[Vol. 6 



8 ANNALS OF THE MISSOURI BOTANICAL GARDEN Gain 



Flaak over 

 content control 



Series V (without potassium and phospho- 



(mgs. N) (rags. N) 



rus) 



2.9 0.2 



Series VI (without sodium) 18.0 15.3 



Series VII (without potassium and sodium) 21.2 19.5 

 Series VIII (all nutrients, uninoculated) 2.7 



In addition to these essential elements it is quite evident 

 although absolutely definite data are lacking — that the re- 

 action of the medium is quite important. It is known beyond 

 any question that the medium must not be acid, but the exact 

 concentration of hydrogen as ion has not been properly stud- 

 ied. It has been more or less generally considered that the 

 reaction should be faintly alkaline, and the fact that the addi- 

 tion of solid calcium carbonate to the culture medium, which 

 is then maintained or " buffered' ' to a reaction of approxi- 

 mately P H 8.0, has found quite wide favor, is in accord with 



this idea. 



Now it is evident at once that these three requirements, 

 phosphates, calcium, and a faintly alkaline solution, are very 

 difficult to obtain, owing to the formation of the but slightly 

 soluble tricalcium phosphate. For instance, in Ashby's me- 

 dium, which is prepared from distilled water, mannite (or dex- 

 trose), potassium phosphate, sodium chloride, calcium and 

 magnesium sulphates, a trace of ferric chloride and solid cal- 

 cium carbonate, the phosphates are essentially quantitatively 

 transformed into the almost insoluble tricalcium phosphate. 

 In Beijerinck's medium, which is made from tap water, man- 

 nite (or dextrose), and dipotassium phosphate, the calcium 

 (contained in the tap water) is precipitated during the steril- 

 ization processes owing to the hydrolysis of the dipotassium 

 phosphate. 1 Thus Ashby's solution is deficient in soluble 

 phosphates, but is well buffered slightly on the alkaline side, 

 while Beijerinck's solution is lacking in soluble calcium salts, 

 its only buffer being a relatively low amount of dipotassium 

 phosphate, which maintains its reaction approximately at neu- 



1 In Winogradsky's medium for nitrifying bacteria the phosphorus is pre- 

 cipitated as ferric, ferrous, and magnesium phosphates. 



