52 



oping in the pots was determined by the methods de- 

 scribed by Brown (2), using his albumen agar. Each 

 soil was plated in duplicate, using dilutions of 1 to 

 20,000 and 1 to 200,000, as described by Brown. 



The Number of Microorganisms. The number of 

 microorganisms developing in the pots are given in 

 Table I. 



Table I — Microorganisms in Millions per 0.25 gm. of 



Air Dry Soil. Soil Potted June 30th. Compounds 



Added to Pots July 29th. 



From Table I it can be noted that in all cases the 

 compounds have caused a marked increase in the 

 numbers of microorganisms. In the case of vanillin 

 and quinoline this occurs after an initial depression. 

 In the case of pyridine and cumarin no such depression 

 is evident. It should be remembered, however, that 

 if a determination had been made sooner after the 

 addition of the compound to the pots, a depression in 

 numbers might also have been found in the case of the 

 latter two compounds. 



The maximum number of organisms developing in 

 four days at room temperature on the medium used 

 is 178.20 millions per gm. of air dry soil in the cumarin 

 treated pots 17 days after treatment. This is almost 

 40 times as many organisms as were present in the 

 untreated pots. The maximum number of organisms 

 observed was 218.24 millions in one of the cumarin 

 treated i)ots at the end of 17 days. The actual number 

 in any of the treatments may have been larger than 

 is indicated in the table as the greatest increase may 

 have occurred in the case of some of the treatments 

 at a time which fell between determinations. 



The maximum number of organisms occurs at difTer- 

 ent times depending on the treatment. In the cumarin 

 and pyridine treated soils the organisms reach their 



