OXIDATION OF SULFUR IN THE SOIL 247 



for twelve days. At the end of that period, the pH was deter- 

 mined by the colorimetric method, titration was obtained from the 

 amount, in cubic centimeters, of j^ NaOH necessary to neutralize 

 1 cc. of the filtered culture using phenolphthalein as an indicator. 

 The total sulfates were obtained by adding the amounts of 

 soluble and insoluble sulfates : the latter were obtained by digest- 

 ing the filtered residue in acidulated water and determining the 

 sulfates in an aliquot portion. 



In the presence of calcium phosphate, the largest amount of 

 sulfur oxidized by a pure culture of the organism was obtained in 

 medium 1, to which no stimulating agent has been added. In 

 the absence of the tri-calcium phosphate, the amount of sulfur 

 oxidized was appreciably less, oxidation in this case being stimu- 

 lated by various substances. The most beneficial influence was 

 exerted by the addition of a small amount of soil: this may be 

 due to the introduction, with the soil, of a small amount of the 

 lacking calcium salt or of some vitamine-like substance. The 

 favorable action of the organic substances, aluminum and 

 manganese sulfates, may be of a stimulating nature; however, 

 this beneficial action is only very small and lies within the range 

 of natural variability of the organism. 



Influence of temperature. The optimum temperature for the 

 activitiesof Thiobacillus thiooxidansn.sp. lies at about 28° to 30°C. 

 Growth and sulfur oxidation are much slower at lower tempera- 

 tures (18°) and at 37°C. Temperatures of 55°-60°C. are sufficient 

 to kill the organism. 



THE NATURE OF ACID FORMED AND THE INFLUENCE OF REACTION 

 UPON THE GROW^TH OF THIOBACILLUS THIOOXIDANS N. SP. 



To get an insight into the true nature of the acid formed, 

 particularly in the presence of tri-calcium phosphate, a series of 

 tubes containing 2 cc. portions of the culture were arranged; 

 measured quantities of j NaOH were added to these, then 

 the volume of the liquid was brought, in all tubes, to 3 cc. by 

 the addition of distilled water. The hydrogen ion concentra- 

 tion of the tubes was then determined, by the colorimetric method. 

 The results are tabulated in table 3 and graphically presented 

 in figure 2. 



