160 NINTH REPORT. 



THE INFLUENCE OF THE COMPOSITION OF THE MEDIUM UPON 

 THE SOLVENT ACTION OF CERTAIN SOIL BACTERIA. 



Charles W. Brown. 



If phosphate fertihzers, which are insokible in water, are to be used eco- 

 nomically upon soils deficient in phosphorus, it is necessary to know what 

 agents are active in rendering the insoluble phosphates soluble ; and to what 

 extent these agents act. It is known that soil moisture, that chemical 

 compounds in the soil, that acids secreted by the roots of plants, that the 

 carbon dioxide held by the soil water, and that bacteria are the most im- 

 portant factors in the dissolution of insoluble phosphates. It is toward the 

 soil bacteria that I shall direct my remarks. 



When bacteria are isolated from the soil, it is obvious to say that we are 

 cultivating them upon a medium somewhat different than the soil itself; 

 but if we find that a certain soil bacterium, when grown upon culture media, 

 is capable of dissolving insoluble phosphates, we may suspect that this germ, 

 even while 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 the solvent action of certain soil bac- 

 teria. The materials experimented upon are rock phosphate, bone, tri- 

 calcium phosphate, dicalcium phosphate and calcium carbonate. After 

 these minerals are finely ground, the powders are shaken up with water 

 and those particles which remain in suspension for over half a minute are 

 poured into a filter and washed with water. A little of the washed powder 

 is put into a flask of medium and sterilized in live steam under 15 pounds 

 pressure for fifteen minutes. The sterilized medium is allowed to cool to 

 l)etween 50 °C., and 60° C. ; then shaken to distribute the suspended particles 

 equall}'. Plates are poured by using a sterile pipette to transfer about 

 eight cubic centimeters of the medium to a sterile Petri dish. They are 

 inoculated by a stroke on the surface of the solidified medium, and placed 

 in a temperature room at 22° to 23° C, to wait development. Here they 

 are examined daily for a visible solvent action which is made apparent by 

 the suspended particles near the growth of the germ disappearing. This 

 is a crude comi:)arative quantitative method for detecting the solvent action 

 of bacteria, but if minute dissolution is to be detected, a more sensitive 

 method must be resorted to. 



Ordinary nutrient agar was tried as a medium, ])ut 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 and the tricalcium phosphates, but no visible action was seen on 

 either the bone or the rock phosphates. 



A synthetic agar medium composed of .02% magnesium sulphate and 

 ammonium sulphate respectively and 2% agar was tried with and without 

 sugars. Into the first set of plates was poured the synthetic medium alone; 



