218 THE SCIENTIFIC FEEDING OF ANIMALS 



of the organic fertilizers used supply enormous numbers of bacteria 

 which serve as food for Infusoria, etc., and even the Mollusca. 



The character of the fertilizers used is much the same as that of 

 the manures applied to fields and meadows. Since the supply of nat- 

 ural manures is usually limited, chemical manures are resorted to, much 

 the same as in agricultural operations, viz., lime, potash, nitrogen (am- 

 monia) and phosphoric acid. The other elements, like sodium, iron, 

 magnesia, sulphur, silicic acid, etc., are usually present in sufficient 

 quantity and need not be supplied artificially. Carbon, which is neces- 

 sary for the support of all life, is usually supplied in abundance, when 

 nature is not interfered with, but when artificial fertilization is practiced 

 this important element may be reduced below the minimum required, 

 as compared with the proportion of other available food, and thus coun- 

 teract the benefits of fertilization (Zuntz, Gensny). When artificial 

 fertilization is practiced care must be observed that all the nutrient 

 elements or substances are supplied in sufficient quantity, or rather in 

 the relative proportions required, thus maintaining the chemico-physico- 

 biological equilibrium, or restoring it, as the case may be CDemol). 

 Practical pond or laboratory experiments may be conducted to obtain the 

 necessary information upon which to proceed with the fertilizing oper- 

 ation, much the same as in agriculture. 



According to Zuntz-Knanthe, tlie laboratory experiments for "bonitizing" fish 

 ponds are conducted as follows: The pond water in question is filtered through 

 paper filters and one-half liter of the filtered water placed in each of 14 flasks of ^ 

 liter capacity. These flasks are inoculated with one-celled green algae (e.g., Bacillus 

 profococcus). which are easily obtained from ponds with the aid of dipping nets. 

 After inoculation the flasks are treated with various solutions of nutrient salts, 

 etc., combined in definite proportions. Zuntz uses twelve different solutions,^' 4 

 drops of each solution being added to a flask. All flasks are then placed under the 

 same conditions of temnerature and light and kept under observation. After deter- 

 mining which of the flasks produces the most rapid or luxuriant growth, further 

 trials are made to determine the proper concentration of the solution in question. 

 After the algse have reached a certain degree of develooment, 2 daphnids are placed 

 into each flask, to test the effect of the fertilizers on the development of these very 

 important food organisms. In the course of from 8 to 10 days several generations 

 of daphnids will have developed and the effects of the different solutions become 

 very apparent. 



Zuntz's method above described, while not without certain imperfec- 

 tions, gives results that constitute a valuable basis for practical work. 

 The manifold and complicated conditions that influence the biological 

 processes in ponds (light, temperature, precipitation, currents, carbon 

 dioxid and bacterial content) cannot be reproduced in the laboratory. 



In the artificial fertilization of ponds it is necessary to have knowl- 

 edge of the minimum, optimum and maximum amounts of fertilizing 



12The followinET solutions, with the exceptions noted, are all of 5 per cent strength (No. 9, 2 

 per cent; No.s. 11 and 12 are used undiluted, prepared according to directions): 1. Glauber salts 

 (sodium sulphate). 2. Di-basic sodium phosnhnte. 3. Chile .saltpeter (sodium nitrate). 4. Com- 

 mon salt (sodium chlorid). S. Potassium sulphate, fi. Ammonium sulphate. 7. Calcium chlorid. 

 R. Ensom salt (maenesnim sulphate). 9 Conperas (ferro-sulphate) . 10. Milk of lime (caustic 

 lime). 11. Decomposed urine of animals (linuid manure). 12. Straw infusion (10 grams of 

 straw treated with 100 grams of boiling water, supernatant liquid poured ofF after twenty-four 

 hours and preserved for use). 



