164 Theories and Techniques of Management 



As yet the fertilization of waters is far from an exact science; however, 

 some of the information now available is given below. 



Manganese. Hasler and Einsele ^^ described the use of manganese to 

 release phosphates from iron. Thus, manganese dioxide (Mn02), which 

 is not a fertilizer, may release phosphate (PO4) from an insoluble bond 

 with iron, so that the eflFect is the same as though phosphate were added. 



Lime. Many authors stress the importance of lime in pond fertilization 

 where there is a natural shortage of calcium.-"' ^^' ^^' ^^' ^^^' ^^^' ^-^ In 

 waters containing less than 10 ppm the addition of lime may be followed 

 by a large increase in fish production. Lime is believed to have many 

 eflfects, particularly on the bottom mud where it changes the colloidal 

 and adsorptive properties and creates an alkaline environment, which is 

 more suitable than an acid environment for bacteria and fungi. Thus, it 

 increases, indirectly, the rate of decay. It is believed to have several pos- 

 sible chemical actions, such as the precipitation of iron compounds, and 

 may counteract the poisonous properties of sodium, potassium, and 

 magnesium ions. The calcium in lime may displace other fertilizing 

 substances from organic colloidal systems, making available K+, and 

 ~P04.^^' ^^* The calcium in stripmine waters may be responsible for the 

 establishment of a strong buffer system that keeps the high sulfate ( ~S04 ) 

 from being toxic to fish and other aquatic organisms. In Europeon fish 

 ponds, enough lime is added to the drained pond basin to give a slight 

 alkaline reaction and a crumbly mud structure. When unslaked lime is 

 used on a drained pond basin, it is believed to have a toxic effect on 

 aquatic organisms and fish parasites. 



In soft water, the addition of lime may be followed by an increase in 

 carbon dioxide storage in the form of bicarbonate. Swingle ^^^ believed 

 that calcium competes with the algae for the free carbon dioxide, but 

 Nielsen ^^' ^^ demonstrated that aquatic plants used bicarbonate ( HCO3 ) 

 directly in photosynthesis, up to one half of the amount present. Bi- 

 carbonate was used more slowly than free carbon dioxide because the 

 latter diffuses about 8 or 9 times as fast as bicarbonate. 



Calcium may be applied in the form of "quick lime" (CaO) or as 

 agricultural limestone. It should never be applied at the same time as 

 phosphate, and "quick lime" should be applied 2-3 weeks before fish are 

 stocked. 



Potassium. Ponds with sandy bottom soils are often poor in potassium 

 and respond markedly when this element is added. Usually it is difficult 

 to measure the effects of adding potassium. These effects may be direct 

 if there is a potassium scarcity or indirect if the addition of potassium 

 displaces hydrogen from soil colloids, forming dilute acids in which 

 phosphorus becomes soluble, i.e., potassium may indirectly make phos- 



