ABSTRACTS 99 



Comparisons made of treated and untreated soils under various 

 conditions failed entirely to give any evidence in support of the theory 

 that there exists in soil a harmful biological factor which is destroyed 

 by the action of volatile antiseptics. 



The Relation of Protozoa to Certain Groups of Soil Bacteria. T. L. Hills. 



In this work studies were made of the effect of protozoa on ammoni- 

 fication, nitrification and free nitrogen fixation in soil. 



Ammonification. Three sets of the same sandy loam soil were used. 

 One set was left untreated, another heated to 90°C. for one hour and 

 the third heated as above and later inoculated with 1 per cent of the 

 original untreated soil thus introducing the supposed harmful factor, 

 the protozoa. The ammonia and nitrate determinations after 30 days 

 revealed the following: in the untreated soil the ammonia content 

 remained about the same while the nitrate increased slightly; in the 

 heated soil the ammonia increased considerably but the nitrate re- 

 mained the same, as the nitrifiers had been destroyed by heating the 

 soil. In the heated and reinoculated soil the ammonia decreased 

 slightly but there was a decided increase in nitrate formation. The 

 protozoa introduced did not seem to have any detrimental effect on the 

 production of ammonia and its subsequent oxidation to nitrate. 



Nitrification. Flasks containing soil were sterilized by heating at 

 15 pounds pressure for two hours. Then one half of them were inocu- 

 lated with a suspension of normal soil in sterile distilled water and the 

 remaining half were inoculated with the same amount of protozoa free 

 soil. This latter soil was obtained by sterilizing soil and inoculating 

 it with as many different kinds of bacteria as could be isolated by the 

 plate method. All flasks were then inoculated with protozoa free 

 cultures of Nitrosomonas and Nitrobacter. Then a definite amount 

 of ammonium sulphate was added. The ammonia and nitrate were 

 determined after 28 days' incubation. There was but very little 

 difference in the rate of conversion of ammonia to nitrate in the two 

 different soil cultures. 



Free nitrogen fixation. Sterile soil cultures were inoculated one 

 half with normal soil and the remainder with protozoa free soil. Then 

 a suspension of Azotobacter (and I per cent mannite in sterile solution) 

 was added and after incubation of 21 days at 25°C. the cultures did 

 not show any appreciable difference as regards the amount of nitrogen 

 fixed, the difference being quite well within the limit of experimental 

 error. In liquid cultures sterile Ashby's solution was used, one set 

 being inoculated with soil containing protozoa and the other set with 

 soil free from those organisms. Suspensions of Azotobacter were also 

 added. After 21 days' incubation at 25°C. the total nitrogen analyses 

 revealed a noticeable difference in free nitrogen fixation; those cultures 

 free from protozoa fixing 2.05 mgs. as an average per 100 cc. of solution 

 in excess of those containing protozoa. It seems evident that the 

 protozoa finding here a medium suitable for their development de- 

 stroyed many of the Azotobacter cells. 



