheboygan County, 

 Michigan) stimulated the growth of filamentous algae on the bottom 

 which appeared to interfere with the nest building of sunfishes. 



Although no controlled experiments have been projected to date, it 

 seems likely that undissolved salts of commercial fertilizers falling into 

 nests of centrarchids containing developing eggs or yolk sac fry would 

 cause the embryos to die. Commercial fertilizer is usually broadcast in 

 shallow water over an area that corresponds roughly with that selected 

 by bluegills and other sunfishes for nesting. If a fertilization schedule 

 called for an application of fertilizer to the shoal waters of a pond at two- 

 week intervals from early spring to September or October, it is probable 

 that many centrarchid embryos would be killed. This might give sub- 

 stantial assistance in keeping the bluegills or other sunfishes from becom- 

 ing overly abundant. 



One of the techniques of pond management often suggested is the 



