388 STATE BOARD OB" AGRICUI/rURE. 



THE INFLUENCE OF THE COMPOSITION OF THE MEDIUM 

 UPON THE SOLVENT ACTION. 



When bacteria are isolated rroin tlie soil, it is obvious to say that we 

 are cultivatiug them upon a medium someAvhat dili'erent than the soil 

 itself; but if we find that a certain soil bacterium, when j^rown upon 

 culture media, is capable of dissolving insoluble ]>hosphates, we may 

 suspect that this germ, even Avhile growing in the soil, does have the 

 power of making phosphorus available to plants. 



Several experiments have been carried on to determine the influence 

 of the composition of the medium upon tlie solvent action of certain 

 soil bacteria. The materials experimented upon were rock phosphate, 

 bone meal, tri-calcium phosphate, di-calcium phosphate and calcium 

 carbonate. After these minerals were finely ground, the powders were 

 shaken up with water and those particles which remained in suspension 

 for over half a minute were poured into a filter and washed Avith 

 water. A little of the washed powder was put into a flask of medium 

 and sterilized in live steam under 15 pounds pressure for fifteen minutes. 

 The sterilized medium was alloAved to cool to between 50°C. and 60°C., 

 then shaken to distribute the suspended particles equally. Plates were 

 ])Oured by using a sterile pipette to transfer about 8 c. c. of the medium 

 to sterile Petri dishes. They were inoculated by a stroke on, the surface 

 of the solidified agar and placed in a constant temperature room at 

 22° to 23^C. to wait developments. Here they were examined daily for 

 a visible solvent action which was made apparent by the suspended 

 particles near the growth of the germ disappearing and the zone adjacent 

 to the streak becoming clear and transparent. This is a crude quantita- 

 tive method for detecting the solvent action of bacteria, but if a minute 

 dissolution is to be discovered, a more sensitive method must be re- 

 sorted to. 



Ordinary nutrient agar was tried as a medium, but in no case was 

 there a visible dissolution of any of the five minerals tried. 



Nutrient agar containing 2% dextrose was used. This time several of 

 the germs showed an action upon the calcium carbonate and the di- 

 calcium phosphate, but no visible action was seen on either the bone 

 or the rock phosphate. 



A synthetic agar medium composed of .02% magnesium sulphate 

 and ammonium respectively and 2% agar in tap water was tried with 

 and without sugars. Into the first set of plates was poured the synthetic 

 medium alone; into the second set the synthetic medium plus 4% saccha- 

 rose; into the third, the fourth and the fifth .sets of plates was 

 poured the synthetic medium plus 1%, 2%, and 4% of dextrose respec- 

 tively. Not a germ was found which gave any visible action in the 

 plates containing no sugar; but, in the pr(>senre of Ihe 4% saccharose, 

 the 1%, 2% and the 4% dextrose, some of the germs gave a marked 

 action upon the calcium carbonate and the di-calcium and tri-calcium 

 phosphates. Some even dissolved those suspended particles which were, 

 in cases, two centimeters from the groM'th of the germ. As before, 



