Apr. 3, i9i6 Relation of Carbon Bisulphid to Plant Growth 3 



At first the antiseptic causes a great reduction in the number of organ- 

 isms capable of developing on Heyden agar. The period of depression 

 lasts for only a short time — in this experiment about five days. From 

 that time until the end of the test the number of organisms in the treated 

 series far exceeded that of the control. The highest number in the 

 carbon bisulphid evaporated and unevaporated soil occurred about the 

 thirteenth day; while the carbon bisulphid evaporated soil plus control 

 soil gave the highest count on the fifth day. At the time of the last 

 count, 60 days after carbon bisulphid was added, the organisms in the 

 treated series far exceeded those in the original soil. Apparently the 

 effect of carbon bisulphid on the number of bacteria is noticeable for a 

 long period of time. 



If the results of the counts with carbon bisulphid unevaporated are 

 compared with those of carbon bisulphid evaporated, it appears that no 

 very marked difference exists. The greatest reduction in numbers 

 occurred in soils with the carbon bisulphid evaporated. It is significant 

 that soil with carbon bisulphid evaporated should prove more injurious 

 to micro-organisms than the unevaporated. This agrees with Gainey (2, 

 p. 592), who reports that the combined effect of the two processes seemed 

 more injurious to nitrification than treatment with carbon bisulphid 

 unevaporated. 



After the thirteenth day the treated and reinoculated soil did not 

 show as many organisms as the treated series. This difference is shown 

 very distinctly in Plate I, which is reproduced from a photograph of 

 a number of colonies developing on agar. Four parallel plates were 

 made from the same dilution of each soil. 



On this date samples were also drawn for ammonification tests. The 

 purpose of this was to measure the rate of the decomposition of casein 

 in the various series, and i per cent of casein was added to the soil and 

 the ammonia determined after 12 and 24 hours. The beneficial effect 

 of carbon bisulphid on ammonification is very evident. If after 12 hours 

 the untreated is 100, then carbon bisulphid unevaporated is 154, carbon 

 bisulphid evaporated is 212, and carbon bisulphid reinoculated is 190. 



After 24 hours the untreated is equal to 100, carbon bisulphid un- 

 evaporated is 149, carbon bisulphid evaporated is 171, and carbon 

 bisulphid reinoculated is 153. The data show very clearly that casein 

 is decomposed more rapidly in treated than in untreated soils. This 

 difference is most prominent in the 12 -hour tests. 



Protozoa. — Counts at the beginning showed the presence of protozoa 

 in dilutions representing i to 1,000 gm. of soil (13, p. 626). Two weeks 

 after treatment the soils were recounted. At this time numerous small 

 flagellates were found in dilutions of i to 1,000. It is evident that the 

 different treatments with carbon bisulphid had not seriously injured this 

 group of organisms. 



